Drainage of earthwork slopes PPR341 - trl.co.uk · TRL Limited PPR 341 4.1.7 Electro-osmosis 43 4.1.8 Vegetated slopes 45 4.2 Maintenance 45 4.2.1 Deep trench drains, counterfort

Published Project ReportPPR341

Drainage of earthworks slopes

D R CarderG R A Watts (TRL Limited)L CamptonS Motley (Halcrow Group Ltd)

Drainage of Earthworks Slopes

by D R Carder, G R A Watts (TRL Limited), L Campton and S Motley (Halcrow Group Ltd)

PPR 341 153(666)

PUBLISHED PROJECT REPORT

TRL Limited

PUBLISHED PROJECT REPORT PPR 341

DRAINAGE OF EARTHWORKS SLOPES Version: Final

by D R Carder, G R A Watts (TRL Limited), L Campton and S Motley (Halcrow Group Ltd)

Prepared for:Project Record: 153(666) HTRL Drainage of Earthworks Slopes

Client: T Messafer (SSR, Highways Agency)

Copyright TRL Limited May 2008 This report has been prepared for the Highways Agency. The views expressed are those of the authors and not necessarily those of the Highways Agency. Published Project Reports are written primarily for the Client rather than a general audience and are published with the Client’s approval.

Approvals

Project Manager Derek Carder

Quality Reviewed Bill McMahon

This report has been produced by Halcrow/TRL under a Contract placed by Highways Agency. Any views expressed are not necessarily those of the Agency.

TRL is committed to optimising energy efficiency, reducing waste and promoting recycling and re-use. In support of these environmental goals, this report has been printed on recycled paper, comprising 100% post-consumer waste, manufactured using a TCF (totally chlorine free) process.

TRL Limited PPR 341

CONTENTS

Executive summary i

1 Introduction 1

2 The distribution of pore water pressure in highway slopes 2

2.1 The distribution of pore water pressures in embankments 2 2.1.1 M1 Toddington 2 2.1.2 M26 Dunton Green 4 2.1.3 A45 Cambridge Northern Bypass (2 slopes) 4 2.1.4 M26 Nepicar 5 2.1.5 M25 Franks Farm 6

2.2 The distribution of pore water pressures in cuttings 7 2.2.1 M25 Godstone 7 2.2.2 A12 Colchester 9

2.3 The effect of vegetation upon pore water pressures 10 2.4 Predictions of the pore water pressure regime 12

2.4.1 Embankments 12 2.4.2 Cuttings 13

3 Critical review of methods of slope drainage 15

3.1 Deep trench drains, counterfort drains and rock ribs 15 3.1.1 A12 Romford 15 3.1.2 A120 Colchester 18 3.1.3 A45 Cambridge Northern Bypass 19

3.2 Shallow slope drains 20 3.3 Embankment drainage blankets and sand drains 20 3.4 Filter and fin drains 20

3.4.1 Filter drains 20 3.4.2 Narrow filter and fin drains 22

3.5 Open ditch and surface water channels 23 3.5.1 Applicability of channels for slope drainage 25 3.5.2 Concrete channels 25 3.5.3 Grassed channels 25

3.6 Vertical and horizontal drains 28 3.6.1 Vertical drains 28 3.6.2 Horizontal drains 28

3.7 Electro-osmosis 30 3.8 Vegetated slopes 31 3.9 Interaction with highway drainage systems 32

4 Best practice guidance in design and maintenance 35

4.1 Design 35 4.1.1 Deep trench drains, counterfort drains and rock ribs 35 4.1.2 Shallow slope drains 36 4.1.3 Embankment drainage blankets and sand drains 37 4.1.4 Filter and fin drains 37 4.1.5 Open ditch and surface water channels 38 4.1.6 Vertical and horizontal drains 40

TRL Limited PPR 341

4.1.7 Electro-osmosis 43 4.1.8 Vegetated slopes 45

4.2 Maintenance 45 4.2.1 Deep trench drains, counterfort drains and rock ribs 45 4.2.2 Shallow slope drains 46 4.2.3 Embankment drainage blankets and sand drains 46 4.2.4 Filter and fin drains 46 4.2.5 Open ditch and surface water channels 46 4.2.6 Vertical and horizontal drains 47 4.2.7 General 48

4.3 Interaction with highway drainage systems 48

5 Survey of practitioners 50

6 Selection of Drainage Measures 51

6.1 Selection of drainage measures 51 6.2 Impact of climate change 52

7 Conclusions 55

8 Recommendations for further research 55

9 Acknowledgements 57

10 References 58

Appendix A. Questionnaire to practitioners 63

Appendix B. Costs of drainage remedial measures at 2002 prices 66

TRL Limited i PPR 341

Published Project Report Version: Final

Executive summary The stability of highway embankments and cuttings is critically dependent on the magnitude and distribution of pore water pressures within the soil. Pore pressures arise from the presence of water within the soil matrix, which may be introduced via natural precipitation and/or recharge from nearby external sources. For all earthwork slopes, rainfall will enter the underlying soil through the surface, in quantities which depend on the soil type, topography, and vegetation cover. Slope drainage can control the movement of surface water and also the subsurface pore water pressure in the slope. Drainage can be very effective if installed at the correct location on or within the slope. In the long term, systems need to be designed with maintenance operations in mind so that a sustainable system is installed with a design life comparable to the 60 year design life of a highway slope.

Although it is widely recognised that robust, sustainable and cost-efficient slope drainage measures are of critical importance, there is comparatively little information on slope related issues in current Highways Agency documents. There are references in a number of the Advice Notes and Departmental Standards within the DMRB relating to earthwork drainage, but none deal specifically with this subject. This study has aimed at addressing this issue by highlighting references in the DMRB, identifying other sources of information and noting relevant documented case/field studies.

The report commences by reviewing the measured distributions of pore water pressures in both embankment and cutting slopes. A critical assessment is then made of the available slope drainage techniques and related issues, these include:

• slope drains, counterfort drains and rock ribs,

• embankment drainage blankets and sand drains,

• filter and fin drains,

• vegetated slopes,

• open ditch and surface water channels,

• vertical and horizontal bored drains,

• electro-osmosis,

• interaction with highway drainage systems.

The current design practices for each of the above techniques are given in the report and a look-up table is presented that relates the development of pore water pressures in slopes and drainage issues to appropriate remedial measures, and provides a crude comparison of relative costs.

The increasing frequency of extreme rainfall events caused by climate change requires greater emphasis to be placed on a consideration of storm water and surface run-off at the design stage. This is to avoid in-service problems with mud and debris slides and to minimise the infiltration of water into clay slopes after the development of shrinkage cracks during prolonged hot spells.

The report concludes by recommending an increased use of surface water channels and a more proactive bioengineering approach in the design of slope drainage systems. These measures offer the advantages of ease of inspection, reduced maintenance costs and a sustainable technique.

TRL Limited ii PPR 341


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TRL Limited 1 PPR 341


1 Introduction For all earthwork slopes, rainfall will enter the underlying soil through the surface, in quantities which depend on the soil type, topography, and vegetation cover. Slope drainage can control the movement of surface water and also the subsurface pore water pressure in the slope. Drainage can be very effective if installed at the correct location on or within the slope. In the long term, systems need to be designed with maintenance operations in mind so that a sustainable system is installed with a design life comparable to the 60 year design life of a highway slope.

The impact of climate change upon drainage and slope stability also needs to be considered in the design. Advice on this is given by the UK Climate Impacts Programme (UKCIP02) and impacts include:

• higher temperatures,

• changing precipitation patterns (including more extreme events),

• changes in other variables (e.g. number of storms, changes in sea level, regional effects).

Currently the number of slope failures on the highway network is limited and failures which do occur are mostly associated with the high plasticity, over-consolidated clays which are common in England (Perry, 1989). Clay slopes are particularly susceptible to drying during prolonged dry summers and this leads to the formation of shrinkage cracks which rapidly fill with water during intense rain storms. This ingress of water can lead to softening of clay slopes and adversely affect slope stability in critical cases. More severe winter conditions with heavier extreme rainfalls will lead to a rise in the water table with increased pore water pressures in the slope leaving soils, especially those that are clayey, vulnerable to reductions in soil strength which adversely affect slope stability.

Although the focus of this report is on soil slopes, a wider range of slope failures occurs in the north of England, Wales and Scotland because of the range of geologies. For example, the issue of landslides, slumps, and mudflows has been extensively considered by Transport Scotland and a summary of the findings is given by Winter et al (2005).

Slope instability affects the infrastructure foundation and can damage other assets located on the embankment or in the cutting. Large slope movements or settlements lead to traffic speed restrictions or route closure, and can damage the carriageway and any footway. Hence the understanding and management of drainage provisions to improve the longevity and stability of slopes is of prime importance to all infrastructure owners. For this reason, although the emphasis of this study is on highway slopes, some information from other slopes has been included where relevant.

This report commences by reviewing the measured distributions of pore water pressures in both embankment and cutting slopes, so that a critical assessment can be made of the applicability of different slope drainage techniques.

The various drainage techniques and issues covered by the report include:

• slope drains, counterfort drains and rock ribs,

• embankment drainage blankets, sand drains and wick drains,

• filter and fin drains,

• vegetated slopes,

• open ditch and surface water channels,

• vertical and horizontal bored drains,

• electro-osmosis,

• interaction with highway drainage systems.

The report concludes with best practice guidance in the design and maintenance of the various drainage systems.

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2 The distribution of pore water pressure in highway slopes An understanding of the distribution of pore water pressures within a slope is of particular relevance in determining the type and location of suitable slope drainage systems. Methods for the field measurement of pore water pressures are described by Ridley et al (2003). Pore water pressure is one of the dominant factors to be controlled to ensure the stability of slopes and a review of the measured distributions of pore water pressure in typical earthworks slopes has therefore been undertaken. This is based largely on previous TRL measurements at a number of sites on highway schemes. In general, these sites are at locations of incipient instability and are therefore very relevant to an understanding of how drainage provisions can be optimised with respect to stability improvement, and where they are best located.

The interpretation of the measurements at the TRL sites takes into consideration previous numerical analyses of pore pressures which have identified potential for water pressures to build up, possibly leading to delayed collapse of clay slopes. The prediction of moisture migration in highway slopes is an important aspect.

The groundwater regime in embankments and cuttings is very different and for this reason the case history studies described in this report are separated into these two categories. It must be noted that for a vegetated slope, water will be removed by transpiration: these issues are separately discussed with case history studies in Section 2.3.

2.1 The distribution of pore water pressures in embankments

2.1.1 M1 Toddington

TRL has investigated the pore water pressures in a 6.5m high Gault Clay embankment on the M1 (Carder et al, 2001). The embankment was constructed as part of the section of the M1 (Luton to Ridgmont) which was opened in 1959. The embankment was constructed using clay excavated from neighbouring areas of the works and the slope angle was approximately 1 in 2. An open ditch was used for drainage purposes in front of the toe of the slope and there is no record of any drainage blanket being used beneath the embankment. Strength profiles obtained from tests carried out using the Panda penetrometer through the embankment material and into the clay foundation found no evidence of a layer of higher strength. This tended to confirm that a drainage blanket had not been installed below the clay fill at the time of embankment construction.

In recent years the site had shown some instability with numerous very shallow slips being evident on the grassed slope giving it an undulating appearance. In Figure 2.1 the measured pore water pressures at the piezometer locations are shown along with contour plots of the pore pressure distribution. The piezometers used incorporated vibrating wire transducers with hydraulic twin tubes for de-airing purposes so that small suctions could be recorded.

The measurements in Figure 2.1 show the contours of positive pore pressure which extend from the body of the embankment into the side slope. Within the body of the embankment the pore pressure initially increases with depth as would be expected because of the incidence of rainfall on the central reserve and other unsurfaced areas at the top of the embankment. Below about mid-height these positive pressures reduce as drainage occurs into the embankment foundation. Figure 2.1 shows the contours of pore pressures of 5kPa and 10kPa extending from the body of the embankment into its side slope by an amount primarily related to the seasonal rainfall. For example, the contour of 10kPa pore pressure is far more extensive in May 2000 and February 2001 after periods of heavy rainfall. Although generally the pore pressures within 1m of the slope surface are in suction, the contour plot in February 2001 indicates the possibility that after prolonged and intense heavy rainfall, positive pore pressures could occur near the slope surface at about mid-height. This could account for the incidence of shallow failures at this site. It is also evident in Figure 2.1 that pore pressures measured on the piezometer at foundation level and nearest to the toe of the slope fluctuated considerably depending upon the season. In February 2001 a maximum value of 10.2kPa was recorded at this location and, after prolonged rainfall events, it is envisaged that even higher pore pressures could be developed at the toe adversely affecting the slope stability.

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(a) May 1999

(b) June 1999

(c) August 1999

(d) May 2000

(e) August 2000

(f) February 2001

Figure 2.1. Contour plots of pore pressure (kPa) in the Gault Clay embankment on the M1

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2.1.2 M26 Dunton Green The embankment slope on the M26 at Dunton Green is 8.3m high with a slope angle of 1 in 4. The embankment fill was Gault Clay. Some while after construction, TRL installed an array of flushable twin tube hydraulic piezometers with hydraulic heads being measured utilising hydraulic scanning valves which switched each piezometer in turn to a pressure transducer in the gauge house. Each piezometer was pushed into the base of pre-formed boreholes so that intimate contact was obtained between the piezometer tips and the clay. In this way the distribution of pore water pressure in the slope could be determined.

Crabb and Hiller (1993) produced plots of the variation of pore pressure with time as part of an investigation into the mechanism of shallow failure in Gault Clay embankment slopes, although it must be noted that no visible signs of failure were evident at the time of monitoring.

The data presented by Crabb and Hiller (1993) showed that there is an annual cycle of pore water pressure due to seasonal wetting and drying, which penetrates to the depth of about 1.5m. This depth correlated with that at which shallow failures in Gault Clay slopes commonly occurred and a potential failure surface is indicated in Figure 2.2.

+20

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Figure 2.2. Contour plot of pore pressures (kPa) in the Gault Clay embankment

on the M25 (March 1988)

It is noticeable that pore pressures at depth in the slope are much less than those reported for the Toddington M1 site (see Section 2.1.1). This may be related to the age of the earthworks as the suctions in the M1 clay fill have had much longer to dissipate. It is expected that more positive pressures will develop at depth in the M25 slope in the course of time.

2.1.3 A45 Cambridge Northern Bypass (2 slopes) A trial of seven different slope stabilisation techniques was carried out on the A45 (now A14) and reported by Johnson (1985). The purpose of the trial was to identify the most satisfactory and economical method of repairing and preventing the numerous shallow failures that commonly occur on Gault Clay embankments. The embankment was 7.8m high with a nominal side slope angle of 1 in 2. The section of the slope chosen for the study was an unfailed section of about 35m length. Pore

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water pressure measurements using flushable hydraulic piezometers were initiated in January 1984 and the first signs of movement were apparent in May 1985 when a 600mm deep crack appeared near the crest. By July 1986, heave was visible near the toe of the slope. The pore water pressures recorded in April 1987 and the shape of the failure surface are shown in Figure 2.3.

0 1 2

Metres

0-10

-40

-80

Original surface

Failure surface

Ground surface (post-slip)

Piezometer

+10


on the A45 (April 1987)

As a result of the failure it is difficult to draw conclusions about the pore pressures within the failed zone, however the contours in Figure 2.3 clearly indicate the high suctions beneath the embankment which extend well into the side slope. It is known that the fill at Cambridge came from a deep borrow pit at Milton. In the core of the embankment this suction has probably been sustained by the combination of low infiltration through the pavement and the elevation above the natural groundwater level, whereas in the slope greater infiltration and lower elevation have allowed the pressure to increase. Given the high levels of suction, the potential for deeper seated future failures is particularly low provided that water ingress is limited.

2.1.4 M26 Nepicar

The Gault Clay embankment at Nepicar on the M26 was located on the north-facing slope of the west approach embankment, where it is 6.7m high and at a slope angle of 1 in 3.3. The length of slope was considered to be at risk as it was adjacent to an existing, unrepaired failure towards the bridge abutment end of the embankment. Flushable, twin-tubed hydraulic piezometers were installed in January 1985 (Crabb and Hiller, 1993). Following piezometer installation, the first evidence of failure in the instrumented section occurred in June 1988 when cracking and vertical movement were observed over several metres, close to the crest of the slope. The contours of pore pressures measured within the slope in December 1988 are shown in Figure 2.4. Further movements and cracking at the crest occurred subsequently in 1989.

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Ground Surface

Piezometer

0

Metres

5


on the M26 (December 1988)

High suctions were again measured beneath the embankment and these extended into its side slopes. These observations were similar to those at Cambridge and although the source of the Gault Clay fill at Nepicar is not known it is presumed that it came from a deep borrow pit in much the same way. The contours in Figure 2.4 suggest that positive pressures develop seasonally near to the surface as water migrates into the clay to dissipate its high suction.

2.1.5 M25 Franks Farm The embankments in the vicinity of Junction 29 of the M25 motorway were constructed in 1981 using high plasticity over-consolidated London Clay as fill material. Following a series of slope failures, both shallow and deep, in the area, instruments were installed in 2003 to monitor the performance of the slopes at three locations to the south of Junction 29. Flushable twin tube hydraulic piezometers connected to a vibrating wire pressure transducer were again installed to monitor the seasonal variation of pore water pressures. However as the main focus of the investigation was to measure slope movements, an insufficient number of piezometers were actually installed to produce reliable contour plots of pore pressure. Nevertheless some useful conclusions on the pore water regime could still be made. The main conclusions were:

• measurements of pore water pressures at the central reservations of the three slopes indicated the existence of a perched water table in a clayey sand layer, which overlies the more impermeable clay. Some cyclic change in pore pressures was measured which was considered to be related to infiltration of rainfall.

• measured pore pressures within the clay fill of the side slopes were, with a few exceptions, generally in the range +5kPa to -10kPa. Suctions appeared to be particularly prevalent near the level of the drainage blanket where some under-drainage occurred and also at shallow depths beneath the grassed slopes.

• at all three slope locations, progressive lateral movements of the slope surface associated with shallow failures were measured over a 28 month duration. Superimposed on the progressive trend of downslope movement is a cyclic

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seasonal effect related to swelling of the clay during the wet season with shrinkage of the surface during the summer season.

Movement data were similar at all three locations even though at two of these drainage ditches exist at the toe of the slope, whilst at the toe of the other there is no drainage ditch with some flooding of the adjoining field occurring during the winter.

2.2 The distribution of pore water pressures in cuttings

2.2.1 M25 Godstone

Following a deep-seated failure of a Gault Clay cutting slope near Junction 6 of the M25, the slope was remediated using a single row of spaced bored piles (1.05m diameter and 16m long piles at 2.5m centres) accompanied by extensive drainage works comprising the construction of deep counterfort drains downslope of the piles and a cut-off drain upslope of the backscarp. Details of subsequent ground and pile movements are not the subject of this study but are reported by Carder and Barker (2005a). One aspect of this study however included the measurement of pore water pressures after remediation and these are of significance when looking at the effectiveness of the installed drainage measures.

Generally the side-long slope comprised stiff fissured Gault Clay, in some areas weathering of the upper zones was readily identified and in others the clay was overlain by a few metres of poorly sorted Head deposits. Gault Clay cutting slopes are known to be a geology associated with a high percentage of shallow failures (Perry, 1989). On the motorway network they tend to have a predominant slope angle of 1:2.5. In this case, the relatively deep-seated failure occurred even though the slope angle was only 1:4 to 1:5.

Figure 2.5 shows the variations of pore water pressure at three selected dates. Two of these (March and October 2002) were selected to illustrate the normal seasonal range of pore pressure conditions, whilst the values recorded in January 2003 are shown as pore pressures peaked at this time. On the latter date, pore pressures at 20m upslope and those close to the pile line exceeded the normal range of values, whilst any effect was less noticeable at 36m upslope possibly because of the proximity of the cut-off drain.

Also indicated in Figure 2.5 is the mean level of the water table in the underlying unweathered Gault Clay; below this level the increase in pore water pressure with depth was approximately hydrostatic. At all locations a perched water table existed in the weathered clay and head deposits: this was particularly evident at the piezometer locations furthest (i.e. 36m) upslope from the stabilising piles.

The cut-off and counterfort drainage systems were generally effective in controlling the development of excess pore water pressures within the remediated slope. However Carder and Barker (2005a) report continuing lateral movements of the ground at 20m upslope from the pile line. The development of these movements is strongly related to the cumulative effect of any persistent and heavy rainfall. In addition to surface lateral movements, peaks in subsurface movement at this location developed at about 152mAOD (5m depth) within the weathered clay and at 148mAOD (9m depth) in the unweathered clay. Because these movements are at depth, it is not clear if further drainage measures on their own would be adequate in controlling them without additional stabilising measures.

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2.2.2 A12 Colchester The cutting slope is located on the southbound carriageway of the A12 Colchester Northern Bypass. The slope has a maximum height of 9m and a slope angle of approximately 1:2. The cutting slope is mainly constructed in London Clay, although various repairs have been carried out by reinstating the slope using imported granular material. Following the last reinstatement, the slope failed again by deep-seated slippage in the underlying clay. A more permanent structural solution was therefore sought by using a line of spaced piles (900mm diameter and 13m long piles at 3m centres) and improving top of slope drainage (Carder and Barker, 2005b).

As part of the performance monitoring of the remedial works, piezometers were installed at various depths within the slope. Figure 2.6 shows the measured pore water pressure distributions after completion of the remediation.

A cut-off filter drain at the top of the slope was installed immediately after piling was completed and this controlled the apparent water table to about 2m to 3m depth below the slope surface. There was evidence of a perched water table in the weathered clay of the slope, as pore water pressures increased with depth but then reduced again nearer to the weathered/unweathered clay interface at about 9m depth below the crest of the slope.

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Cut-off drain

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01-06-2003

01-09-2003

01-12-2003

01-03-2004

01-06-2004

01-09-2004

01-12-2004

16-02-2005 Line of spaced piles

Figure 2.6. Pore water pressure distribution in a failed London Clay cutting slope on the A12 after its remediation

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2.3 The effect of vegetation upon pore water pressures

The presence of vegetation on a slope brings both hydrological and mechanical benefits to the stability of slopes. Vegetation modifies the moisture content of the soil, so influencing soil strength, and the presence of roots in the soil also increases soil strength and, therefore, its stability.

The effect of vegetation can be viewed as an “indirect” drainage measure as its presence can reduce the soil moisture content and the pore water pressure near the slope surface and prevent ingress of water into and possible erosion of weathered soil at the surface. Vegetation cover can reduce soil moisture content through:

• foliar interception and direct absorption and evaporation of rainwater, which reduces the amount infiltrating into the soil; and

• extracting soil moisture via the transpiration stream, thus reducing pore water pressure and counteracting the reduction in soil strength that wetting causes.

Transpiration is greater when plants are growing rapidly in the summer, under non-limiting moisture conditions. Many plants that live in damp habitats are characterised by high transpiration rates, and so have a high capacity to remove water from the soil. Such plants are potentially useful for reducing high pore water pressures, but their tolerance of drier soils may be limited.

MacNeil et al (2001) reported a survey of the plant groups relevant to soil stabilisation, the form and function of the root systems, and the processes by which vegetation may enhance stability. Soil water depletion is strongly influenced by root depth and distribution. Grasses are capable of rooting to a reasonable depth in good soils and the ability of many trees to reduce soil moisture to considerable depth is well known1. However planting of trees on highway slopes may possibly give rise to other problems such as leaf fall2, disruption to the network if blown over by strong winds, and the need for coppicing. A suitable compromise may be to plant live willow poles, which have been proven to be effective in preventing shallow failures by both dowelling action and moisture depletion (Steele et al, 2004).

As plant metabolic activity and transpiration are lower over the winter, this reduces the impact vegetation has on soil/water relationships during the period when rainfall is at a maximum. This led Coppin and Richards (1990) to argue that the ability of trees to reduce soil water and affect soil strength would be less than the effect their roots had on soil reinforcement, especially at the times critical for slope stability, i.e. in the spring when soil moisture is high and before transpiration rates begin to increase. However with the increasing impact of climate change and the possibility of intense summer storms, the use of vegetation may become more important.

Examples of tensiometer measurements of soil suction beneath the north slope of a trial embankment planted with different types of vegetation are shown in Figure 2.7. As would be expected higher suctions were recorded under trees and shrubs than those located under perennials and grasses. Suctions generated by the vegetation varied seasonally with only low suctions being measured between late autumn and late spring, when earthwork slopes are most vulnerable to failure. Further examples of suction measurements below vegetation and live willow poles are given by MacNeil et al (2001) and Steele et al (2004) respectively.

1 Network Rail has identified common high water demand trees as elm, oak, poplar, willow and hawthorne. 2 Network Rail has identified problem leaf fall species as poplar, ash, chestnut, lime and sycamore.

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Figure 2.7. Tensiometer data for trees, shrubs, perennials and grasses on the north slope of an embankment

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2.4 Predictions of the pore water pressure regime

2.4.1 Embankments

The pore water pressure distributions in embankments are very dependent on whether a drainage blanket was installed at the time of construction and if it is still operative. Coupled consolidation3

analyses using an elastic perfectly plastic soil model were carried out by Geotechnical Consulting Group (1993) and Carder and Easton (2001). The former found that when an embankment is constructed directly on a clay foundation, the initial rupture surface can develop rapidly from the toe of the slope along the foundation. When an embankment is constructed on a drainage blanket, the potential rupture surfaces are confined to the slope above the drainage blanket and are shallower, particularly if the surface of the slope is weathered and more permeable.

Carder and Easton (2001) reported that after long term consolidation the final pore water pressure distributions are as shown in Figure 2.8. In Figure 2.8a the pore pressure in the centre of the embankment initially increases with depth and then reduces again at the level of the drainage blanket. Pore water pressures in the foundation then increase again approximately hydrostatically with depth. Under this pore water pressure regime, suctions develop near the slope face as would be expected.

In some situations, vertical ingress of water to the embankment may result in higher pore pressures within the clay fill and the pore pressure fixities were therefore modified to produce the final regime shown in Figure 2.8b.

More significant differences in slope behaviour are predicted if either the embankment is constructed directly on its soil foundation without using a drainage blanket or if the blanket fails to operate satisfactorily. The final pore water pressure regime then follows the pattern shown in Figure 2.8c with the drawdown in water pressure near to the foundation not being observed. In this case the ground movements which occur are not only larger but more deep-seated.

3 Coupled consolidation allows dissipation of pore pressure according to the specified permeabilities of the different soil layers. In this way the magnitudes of both the effective stress and the pore pressure can be determined at different time increments.

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(a) Drainage blanket present

(b) Drainage blanket present plus increased pore pressure in slope

(c) Case (a) without drainage blanket

Pore pressure

Figure 2.8. Predicted pore water pressure regime after 60 years in service (embankment slope)

2.4.2 Cuttings

Geotechnical Consulting Group (1993) looked in some detail at delayed failures in clay cuttings. The equilibrium pore water pressures predicted in a clay cutting with an initial earth pressure coefficient (K) of 1.5 are shown in Figure 2.9. The contours of deviatoric plastic strain in Figure 2.9a show the development of rupture from the toe. Figure 2.9b shows the equilibrium pore pressures in the slope. These data were also included in the Rankine lecture on natural slopes and cuts reported by Leroueil (2001).

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(a) Accumulated plastic strain 14.5 years after excavation, just before collapse

(b) Equilibrium pore water pressures

Figure 2.9. Analysis of 10m high cutting slope (1:3) assuming K of 1.5 and surface suction of 10kPa

Geotechnical Consulting Group found considerable variability in the predicted time to collapse of the slopes analysed. The parametric studies for a 1:3 slope gave collapse times ranging from 14 to 45 years. The surface hydraulic boundary condition had a strong effect on stability. An increase in surface suction from 10kPa to 20kPa went more than half way towards stabilising the 1:3 slope, and it had almost as much effect as significant under-drainage below the cutting foundation. Further examination of the role of vegetation in increasing evapo-transpiration and surface suction may therefore assist in controlling the risk of long term deep-seated failures.

The authors also concluded that surface pore pressures can be reduced by counterfort drains, although these need not be to the depth of a potential deep-seated slide. Drilled drainage systems which can be installed horizontally into slopes could also reduce pore pressures sufficiently to eliminate the risk of delayed collapse.

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3 Critical review of methods of slope drainage All methods of slope drainage have been critically reviewed including deep and shallow trench drains, counterfort drains and rock ribs, embankment drainage blankets, fin and filter drains, vertical and horizontal bored drains, electro-osmosis, and the use of vegetation control measures. The effects of climate change which are likely to involve wetter winters and more intense rainstorms in the summer are also important as slope drainage systems will need to have the capability of dealing with extensive surface water run-off.

Current design practice and maintenance is dealt with separately in Section 4.

3.1 Deep trench drains, counterfort drains and rock ribs

Drains that run down the slope, perpendicular to the crest, draining the upper 2m (or so) of soil are often referred to as slope (or trench) drains. Deep slope drains, which sometimes penetrate to the full depth of the embankment below any potential slip surface, are often known as counterfort drains. Counterfort drains are normally lined with a geotextile layer to prevent erosion behind and fouling within them, although detailed design needs to take account of geology. Where the counterfort drains are filled with coarse material (stone-fill) they are often known as rock ribs: these also provide a resistance to soil shear and therefore have a stabilising effect. In general this type of drain requires a positive fall to a collection point to prevent ponding and to avoid introducing water into the slope. A slotted drainage pipe is sometimes laid in the base to help water flow through the drain.

Information on slope drain performance is available from monitored trials at Romford and Colchester, and a further trial of a counterfort drainage system undertaken at Cambridge Northern Bypass. The results of these studies are reported below.

3.1.1 A12 Romford

The A12 trunk road at Romford, which was completed in 1934, was constructed with its south side in a 6m deep London Clay cutting with an average slope angle of 1:4 (14o). After some earlier minor movements, the lower half of the slope failed in 1969. The affected part was regraded to about 12o, an interceptor drain about 1m deep was laid along the top of the regraded slope, and herringbone drains about 0.6m deep were installed in the slope. Further problems occurred in 1975 when the middle of the regraded slope sank and there was distortion of the pavement. A softened zone, which may have been the slip zone, was subsequently identified at a depth of 1.8m in the middle of the slope.

In 1981 deep trench drains were installed running down the slope at 5m centres (Figure 3.1) and depths of 3m at the top and middle of the slope and between 1.2m and 2m at the bottom. The slope drains discharged into a filter drain, which discharged into a surface water drain at the west end of the site. Both the slope and filter drains were 0.6m wide, and backfilled with granular material (Type B filter material; Specification for Highway Works (SHW, MCHW 1)) which was wrapped round in a geotextile filter (Dupont Typar 3407 spunbonded polypropylene).

Standpipe piezometers were installed during the drainage works at the locations shown in Figure 3.2 and the pore pressures monitored over the following seven years. To maintain observations between site visits, a device to record minimum depth to water was placed in some piezometers. The results are reported in detail by Farrar (1990).

Water tables at depth within the slope were lowered by the installation of drainage, except near the upper end of the drain. In contrast, shallow perched water tables midway between the drains were not lowered. Permeability measurements showed that this could be explained by the presence of a more permeable zone near the ground surface.

Farrar (1990) compared the pore pressures at 3m depth in the drained and undrained slope and

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Figure 3.1. Plan of site at Romford, Essex

found that the slope drains did have a significant effect in reducing pore pressures. Hutchinson (1977) gives charts for the estimation of the effect of slope drains on the water table and hence on pore pressures. Predicted maximum pore pressures obtained by this method for a drain in uniform soil are shown in Table 3.1. There was generally good agreement between predicted and measured values for piezometers at 3m depth.

Table 3.1 Minimum depths to water table from 3m deep standpipe piezometers

The measured water table depths using the piezometers at 1.5m depth were again compared with the predicted values using Hutchinson’s charts by assuming a drain penetration of 1.5m (Table 3.2). The

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(a) Between slope drains 3 and 4

(b) Between slope drains 10 and 11

Figure 3.2. Location of piezometers at Romford site

charts suggest that the drains would have little effect on water table and pore pressures midway between the drains and this is confirmed by the measurements. As previously discussed high pore pressures were measured in the undrained slope and also midway between the drains because of the perched water table.

Farrar (1990) reported that there was no sign of further instability in the slope since the drains were installed.

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Table 3.2 Minimum depths to water table from 1.5m deep standpipe piezometers

3.1.2 A120 Colchester

The immediate and long term effectiveness of slope drainage in a new cutting constructed in London Clay on the A120 Colchester Eastern and Elmstead Market Bypass in 1980 was reported by Farrar (1992). The cutting was about 8m high and drainage was omitted from two experimental lengths to act as control sections.

The cuttings were made with 1:3.5 (16o) slopes, and the deep trench drains (width ~0.5m) had a design spacing of 7m with a depth of about 4m as shown in Figure 3.3. Type A filter material (SHW: MCHW 1) was used for the drains located to the west of a railway bridge, whilst more permeable Type B was used for drains to the east of the bridge.

Figure 3.3. Cross-section showing design of drains at Colchester

Hydraulic and standpipe piezometers were installed at both the eastern and western sites, in the experimental lengths for which drainage had been omitted, and in the adjacent lengths in which slope drainage had been installed. Some standpipe piezometers had an incorporated device to record

19



maximum water tables. The piezometer locations and the observations are reported in detail by Farrar (1992).

At the western site, high pore water pressures were measured in the clay a year after construction and remained high throughout the period of observation. The slope drains did not function properly until the exit from the slope drain into the verge drain was excavated and replaced with clean Type B filter material in 1989. However, even after repair, reductions in pore pressure were small. This is probably because the drains intercepted a surface deposit of water-bearing granular material. This deposit introduced water into the drains which was retained by the relatively poorly draining filter drain material used on this site.

At the eastern site, pore pressures in the weathered clay were initially low but increased over a period of years. Although the slope drains were not effective in reducing pore pressures initially, in the longer term they did produce some limited reduction in pore pressures. The time required for equilibrium conditions to be attained was found to be about 5 years for a distance to the nearest drainage surface of 2m and 10 years for a distance of 4m.

3.1.3 A45 Cambridge Northern Bypass The embankment, which was about 7m high with side slopes of 1:2, was constructed from over-consolidated Gault Clay taken from a deep borrow pit nearby. Generally embankments in the area have been prone to shallow translational failures. Rock ribs were cut into a length of embankment to provide drainage and buttressing, both of which were intended to improve the slope stability. Prior to installation of the rock ribs in 1983, tension cracks up to 100mm wide were found in the slope indicating that it was close to failure.

As shown in Figure 3.4 installation of the rock ribs comprised cutting trenches 1m wide by 1.8m deep extending from the toe of the slope to the vehicle restraint barrier of the carriageway (Johnson, 1985). The base of each trench was sloped near the toe to ensure water drained into the existing ditch. The trenches were cut at 4m centres and filled with locally available crushed limestone having a maximum particle size of 125mm.

Figure 3.4. Layout of rock ribs on the A45 embankment

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By 1989 an untreated section of slope had failed to the east of the treated section. The rock rib section was unaffected and appeared to have good stability. Although no measurements of pore water pressure were taken at the site, Boden (1995) concluded that rock ribs were a cost effective solution worthy of further investigation.

3.2 Shallow slope drains

Whereas the deep trench drainage systems discussed in Section 3.1 serve two purposes in both reducing pore water pressures in a slope and intercepting run-off, shallow slope drains are only effective in intercepting run-off and controlling infiltration into the slope. Shallow slope drains, often known as herringbone drains because of the pattern in which they are laid, are typically about 300mm wide and 300mm deep and stone-filled. Nevertheless the principle of their installation by trench, albeit shallow, and backfilling with free-draining granular material is similar to that of deeper drains. Reference should also be made to Section 3.4 on filter drains.

A chevron or herringbone pattern on the face of the slope can be effective in catching run-off, although other patterns can be employed. Unless discharges are expected to be high, the cost of providing open-jointed or perforated pipes may not be worthwhile. These types of shallow drain, whether including a pipe or not, are very prone to clogging and must be considered to have a very limited service life. Some engineers consider that in many cases trenching for the installation of herringbone drains may increase the risk of water infiltration into the slope (via the backfilled trenches) and therefore cause more problems than are solved.

3.3 Embankment drainage blankets and sand drains

Drainage blankets (or starter layers) installed at the base of an embankment are widely used to dissipate excess pore pressures during construction and they also have a permanent function in providing a drainage path. The importance of drainage blankets in controlling pore water pressures and ensuring slope stability in the longer term has already been discussed in Section 2.4.1.

Drainage blankets are often employed in conjunction with vertical sand drains particularly when constructing embankments on alluvial deposits. This form of construction was reviewed by O’Riordan and Seaman (1994). The installation of vertical sand drains is discussed in more detail in Section 3.7.

Finlayson et al (1984) describe a failure resulting from the inadequate design, specification and monitoring of the drainage blanket below a newly constructed embankment. The drainage blanket comprised a granular layer with a geotextile filter as wrapping. It was shown that, although not the only possible cause of failure, the drainage blanket was insufficiently permeable leading to the formation of a softened zone in the clay fill adjacent to the geotextile. The drainage blanket acted as a reservoir of water under pressure resulting in a zone of movement along a near horizontal plane at the base of the clay fill. Immediate remedial action had to be undertaken to avoid costly disruption of the works programme.

3.4 Filter and fin drains

3.4.1 Filter drains Filter (French) drains essentially comprise a trench, with a perforated carrier pipe supported on a concrete bed at the bottom of the trench and backfilled with a free draining filter material. The concrete bed helps to prevent standing water in the trench infiltrating into the ground. In some cases this may not be deemed necessary by the designer depending on the underlying ground conditions. The trench may be lined with a geosynthetic filter/separator to prevent the ingress of fines and detritus into the drain. A schematic drawing of a filter drain is shown in Figure 3.5.

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Filter drains are primarily sub-surface drains. However, they are widely used as pavement edge drains to collect surface runoff and intercept both groundwater and water draining from the unbound layers within the pavement. Filter drains can function very effectively as a combined surface and sub-surface drain.

Filter drains are described in HD 33 (Design Manual for Roads and Bridges, DMRB 4.2.3) Surface and sub-surface systems for highway drainage, and comments on their performance are included in HA 39 (DMRB4.2.1) Edge of pavement details. The ‘F’ Series of standard drawings in the HCD (MCHW 3) provides details of three types of filter drain: combined surface and groundwater filter drains, fin drains and narrow filter drains. Narrow filter drains and fin drains are discussed in the next Section.

Filter drains can accommodate high volumes of flow, all of which should be within the carrier pipe.The designed size of the pipe should be sufficiently large such that static water and longitudinal flow should not occur within the backfill filter material.

Figure 3.5. A schematic representation of a filter drain

Requirements for the backfilling of filter drains are specified in Clause 505 of the SHW (MCHW 1).

Where geosynthetics are used in drain construction, consideration must be given in its selection and installation to the following factors:

• The pressure head required for water to pass through the geosynthetic filter material must not exceed expected hydraulic head (too high an entry head will result in water running along the surface of the filter leading to the washout of fine soil particles).

• Damage to the geosynthetic is likely to occur during maintenance operations. • Construction and/or maintenance operations that result in soil smearing on the

geosynthetic may cause clogging of the filter. • Clogging of the filter due to the migration of fines and detritus.

Much published information is available to assist with specifying the requirements for geosynthetic filter products. It is recommended that designers seek expert advice.

For slopes with large catchments, i.e. where large surface flows are predicted, consideration should be given to the separation of surface and ground water, so as to avoid introducing large quantities of water into the ground. One solution, which is preferred by the HA for edge of pavement drainage, is to construct a channel with an impermeable invert directly over the filter drain. This permits the rapid removal of surface water and stops it entering the subsurface drain. Combined surface and sub-surface drains are described in HD 33 (DMRB 4.2.3) and HA 39 (DMRB 4.2.1).

The dimensions of the carrier pipe are selected to accommodate the design flow. Requirements for the selection of the carrier pipe, and the selection and compaction of the free draining backfill material are provided in the 500 Series of the SHW (MCHW 1) and the associated Notes for Guidance (MCHW 2). Slope drainage is not specifically covered within the DMRB or MCHW but much of the

Existing ground surface

Free draining backfill material

In situ soil

Perforated carrier pipe

Geosynthetic filter/separator

Concrete

Existing ground surface

Free draining backfill material

In situ soil

Perforated carrier pipe

Geosynthetic filter/separator

Concrete

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information on pavement edge drainage presented in these documents is also suited to this application. A review of the DMRB and MCHW and their application to the drainage of earthwork slopes is provided by Farrar and Brady (2000) and contains advice on the performance of filter drains.

Filter drains are well suited for slope drainage applications and are often sited at the toe and/or the crest of a slope. However, it is of crucial importance that the location of outfalls and the gradient of the invert are selected to avoid instances of standing water within the backfill that would result in the infiltration of water into the slope.

It is well known that defects in edge of pavement filter drains can lead to deterioration of the pavement. It is equally true that defects in slope drains may increase the moisture content in the slope leading to instability. Site investigations should take special note of existing land drains and any interaction with these should be carefully considered at the design stage. Adherence to designs and close supervision during construction are recommended. Special attention to detail is required for joints between adjacent sections and at outfalls etc.

Todd and Stephens (1997) reported the results from several surveys undertaken to try to correlate defects in subsurface drains with deterioration of the road pavement. The two primary causes of premature deterioration due to increased moisture in the unbound materials were poor hydraulic conductivity of the material and blocked or defective drainage. The actions that caused the defects appeared to result from:

• a possible lack of understanding of the drain’s mode of operation; • poor construction resulting from poor quality workmanship (construction and sometimes

design) and/lack of effective supervision; • lack of maintenance.

It should not be assumed that filter drains are prone to defects. For example, Farrar and Samuel (1988 and 1989) reported that 20 year old filter drains, constructed in Gault clay adjacent to the M20 and A20 in Kent, were in good condition and were effective in lowering the water table in their immediate vicinity, but did not prevent shallow translational slope failures. Road detritus was retained on the surface of the drain restricting the drainage of surface water, but effective operation of the drain at depth continued. Farrar and Samuel (1988 and 1989) also provided useful information on the performance of geosynthetic filters, installed around the drains.

3.4.2 Narrow filter and fin drains The requirements for narrow filter drains and fin drains are presented in Clauses 515 and 514 of the SHW (MCHW 1) respectively and the associated Notes for Guidance (MCHW 2). Narrow filter drains are essentially just that, i.e. similar in construction to a filter drain as described in the previous Section with a maximum diameter of the carrier pipe equal to 100mm.

Fin drains perform the same function as narrow drains. However, they comprise a three dimensional planar geosynthetic composite (geo-composite) with a very high in-plane permeability, and a carrier pipe. The geo-composite is installed vertically; the lower end is fitted into a longitudinal slot in the non-perforated carrier pipe. The geo-composite receives water through the surfaces and rapidly drains into the carrier pipe. Fin drains may be installed by automated machinery.

Details of narrow filter drains and fin drains are shown in Drawing F18 of the HCD (MCHW 3) reproduced in Figure 3.6.

Comparative trials of five different fin drains (also known as geo-composite drains) were reported by Corbet (1990). The site selected for the trials was an area of land adjacent to the A11 Thetford Bypass. Five suppliers provided fin drains and each in turn was placed vertically in a trench parallel to its length, and surrounded by a selected backfill. The fill was clayey silty gravelly sand, selected to have at least 15 percent silt or clay particles which would test the ability of the geotextile to retain the finer fraction of the backfill. Water was fed into the sides of the trench and the outflow from the fin drain, and the water table, were monitored.

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The trials demonstrated the value of such comparative studies for highlighting potential problems. For example, the wrong grade of geotextile filter was inadvertently supplied for one product which adversely affected the permeability. This illustrates the importance of good quality control and routine site testing. The pore size of the remaining four geotextile filters used in the fin drains broadly conformed to a number of the published design criteria, and functioned effectively as filters.

Figure 3.6. Arrangements for narrow filter and fin drains (reproduced from HCD (MCHW 3))

Corbet (1990) also reported that flow normal to the geotextile filter and entering the fin drain when in contact with the backfill was less than a thousandth of that measured in laboratory index tests. If such a test is employed for obtaining design values, a very high factor of safety is essential. Performance testing involving both soil and geotextile is recommended for a more accurate and reliable estimate of design properties.

3.5 Open ditch and surface water channels

Open ditch and surface water channels are widely accepted for edge of pavement drainage to remove surface water and to avoid runoff entering the subsoil or unbound pavement layers. However, in view of the increasing incidence of heavy storms caused by climate change, they may be increasingly valuable in slope drainage applications as shown schematically in Figure 3.7.

Projects on surface water channels, combined surface and pipe systems, and combined surface and subsurface drainage systems (French drains) have been carried out for the Highways Agency by TRL

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(a) Crest drain, aqueduct and toe drain

(b) Spillway

Figure 3.7. Possible arrangement of surface channels, and combined surface channel and sub-surface drainage systems for slope applications

in association with HR Wallingford (HRW). The findings of these studies have been implemented through the publication of various Advice Notes within the Design Manual for Roads and Bridges (DMRB).

Channels are the oldest and simplest form of drainage system. They are easy to construct, inspect, maintain and repair. Over recent years there has been a resurgence of their use both for highway and

Fin drain

Combined surface channel and filter drain

Crest drain

Aqueduct

Toe drain

Fin drain

Combined surface channel and filter drain

Crest drain

Aqueduct

Toe drain

Water flow into the spillway

Discharge from spillway into a receiving or stilling basin, and then to an outfall

Water flow into the spillway

Discharge from spillway into a receiving or stilling basin, and then to an outfall

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urban drainage systems; their usage is often combined with a fin or filter drain to intercept groundwater flow, discharging into either common or separate outfalls.

3.5.1 Applicability of channels for slope drainage Channels will collect surface runoff on an embankment or cutting slope, but are not suited for the collection and drainage of subsurface water. However, with some modifications, they can be used for the following applications.

• Crest of a slope. The channel will intercept surface water flowing onto the slope and reduce the potential for erosion at the crest and on the slope itself. Cracks often form parallel to, and just behind, the crest; the channel should be located uphill of the anticipated location of the cracks to minimise the potential for these to fill with water.

• Toe of a slope. The toe of a slope is a prime location for the potential ponding of runoff; failure to remove water from such locations will result in softening of the underlying soil resulting in failure of the slope. Much detritus can collect at the toe of a slope and in consequence failure of other types of toe drain from clogging can occur. Channels are thus well suited to this location as they can be robust, can accommodate high flow capacities, and are easy to inspect and maintain.

• Aqueducts. Channels can be used at intermediate height(s) on a slope face to collect and reduce surface flows and transport water off a slope and discharge into spillways. Usage in the UK is rare and then only on long high slopes, or where the slope is stepped or benched.

• Spillways. Flows from crest drains and aqueducts need to be transported to the bottom of the slope and this can be achieved with spillways. Spillways are open channels, normally constructed from reinforced concrete, incorporating energy dissipaters, which discharge into a stilling basin.

3.5.2 Concrete channels

One of the HA’s preferred options for draining highway runoff is the use of a concrete surface water channel which offer an economic solution for pavement edge drainage. The solution is based on the concept of separation of drainage of surface water runoff and sub-surface seepage, which minimises the risk of the road layers being saturated and consequently damaged.

Roadside channels usually have a triangular cross-section but trapezoidal sections are used where greater flows are anticipated. Associated fin drains intercept any groundwater and permit the drainage of the unbound pavement layers. Details of a typical concrete channel edge drain, in a cutting situation, are shown in Figure 3.8 which is reproduced from Drawing B3 of the Highway Construction Details (HCD) (MCHW 3). The design of concrete channels is covered by HA 37 (DMRB 4.2.4).

3.5.3 Grassed channels

As a result of a requirement for more sustainable drainage systems, the HA commissioned HRW and TRL to carry out a long-term study to investigate available options for grassed channels and to demonstrate the performance of the proposed system by laboratory experiments and field trials; the study commenced in 1998 and was completed in 2006. Full details of the study, including practical aspects of constructability and maintenance, were reported by Escarameia and Todd (2006).

Early in the study a design procedure and a resistance equation for grass were developed, and subsequent field trials showed that these were fully adequate for design purposes. The findings of the study were implemented by the publication of Advice Note HA 119 (DMRB 4.2.9: Grassed surface water channels for highway runoff).

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Figure 3.8. Detail of a concrete surface water channel edge drain in a cutting situation. (Reproduced from Drawing B3 of the HCD, MCHW 3)

The study also found that in addition to providing an environmentally friendly means of discharging road runoff, grassed channels are an effective means of attenuating the flow peaks. Comparisons between average flow velocities in grassed channels and in concrete channels indicated that mean flow velocities in the former (with grass of 40mm height) are likely to be around 25% of those estimated for concrete channels.

Grassed channels are constructed by placing topsoil to provide the desired profile; turfs are then laid on top. The turfs may incorporate a lightweight (polymer) reinforcement to reduce damage from handling. An alternative to laying turfs is hydro-seeding of the topsoil, but generally the use of turfs is more advantageous (Escarameia and Todd, 2006).

A channel can be constructed on top of a filter drain, as previously mentioned. Infiltration of water through the invert of the grass channel will percolate into the underlying drain. If required, hydraulic separation can be achieved by installing a waterproof membrane over the filter drain and constructing the channel directly on top. Errant vehicles traversing the grassed channel are unlikely to suffer any significant loss in control although heavy vehicles, braking on the channel, may suffer a decrease in control and in some circumstances cause substantial damage to the channel.

Grassed surface water channels are increasingly widely used around the HA network. A view of a grassed surface water channel constructed adjacent to the M2 in Kent and water flowing in the lined channel, are presented in Figures 3.9 and 3.10 respectively.

The surface geometry is similar to concrete channels, but their usage offers certain advantages such as:

• more attractive natural appearance;

• better environmental performance due to the ability to absorb spillages and provide vegetative treatment;

• reduced peak flow downstream due to the greater resistance and storage volume of grassed channels.

The latter advantage is of particular relevance in the light of the predicted increase in the incidence of heavy storms resulting from climate change, and in situations where high velocity flows may be encountered e.g. for slope drainage.

It is known that grassed surface water channels are capable of reducing the level of contaminants in water flows. The primary mechanism of pollutant removal is the deposition of sediments transported

27



by the flow; sediments can act as a means of transportation for contaminants or may be contaminants themselves. The ability of grassed channels to reduce contaminant levels is dependent on channel geometry, flow velocity, channel length and residence time.

Escarameia, Todd and Watts (2006) reviewed the issue of contamination and proposed a method of assessing the pollutant removal ability of grassed surface water channels and swales.

Figure 3.9. A grass surface water channel constructed adjacent to the M2.

Figure 3.10. Water flow in a lined grassed channel on the M2.

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3.6 Vertical and horizontal drains

3.6.1 Vertical drains

Vertical drains are long, thin elements installed through soft clay soils to accelerate the rate of consolidation by reducing drainage path lengths and exploiting the naturally higher horizontal permeability of clay deposits, particularly those with sand or silt layers and lenses (O’Riordan and Seaman, 1994). Vertical drains are normally installed in soft ground at the time of construction; however their continued function in the longer term is of benefit to stability.

Although displacement sand drains are cheap and simple to install, they cause the most disturbance and are prone to borehole smear. Non-displacement installation methods are more effective in limiting disturbance, but there will be a smear zone around the borehole walls. Sandwicks can be considered as prefabricated sand drains. They are usually made of polypropylene and melt-bonded fabric stockings filled with a clean sand. Band or wick drains are prefabricated drains using corrugated polymeric materials (e.g. polyethylene and polypropylene) for the core, and woven or non-woven fabrics, fibre or paper for the filter. Typically they are about 100mm wide and about 4mm thick. Band or wick drains are installed using displacement methods by connecting them to a suitably placed mandrel which is then driven into the ground, either dynamically or statically. In addition to drilling and displacement techniques, McGown and Hughes (1982) included washing (or jetting) methods under their classification of installation techniques.

Apart from controlling consolidation during embankment construction, vertical drains can be effective for in-service slopes where there is a perched water table and a convenient underlying permeable stratum to carry the water away.

3.6.2 Horizontal drains Horizontal drains have been used for many years as a means of stabilising wet unstable ground in slopes, or as a drainage measure to improve the stability of cutting and embankment slopes, and slopes in natural ground that have been displaying movement. The use of horizontal drains is most appropriate where the groundwater is too deep to be reached by more conventional measures, such as trench drains. A literature review has indicated that two types of horizontal drain are used within the industry, namely horizontal bored drains and horizontal wick drains.

3.6.2.1 Horizontal bored drains

Horizontal bored drains most commonly comprise small-diameter wells, typically less than 150mm diameter, drilled into the slope to remove groundwater and seepages. The drilling is normally carried out by helical auger or rotary drilling. During drilling, flushing fluid such as bentonite mud, polymers, foam, water or air is required to reduce friction and aid in removal of cuttings.

Permanent perforated casing (drain liner) is usually installed to line the drilled hole before filling with single size granular material. In the early to mid 20th century steel or asphalt coated iron pipes were used as liner as no alternative material was available. In the long term however, these drains suffer from excessive corrosion. In America in the 1960s polyvinyl chloride (PVC) drains were first used (Royster, 1980) although problems were encountered in regards to their flexibility, often resulting in breakage and spiralling during installation.

The selection of drain casing should be determined based on a number of factors such as:

• length of drain,

• physical ground conditions,

• ground and groundwater chemistry (in relation to corrosion),

• design life of drains.

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Some examples of projects where the above factors have played a part in drain type selection are the Handlova Earth Flow (Slovakia), where steel pipes were used when drain lengths exceeded 170m. Similarly in Hong Kong, in the Po Shan Slide steel pipes were used where drain lengths reached 90m (Martin et al, 1995). In contrast, PVC pipes have been used in shorter lengths at Grenville House, Hong Kong, the Nanyang Technological University (NTU) in Singapore, Snowy Mountain in Australia and New Clear Water Bay Road, Hong Kong where drain lengths did not exceed 35m (Craig and Gray, 1985).

At Blackstone Edge, Rochdale, 35m long PVC drains were designed in order to alleviate pore water pressures within the ground (Hillier et al, 2005). On site, the drain casing was changed to steel at two locations owing to obstructions within the uncased sections of the holes in the Lower Kinderscout Grit bedrock.

Today, evidence points to plastic drains being the preferred option, as they are resistant to corrosion, and with advances in technology, the material is becoming more rigid and therefore more suitable for longer drains. Polypropylene pipes 150m in length have been installed successfully at Albion Lower Tip in South Wales (Maddison, 1991).

On installation of the drain casing, an impermeable invert must be created, in order to convey groundwater to the exit point and ensure it does not re-enter the slope. This can be achieved either by using an unperforated casing invert or alternatively by grouting the area below the pipe.

Drain invert grouting has been successfully used in a number of projects in Hong Kong (Chan, 1987), but has been less successful in the UK. In particular, this grouting technique was attempted at Albion Lower Tip in South Wales, but high water flows required an alteration in the design, and casing with a solid, unperforated, invert was subsequently adopted (Maddison, 1991). In Hong Kong, reports have indicated that solid invert pipes have performed successfully in a number of projects (Whiteside, 1997).

Filters are installed either internally or externally in order to prevent excessive corrosion and sediment accumulation. Whilst the performance of both types of filter is comparable, installing external filters is undesirable as cleaning or replacement cannot be achieved without complete removal of the drain. At Wattstown Roundabout Cutting on the Porth Relief Road in the Rhondda Valley, the presence of fines in the ground surrounding the drains (which were installed in bedrock) was considered so small that no filters were used.

Studies on drainage layout have indicated that for a given area, there is no difference in performance between a fan and a parallel arrangement (Royster, 1980). The drain arrangement is therefore best selected based on local conditions including topography, subsurface materials, configuration of the slope and location of specific groundwater sources.

3.6.2.2 Horizontal wick drains

In North America, horizontal wick drains are being studied by some researchers as an alternative to the more traditional bored drains. Although literature available on these drains is limited to articles by Santi et al (2001a and 2001b), his work on the design and installation of this type of drain is extensive. The drains comprise flat, geotextile-coated plastic channels that are driven into the ground rather than drilled. The drains are installed using a bulldozer or backhoe excavator which pushes a small diameter steel tube or disposable drive cone into the slope to be drained. The channel sections are preloaded with 3m lengths of wick drain, and additional lengths can be spliced with a plier stapler (Santi et al, 2001a).

According to Santi et al (2001a) horizontal wick drains appear to offer a number of advantages over conventional bored drains such as resistance to clogging (correct filter size selection is paramount), they are low cost compared to bored drains, they are flexible and can be stretched by up to 100 percent before rupturing and they can be installed by an unskilled workforce. Despite these advantages however, there are some conditions where horizontal wick drains would not be appropriate (Santi et al, 2001b) and these include ground conditions where standard penetration test

30



(SPT) values greater than 30 have been measured, where drain lengths in excess of 30m in harder soils and 45 to 60m in softer soils are required, and where obstructions such as bedrock, large rocks, dense sand or gravel are anticipated.

From a literature search, only one example of horizontal wick drain use has been found outside of North America, at the Stratford Box in London (Channel Tunnel Rail Link). Here horizontal wick drains were used to reduce the moisture content of tunnel spoil, prior to it being used as landscaping and general fill material (private communication).

3.7 Electro-osmosis

Under an electro-potential gradient water will migrate from the anode towards the cathode, whence it may be removed. This phenomenon, termed electro-osmosis, is a well established technique which was used to stabilise steep railway cuttings in 1939 (Casagrande, 1952). The properties of fine grain soils are strongly dependent on the moisture content of the soil. The application of electro-osmosis to such soils can be used to increase the effective stress, by generating a reduction in pore water pressure, and thus an increase in the shear strength of the soil mass. For construction purposes the ability to regulate the water content of fine-grained soils is of paramount importance. Electro-osmosis is effective for the control of seepage forces, and is therefore highly applicable for the stabilisation of slopes constructed with fine grained soils.

Since the 1940’s for reasons relating to cost and problems with the durability of the electrodes, electro-osmosis has seen little use within the general civil engineering industry. However the last decade has seen a resurgence of interest in electro-osmosis, its usage being made more attractive by recent advances in materials technology that led to the development of electro-conductive geosynthetics.

Electro-kinetic geosynthetics (EKGs) are electro-conductive geosynthetics that also provide the more traditional functions of geosynthetics, such as drainage and soil reinforcement. A description of EKGs, electro-kinetic processes and their application to geotechnical engineering construction is provided by Nettleton et al (1998). To date applications include the use of electro-osmosis for the stabilisation of existing slopes beneath the rail network as shown in Figure 3.11, de-watering of mine tailings and the control of sub-surface soil conditions on sports fields.

The application of electro-osmosis for slope drainage and stabilisation requires electrodes to be installed within the slope. However, whereas metallic electrodes are robust they are prone to corrosion and the new range of EKGs has significantly increased the potential applications for electro-osmosis. EKGs do not degrade, except under extreme conditions, and are available in many different forms, e.g. sheets, strips or 3D products.

Figure 3.11. Installation of EKGs to effect drainage and consolidation of an embankment for London Underground. (Reproduced with the permission of Electrokinetic Limited)

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The installation of EKGs into existing slopes may be achieved without causing disruptions to traffic operations, and will result in rapid drainage and consolidation of the soil. The process is considered to be irreversible; however as the electrodes are unlikely to be removed after use, they may be reused as required simply by reconnecting the electrical supply.

EKGs may be incorporated into new constructions to enable the use of poor fill material that may not otherwise have been deemed suitable for use. A further advantage is that the form of the EKGs will permit them to also perform as soil reinforcement thus providing additional stabilisation as demonstrated by Jones and Pugh (2001).

To date electro-osmotic techniques using EKGs have not been used for the stabilisation of earthworks or structures around the highway network. However, the efficacy of the technique has been proved by full scale trials, and it has been used to stabilise an embankment beneath an active railway line. It is considered that the technique could be used effectively for the drainage of water from within an embankment and to increase the embankment stability. It is recommended that if such usage was to be considered for a highway scheme it should be monitored to provide a greater understanding that could be applied to further schemes, leading to the generation of specific guidance for design, operation and maintenance.

3.8 Vegetated slopes

The influence of vegetation upon the pore water pressure regime in slopes has been discussed in Section 2.3. On the motorway and trunk road network it is generally evident that “most slope failures occur on sparsely vegetated slopes”.

Over the last decade TRL has gained extensive experience in the use of vegetated slopes, including the use of live willow poles (MacNeil et al, 2001; Steele et al, 2004). The bioengineering role of vegetation in determining the moisture regime and stability of earthworks has been discussed in detail by Coppin and Richards (1990), Marsland et al (1998) and Marriott et al (2001).

Although vegetation is not a direct method of providing slope drainage, vegetation generally limits ingress of surface water into slopes and encourages runoff at a controlled rate. Transpiration acts to reduce pore water pressures and the root growth binds the surface layers together minimising both erosion and the formation of shrinkage cracks.

The main roles of vegetation in influencing drainage can be considered to be the following:

• Soil moisture depletion, through the transpiration processes, which will reduce pore water pressure and counteract the reduction in soil strength that wetting causes. Transpiration is greater when plants are growing rapidly in summer. Account must therefore be taken of the reduced effect in the winter when slope stability problems generally arise, although with climate change the frequency of intense summer storms may increase. Foliar interception and direct absorption and evaporation of rainwater may also act to reduce water infiltration into the soil.

• Surface cover shading, of the soil provides some protection against intensive drying in full sun and the formation of shrinkage cracking which allows deep infiltration of rainwater. However, it should be noted that in certain soils prolonged extraction of moisture by plant roots can lead to desiccation and thus to the formation of shrinkage cracks. There is a compensating action, however, in that the root matrix tends to restrict movements by binding cracks together.

Although it may be considered that vegetation increases the permeability and infiltration of the upper soil layers due primarily to the presence of roots and increased surface roughness, Coppin and Richards (1990) state that these effects are generally offset by increases in interception and transpiration. They present infiltration rates for different rainfall intensities based on the work of Nassif and Wilson (1975).

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In addition to the above factors vegetation also has beneficial effects on slope stability. These are primarily:

• Root reinforcement, where shear strength is increased as root fibres are produced and extend across the plane of potential shear.

• Buttressing and arching, where trees and root columns act as piles or dowels to counteract downslope shear movements. Arching of the soil between adjacent trees and root columns depends on their spacing, diameter and embedment.

These latter effects are not the subject of this study on drainage issues, but further information can be found in Coppin and Richards (1990).

There are a number of reviews which cover the general growth of vegetation and the selection of appropriate plant groups, e.g. Coppin and Richards (1990), Bache and MacAskill (1984), Schiechtl and Stern (1992). The advice of a specialist in ground bio-engineering should be sought. The following gives some guidance on the attributes of the different plant groups:

(i). Grasses have a wide range of tolerances and quickly establish through vegetative spread to give good ground cover. Grasses tend to be relatively shallow rooting and more suited to surface protection, binding the surficial layers together, and may either attenuate or promote surface water run-off depending on the species selected. Rhizomatous species are of particular benefit to bio-engineering, because they can form a mat of underground stems. In general terms however grasses rarely grow very deep.

(ii) Herbs and legumes are commonly used in conjunction with grasses. Herbs are broad-leaved, non-woody plants which have a range of growth habits from upright to prostrate. Seeds of herbaceous species can be expensive and certain species may prove difficult to establish. Some species are deeper rooting than grasses, and therefore more suitable for deeper stability, but many die back in winter. Legumes are particularly important where soil fertility is low, because they can fix atmospheric nitrogen. They are cheap to establish and are good companion species for grasses, but they are less tolerant of difficult sites.

(iii) Shrubs and trees are woody perennials, which have a wide range of above and below ground habits. They can be single or multiple stemmed, and some spread by suckers from roots. Rooting habit can vary from deep tap roots to shallow branched roots. Those of deeper rooting habit will have a potential role in stabilising shallow slope failures in the longer term. In general shrubs and trees take longer to fully establish and mature than grasses and herbaceous plants.

Although vegetation can play an important part in reducing pore water pressures and attenuating surface run-off, it is normally employed in conjunction with a more direct drainage measure.

3.9 Interaction with highway drainage systems

An effective highway drainage scheme manages the control of runoff and groundwater within the highway boundary. However, to be fully effective any design must take into account the possibility of water entering the area from an external source. External water flows are typically generated from flooded or ineffective land drains, elevated water levels in watercourses or runoff from catchments outside the highway boundary. Advice on the identification of natural catchment areas is provided in HA 106 (DMRB 4.2.1: Drainage of runoff from natural catchments).

The unrestricted flow of surface water entering the highway boundary can result in soil erosion, slope instability, increased sediment in the highway drains, saturation of the drainage system, degradation of the pavement foundation and localised flooding of the highway. In sustained flooded conditions there may be a loss in ride quality necessitating expensive repairs/reinstatement operations. Therefore, to minimise potential problems, it is preferred that flows from external sources and the highway drainage are separated; however this may not be possible in every situation.

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The first line of defence in minimising the influx of external water flows is generally provided by a deep ditch constructed just inside the boundary fence to intercept both surface runoff and groundwater, shown schematically in Figure 3.12.

Figure 3.12. Schematic diagram showing a ditch used to intercept external water flow encroaching the highway boundary.

Further drains, in addition to the pavement edge drain, may be required where there is a potential for a high volume of external runoff. Typically surface water at the top of a slope is removed via a concrete channel to negate the possibility of water entering the soil at the top of the slope and generating problems of stability. At the slope toe a channel can also be used to intercept runoff, but a more widely used solution is a combined surface and sub-surface drain with a suitably sized carrier pipe especially where large groundwater flows are expected in a cutting or where the road has long lengths of near-zero gradients. Advice on the construction of a combined channel and pipe system for surface water drainage is provided in HA 113 (DMRB 4.2.6: Combined channel and pipe system for surface water drainage). Where surface runoff from external catchments is low or where space is limited, the runoff may be collected by the pavement edge drain.

Figures depicting the construction details for surface water drains, filter drains and edge of pavement drains comprising surface channels, are provided in the Highway Construction Details (HCD) (MCHW 3) at the locations given in Table 3.3.

It should be noted that drawings B9 to B13 in the HCD also include edge of pavement details for highway drainage in an embankment situation. These items have not been included in Table 3.3 as it is assumed that, if they are designed and functioning correctly, there are no implications in terms of slope performance.

Boundary fence

Runoff

External catchment

DitchHighway pavement with edge drain

Boundary fence

Runoff

External catchment

DitchHighway pavement with edge drain

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Table 3.3 Figure reference numbers for drawings in the HCD (MCHW 3)

Drain type Generic form Figure reference HCD (MCHW 3)

Combined surface water and ground water filter drain

Edge of pavement drain in a cutting B1

Concrete channel and ground water filter drain

Edge of pavement drain in a cutting B2, B3,

Concrete channel blocks and drains Edge of pavement drain in a cutting B4

Surface water drains Trench and bedding details F1

Filter drains Trench and bedding details F2

Drainage channel blocks Different types of drainage block F15, F16

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4 Best practice guidance in design and maintenance Following the review of methods of slope drainage, information was gathered on the guidance currently available for their design and maintenance. In general this information was hard to obtain and much reliance is placed by Consultants on the experience of designers and the practice followed on previous highway schemes. The various methods of slope drainage are now considered in turn.

4.1 Design

4.1.1 Deep trench drains, counterfort drains and rock ribs Flow nets can be used to predict the seepage pattern and predict the drainage effect of trench drains. The theory was put into a systematic form following finite element analyses by Hutchinson (1977) who also validated the findings against the then available case records. The definitions of the various parameters are shown in Figure 4.1.

(1) ground surface

(2) original piezometric level on plane EH

(3) piezometric levels on plane FG after drainage

(4) mean piezometric level on plane FG after drainage

(5) mean piezometric level on drain inverts after drainage

(6) trench or counterfort drain

(7) clay seal

(8) impermeable boundary at depth

Figure 4.1. Cross-section of typical trench drains (Hutchinson, 1977)

Figure 4.2 shows the design chart presented by Hutchinson (1977) for the piezometric level between adjacent trench drains. Values are given both for fully penetrating drains (n=1) and for partially penetrating drains (n=4.5). In both cases the horizontal permeability (kh) is assumed to be the same as

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the vertical permeability (kv). The ratios S/D and h/D are also taken to be approximately equivalent to S/ho and h/ho.

Figure 4.2. Variation of piezometric level at drain invert level between trench drains (Hutchinson, 1977)

In Figure 4.2, for example, the maximum head of water (h) in the drain would occur at mid-width of the trench (i.e. at x/S=0.5). To control the water head to a particular level in the drain, the appropriate spacing between trenches (S) can then be determined for a known depth of trench (D). The depth of trench is often decided on first as it may be related to the height of the slope, however the chart in Figure 4.2 can also be used to determine the depth of trench from the other variables if so desired.

Finite difference calculations of seepage were also carried out by Bromhead (1986). The solutions all show that for maximum effectiveness the drains should penetrate to the base of the permeable stratum, although some benefit can be obtained with partly penetrating drains. Further look-up charts of mean piezometric heads between trench drains for impermeable and permeable bases are given by Bromhead (1984, 1992).

The ready availability of software packages to undertake the type of seepage analyses described above now means that for significant schemes a more sophisticated analysis is often undertaken using the geomorphology and data specific to the site (e.g. soil permeabilities, drainage boundaries, measured ground water tables).

4.1.2 Shallow slope drains Little guidance is available on the design of shallow slope drainage systems. This is primarily because they are cheap and easy to install and extensive design costs are not therefore justified.

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The design of their layout is therefore a matter of engineering judgement and very site specific taking into account the ground contours, the presence of ponding or wet spots, the availability of drainage outlets at the toe of the slope, avoidance of significant vegetation4, and any other relevant factors. A typical layout of herringbone drains is shown in Figure 4.3.

Figure 4.3. Typical layout of herringbone drains (after CIRIA Report C592, 2003)

4.1.3 Embankment drainage blankets and sand drains The design of basal drainage layers receives some consideration in HA 44 (DMRB 4.1.1) in so far as it is stated that “their function and permeability must be checked to ensure that water drains freely and they do not act as a reservoir of water which could soften adjacent clayey soils”. HA 44 suggests that requirements for a basal drainage blanket may be met by Class 1C coarse granular material which can be a cheap and effective alternative to the more difficult to produce Class 6B selected coarse granular material.

Little guidance on the design of drainage blankets is given, although a consolidation analysis is normally carried out to confirm the rate at which excess pore water pressures dissipate during embankment construction. Generally it is assumed that adequate performance during construction will result in adequate performance in the longer term, although a geotextile filter is advisable in the latter case to prevent silting and clogging.

4.1.4 Filter and fin drains

4.1.4.1 Filter drains

Filter drains (also called French drains) essentially comprise a carrier pipe bedded in a granular material close to the bottom of a trench. Typical constructions are shown in Figure F2 of the HCD (MCHW 3). The requirements for backfilling of trenches and filter drains are given in Clause 505 of the SHW (MCHW 1); natural or recycled coarse aggregate or recycled concrete aggregate may be used of either Type A, B or C and is classified by the particle size distribution as specified Table 5/5 4 This is not only to avoid damaging the vegetation, but also to minimise root penetration into the drainage system in the longer term.

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of the SHW (MCHW 1). In some cases broken/crushed glass and shredded rubber tyres (Watts and Todd, 2006) have also been used as a filter material.

The carrier pipe and sub-surface drain should be designed in accordance with HD33 Surface and sub-surface drainage systems for highway works (DMRB 4.2.3). The trench may be lined with a geosynthetic filter fabric; the pore size should be selected using the same criteria as that used for fin drains given in Clause 514 of the NG (MCHW 2). The geosynthetic filter fabric is intended to inhibit the infiltration of fine particles from the surrounding soil into the carrier pipe. It is not essential however as in some cases fine particles may actually clog the geosynthetic fabric and its presence is thus likely to hinder future maintenance operations.

The region directly above the surface of a filter material and bounded by the sides of the trench, can act as a channel that will permit the rapid removal of surface water under conditions of high capacity runoff; such drains are known as combined surface and sub-surface drains.

A composite construction employs combined surface and sub-surface drains with a grassed channel similar to that shown as Type V in Figure B15 of the HCD (MCHW 3). Guidance on the hydraulic and structural design of grassed surface water channels is provided in HA 119 Grassed surface water channels for highway runoff (DMRB 4.2.9). The configuration of the channel is similar to that of a concrete channel as described in HD 33 (DMRB 4.2.3) and HA 37 Hydraulic design of road edge surface water channels (DMRB 4.2.4).

4.1.4.2 Narrow filter and fin drains

(Note. Fin drains, because of their construction, are sometimes termed geocomposite drains; this terminology is used in the forthcoming Eurocodes.)

The requirements for narrow filter and fin edge drains are given Clauses 515 and 514 respectively of the SHW (MCHW 1).

The accompanying Notes for Guidance for Clauses 515 and 514 (MCHW 2) cover installation and design problems in some detail. Specific advice on the pore-size and durability requirements for geosynthetic filter materials, are given in Clause NG 514 (MCHW 2).

Standard construction details are provided in the HCD (MCHW 3) for construction and drainage detail as shown in Drawing F18 and reproduced in Figure 3.6 of this report.

It is acknowledged that the above Clauses are intended for edge of pavement drains. However, much of the information is considered relevant for the subsoil drainage of earthworks.

All fin drains and their constituents, and the geosynthetic used in narrow filter drains, must be the subject of a British Board of Agrément certificate for Roads and Bridges which certifies the properties required in Clause NG514.

Fin drains are most easily installed by the use of automated drain-laying equipment, but the ease of installation is dependent on the soil conditions. As for all drains, care must be taken to ensure that the operation of the drain is not compromised by poor connections.

4.1.5 Open ditch and surface water channels The principle of using channels for surface water drainage is to keep the water on the surface for as long as possible. The reasoning is that while water remains at surface level there is less opportunity for surface run off to be introduced into the unbound pavement construction or in some cases an adjacent slope, with the consequent softening of the material.

Surface water channels, whether concrete or grass, follow the same design principles. Surface water is kept on the surface and, in a pavement situation, subsurface water is removed by fin or filter drain.

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Guidance on the design of surface water channels is contained in HA37 (DMRB 4.2.4) which, although originally written to provide guidance on the design of concrete channels, is also applicable to grass channels.

Surface water channels may be either trapezoidal or triangular in cross-section but for vehicle safety reasons the depth of concrete channels is limited to 150mm with the channel slopes no steeper than 1:5 on the carriageway side (HA 83, DMRB 4.2.4). If protected by a safety barrier the dimensions of the channel may be amended. Grass channels that are 200mm deep are permitted and this is because the soft edge to the grass channel poses less risk to damaging vehicles than concrete.

Outlets from the channels are designed to HA 78 (DMRB 4.2.1). Outlet spacing ideally allows all the flow in the channel to be diverted into the outlet, so that the length of contributing carriageway is equal to that of the channel draining to the outlet. Where part of the flow is allowed to bypass the outlet, the outlet spacing must then be decreased by a factor η, the design efficiency of the upstream outlet.

Concrete channels are usually constructed off the pavement sub-base using slip form paving techniques, whereas asphalt or grass channels are constructed by pre-forming the channel in the subsurface material and then overlaying with the asphalt/bitumen or topsoil as required.

Channels are conventionally used for edge of pavement drainage and may be at the top or bottom of a highway slope. However their wider use in a mid-slope location may follow similar design principles.

4.1.5.1 Hydraulic design

The hydraulic design guidance is contained in HA37 (DMRB 4.2.4) and is applicable to channels constructed from a range of materials including concrete, asphalt/bitumen or grass. The hydraulic resistance of a channel depends upon its surface texture, the standard of construction, and the presence of deposited sediment. The flow capacity of the channel can be determined from the Manning resistance equation which has the form:

where Q is the flow rate, A is the cross-sectional area of flow, and S is the vertical fall per unit distance. The hydraulic radius, R, of the flow is given by R=A/P, where P is the wetted perimeter of the channel. The Manning’s roughness coefficient ‘n’ is empirically derived and is a function of the surface roughness of the channel.

Appropriate average values for ‘n’ are contained in HA37 (DMRB 4.2.4) for concrete and asphalt and HA119 (DMRB 4.2.9) for grass surfaces, and these are reproduced in Table 4.2.

Table 4.2. Average values of Manning’s ‘n’

Channel Type Manning’s ‘n’

Concrete 0.013

Asphalt/Bitumen 0.017

Grass Dependent on grass type and calculated for individual channels

Typically 0.050 – 0.100

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The value of n for grass channels is calculated for individual channels where the variables are the longitudinal slope (S), grass height (H), and hydraulic radius (R) in the equation below:

2/13/51

05.0

SRmHn

−=

where: m is 0.0048 for perennial rye grass and 0.0096 for fescues dominated mix.

The design capacity of the channel is defined as when the channel is just flowing full at the design flow. Higher standards of design may be required at changes in alignment to super-elevation or at sag points to minimise the flooding risk. However the maximum length of carriageway that can be permitted to drain to a section of channel is based on the allowable flood widths of the hard strip or shoulder.

In terms of the design of outlets from channels, guidance is contained in HA78 (DMRB 4.2.1) and is equally applicable to channels of all constructions. It is essential in the design that the level of the outlet pipe does not allow the surface water level to impede the inflow from the channel. Outlets may be in-line or off-line. An off-line output may be provided by a weir, but this will require the provision of a safety barrier.

4.1.6 Vertical and horizontal drains

4.1.6.1 Vertical drains

As previously mentioned, vertical drains are normally installed in soft ground at the time of construction to improve its stability; however their continued function is also beneficial in the longer term.

The design of a vertical drain system is normally governed by the time allowed in an embankment construction programme for consolidation to occur. The design procedure specifies the type of drain and also its spacing and length. In practice the use of vertical drains will not greatly improve the rate of consolidation for soft soil layers of thickness less than 3-4m.

There is a combination of radial and vertical pore water flow to a vertical circular drain, which is considered as being within a cylinder of soil. Associated radial and vertical consolidation can be calculated separately from theoretical considerations, but the overall performance of a vertical drain system requires assessment of the effects of drain installation (remoulding and smearing of the surrounding clay), the consolidation and flow characteristics of the soil in relation to its depositional structure and macrofabric and the flow characteristics of the drains and how these will change.

The effective diameter of a circular drain, dw, will be the same as the drain or the hole into which the sand is placed. For a pre-formed band drain of width a and thickness b, Hansbo (1979) suggested that dw=2 (a+b)/π.

In practice, the insertion or construction of the drain will create a smeared outer zone with a reduced permeability kr, compared with the permeability of the undisturbed clay, kh. Barron (1948) produced an expression for calculating consolidation rates for the smear case. For jetted sand drains it can be assumed that kh/kr=1. For pre-fabricated drains the effect of disturbance will increase as the spacing reduces. Holtz et al (1991) report values of kh/kr of between 1.5 and 2 for soft clays. Hansbo (1981) used a value of kh/kr =1.5 for pre-fabricated drains. The guidance for the choice of these parameters is limited and a field trial may be necessary if analyses show smear to be critical to the design.

The effect of drain discharge capacity will become significant, particularly in the lower part of the drain, if the permeability of the drain decreases with time or for very long drains. The drain resistance can also be affected by deterioration of the drain filter, infiltration by soil particles and folding of the

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drains during installation and subsequent settlement of the soil. The permeability of the drainage blanket above the drains should also be sufficient to allow lateral flow to occur without restriction.

The drain should have sufficient capacity to enable the water to discharge to layers above and below the consolidating layer. The rate of consolidation can be reduced if the flow is “choked” by the drain. Hansbo (1981) developed an expression to calculate the degree of consolidation at any depth in a drained layer.

More recent design advice is given by many authors, typical of these is that of Zhu and Yin (2001a; 2001b). They presented design charts for use in determining the required drain spacing according to the radius of the drain, the required degree of consolidation, time available and other geotechnical engineering properties of the soil.

It should be noted that in the unusual event of vertical drains being installed post-construction, the principles of diameter, spacing, etc. given in Section 4.6.2 can be applied.

4.1.6.2 Horizontal Drains

Horizontal Bored Drains

Available information indicates that there are two distinct design approaches related to horizontal bored drains: prescriptive drains and designed drains.

Prescriptive drains

These are installed in conjunction with other stability measures, for example ground anchorages, soil nails or retaining structures. In these circumstances, horizontal drains are intended to provide an improvement in slope drainage and thus contribute to stability enhancement, although they are generally not relied upon in design to provide a quantified increase in slope stability (Martin and Siu, 1996).

The majority of reported case histories detail the use of prescriptive drains, with no detailed analytical basis governing their installation positions and dimensions. Proposed drain locations, lengths and spacing appear to have been determined based on previous practice and knowledge of site conditions, with final layouts also guided by assessment of the conditions encountered during installation. This approach is known as the ‘observational method’ and its use is described in papers by Choi (1974) and Kenney et al (1977). The use of an observational approach was also described by Santi et al (2001b) with reference to horizontal wick drains in North America.

Huculak and Brawner (in Royster, 1980) considered this observational approach in more detail suggesting that both the lateral spacing and length of the horizontal bored drains are dependent on several factors that need to be determined in the field. They suggested that the lateral spacing of drains is dependent on the quantity of water tapped in the first few installations, the suspected internal drainage pattern, the height and volume of the potentially unstable area, and the permeability of the soil. In addition, they suggested that the drain length is dependent on the height of the cut or distance from crown to toe of the slide, the location of the probable slip plane, and the distance from the face of the slope to the location of the water source.

Designed drains

These are designed and installed as critical items to achieve a specific quantitative objective such as an adequate factor of safety or groundwater level reduction or limit. The drains are often designed using a numerical model in order to assess their likely effects on the groundwater regime in their vicinity. The drains can be used together with other stabilising measures or as a ‘stand alone’ feature (Martin and Siu, 1996).

Simplistic methods to enable horizontal drains to be designed to meet quantified criteria were proposed as early as 1958 by Baker and Yoder, with other, more detailed design methods subsequently proposed by Choi (1974), Kenney et al (1977), Prellwitz (1978), Nonveiller (1981), Choi (1983) and Lau and Kenney (1984). More detail on these design methods can be found in a

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report produced for the Highways Agency by Halcrow in January 2007 entitled “Review of the use of horizontal drainage systems”.

The design methods are based on theoretical considerations of groundwater flow but owing to the need to make a number of assumptions that may be difficult to substantiate, and then to perform detailed calculations based on these assumptions, the design methods appear to be little used in practice. Detailed design of drain installations is however recommended in Hong Kong literature, with examples at Tsz Wan Shan Estate and Grenville House (Martin and Siu, 1996).

Horizontal wick drains

As an alternative to horizontal bored drains, Santi et al (2001a) studied the use of geosynthetic horizontal wick drains, which are driven into the ground rather than drilled. The majority of horizontal wick drain design has been achieved using the ‘observational method’ whilst installing test drains, although preliminary design of the reduction in water level can be undertaken using the following design process.

The design process assumes that the water table surface between any two horizontal wick drains (or horizontal bored drains) will take the form of an inverted parabola. Low points will exist at the drains, and midway between them, a high point (hmax) exists which is defined as the height of the water table above the level of the drains.

In addition to the assumption regarding the geometry of the water table surface, Santi et al also assumed that the drain is horizontal and offers no resistance to flow, the water table coincides with the drain along its entire length, and that Darcy’s law is valid for the situation, in order to construct an equation.

Values relating to drain length (b'), drain spacing (L), flow rate (Q) and permeability of the soil (K) are then used in the following equation in order to determine a value for hmax.

Values of hmax can only be calculated once site-specific parameters have been recorded, subsequent to installation of the horizontal wick drains. However, the equation can be used as a preliminary design tool prior to drain installation in order to work out approximate lengths and spacings required to reduce the water table by a set amount.

Specifications

Martin and Siu (1996) provided four criteria which stated the basic requirements for horizontal bored drains to perform successfully. In summary, these are:

• The size of the drain should be adequate to carry the maximum water flow without disturbance to the adjacent ground or development of excessive outflow pressures.

• There is no significant loss of flow along the drain length owing to re-infiltration into the ground.

• Any drain should be sufficiently strong and rigid to be easily installed to the designed length and orientation, and capable of supporting the borehole without deforming or collapsing.

• The slotted or perforated length of any drain should be designed so as to prevent soil ingress, or it should be provided with an appropriate filter.

Details of available ‘standard’ documentation, guidelines and bespoke specifications are summarised below.

Standard Documentation

No ‘standard’ specifications were found within the national or regional authorities in the UK, including the Highways Agency, or mainland Europe.

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In North America, the California Department of Transport (Caltrans, 2001) has significant experience of installing horizontal bored drains, and has developed installation and design methods, contained within Chapter 4, Section 68-2 of their standard specifications. In addition, Caltrans recommends maintenance procedures including an ongoing inspection programme starting immediately after installation, complete with a cleaning schedule if a reduction in discharge is noted.

Guidelines

No published guidance was found relating to horizontal drains in UK or mainland Europe.

Dr Paul Santi however, of the Colorado School of Mines, has developed guidelines, which are intended to apply to the installation and maintenance of horizontal wick drains (Santi et al, 2001a).

Government guidance has been published in Hong Kong relating to horizontal bored drains. The documentation is based on previous experience and is provided only as guidance, rather than as a mandatory standard. Three documents were found from Hong Kong providing technical guidance on the use, monitoring and maintenance of horizontal bored drains.

• Groundwater lowering by horizontal drains (Geotechnical Control Office 2/85: Craig and Gray, 1985).

• Performance of horizontal drains in Hong Kong (Geotechnical Engineering Office 42: Martin et al, 1995).

• Monitoring and maintenance of horizontal drains. Technical Circular of the Environment, Transport and Works Bureau, Hong Kong, Works Branch Technical Circular No 10/91.

Although many of the published case histories do not describe design guidelines, those that do have tended to refer to Hong Kong practice, specifically GEO 42. For example, the GEO suggests that maintenance is undertaken on horizontal bored drains six months after installation and then once every year. Chan (1987) examines the use of a multi purpose valve, installed in horizontal bored drains, which can be used to flush the drains. The valve is closed for a few days to allow an accumulation of water, and is then opened in order to flush the drains. Care must however be taken not to destabilise the slope by preventing drain flow. In addition to using water flow to flush the drains, the GEO states that a thin brush or rake can be used to clean dry soil from the drain invert (Martin and Siu, 1996).

In WBTC No 10/91, it is stated that monitoring should be undertaken every two months during the dry season, weekly during the wet season and at a greater frequency during heavy storms.

Bespoke Specifications

As a result of informal correspondence with consultants, it appears that, where used, specifications in the UK tend to be bespoke and (largely) based on Hong Kong guidance. It is understood that specifications tend to relate to installation, rather than performance of drains. Examples of this were found at the Barnstaple Bypass and at Albion Lower Tip in South Wales. For the Albion project, the installation specifications were produced in consultation with the contractor to ensure ‘buildability’. In addition, at Albion Lower Tip, a different monitoring strategy was adopted to that recommended in WBTC No10/91, with the flow rates monitored every month between May and September inclusive and every week between October and April inclusive.

4.1.7 Electro-osmosis The electro-osmosis of fine grained soils will increase the effective strength by the removal of excess pore water. The reduction in pore water pressure drives the consolidation of the soil/fill material. It has been demonstrated that significant negative pore water pressures, e.g. up to -180kN/m2, can be generated through electro-osmosis (Hamir et al, 2001). Some bonding and/or cementation of particles may occur. The procedure is thought to be irreversible.

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The water flow is driven by the applied electromotive force that has to overcome the resistance to flow generated by friction between the water and the surface of the soil particles. An equation describing the flow established by Mitchell (1993) is directly analogous with Darcy’s Law for hydraulic flow.

qA = ke. A. ∆V∆L

where: qA rate of flow ke electro-osmotic permeability ∆V/∆L voltage gradient A cross sectional area of the flow.

The electro-osmotic permeability for most soils lies in the range 1×10-9 to 1×10-8m2/Vs (Lageman et al, 1989) and is independent of pore size (Casagrande, 1949).

Electro-osmosis will drive the water to the cathode, which might typically take the form of a hollow tube to allow the water to be pumped away, or where conditions permit (e.g. in a slope) the cathode may installed so that the water drains out naturally. The anode may be of similar form to the cathode, but if the anode is closed high negative pore water pressures will be generated (Nettleton et al, 1998), and the estimated change in pore water pressure (∆U) is given by:

∆U = ke. γw Vkh

where: kh hydraulic permeability γw bulk unit weight of water V applied voltage.

For design, the required change in pore water pressure to achieve the design strength of the soil can be calculated. The degree of consolidation that will be achieved, with an estimate of the anticipated volume of water that will be drained from the soil, can be estimated from oedometer tests in the laboratory. The voltage gradient necessary to achieve the change can be readily calculated, and a suitable arrangement of electrodes, electrode spacing and applied voltage can also be determined.

The applied voltage and the electrode spacing define the rate at which de-watering will take place. The duration of the application of electricity is dependent on the required change in soil conditions. High voltages and close spacing can provide rapid dewatering but desiccation of the soil surrounding the anode, under high voltage gradients, must be avoided or the treatment will cease to be effective. In most instances designs for the dewatering scheme will be tailored to the requirements of the client. Typically, substantial improvements to the soil condition will be achieved in one week and the treatment will be completed in two weeks.

The effects of electro-osmosis are generally considered to be irreversible. Nevertheless providing the electrodes are not removed or damaged in any way, a system once installed can be re-activated at any time. Such ability could be advantageous in an area subjected to seasonal flooding.

The application of electro-osmosis to improve the shear strength of a fine grained soil is a proven technique. However, no accepted formal design procedures are available and therefore engineers considering usage of the technique on their own schemes are strongly advised to seek expert advice.

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4.1.8 Vegetated slopes Quantification of the benefits of vegetation to slope stability, in terms of reducing the risks of both shallow and deep-seated failures, is given by Coppin and Richards (1990). Other primary sources for numerical evaluation of slope stability include Greenwood (2006) who developed a program for slope stability analysis to include the effects of vegetation, reinforcement and hydrological changes as part of the ECOSLOPES project. These methods primarily include for the effect of root reinforcement in improving the soil strength and in the dowelling effect across the potential shear plane.

Although there is no design standard on evaluating the benefits to drainage of vegetation on the slope, the following principles are commonly adopted:

• An assumed reduction of pore pressures near the slope surface when carrying out the slope stability analysis at the design stage. In some cases it is preferred to implement this by increased soil cohesion due to the soil suction.

• During the winter the effect of vegetation on soil moisture content and in developing soil suction will be at a minimum. This needs to be considered by designers carrying out the slope stability analysis.

• In terms of the selection of vegetation during the design process, some advice on the performance of different plant groups is given in Section 3.8. In general a more open cover results in higher runoff volumes and a more rapid runoff response, whilst shrubs and trees result in more attenuation. These considerations are normally site specific and the advice of a bioengineering specialist is normally sought on this issue.

• The differences in the grass cover within grassed channels are considered in Section 4.1.5. With taller grasses retardance of flow increases and this has an effect on the Manning nvalues used to calculate the flow.

It must again be noted that drainage aspects are generally considered to be secondary considerations in vegetated slope design compared with the benefits of root reinforcement and dowelling effects.

4.2 Maintenance

This report has served to highlight the importance of drainage measures for ensuring the long term stability of highway slopes, yet the long term stability can only be assured as long as the drainage continues to operate satisfactorily. The provision of adequate and appropriate maintenance is required to maximise the life of drainage measures, however there appears to be little guidance on this. There are references in a number of Advice Notes and Departmental Standards within the DMRB and also in BSI Codes of Practice, however the advice given generally relates to the maintenance of drainage systems by inspection and gives little if any guidance on the actual practicalities of undertaking the work. The form and location of many types of earthwork drains, for example, stone filled trenches on sloping ground, essentially preclude the undertaking of routine maintenance and therefore it is not until defects are identified that maintenance and repair works are carried out. BS 6031 Code of Practice for Earthworks (BSI, 1981), states that regular inspection of drainage systems is an essential basis for programming the frequency and extent of maintenance work, with paragraph 11.2.3 giving general guidance on when and how inspection should be undertaken.

The following sections identify some of the issues that may lead to the need to undertake maintenance works on the various drain types discussed in this report.

4.2.1 Deep trench drains, counterfort drains and rock ribs The coarse backfill in these types of drains may become clogged in time and may require replacement. The size and location of these drains, often greater than 2m deep and on steep slopes, makes the replacement of the filter media, by either new or cleaned existing material into a large enough job to warrant a special scheme. To mitigate the effects of clogging and minimise

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requirements for future maintenance works these drains are generally laid to a positive fall and are most commonly wrapped in geotextile to keep clay and silt particles out of the collector system, though these are also prone to clogging. In addition the need for maintenance can be further reduced by introducing carrier pipes into the drain base, as these are designed to be self cleansing. Should blockages occur then rodding or flushing of the pipes from the inspection manholes can be undertaken.

4.2.2 Shallow slope drains

These are generally employed to intercept surface run-off and prevent infiltration, although there is some argument, that as they are rarely wrapped in geotextile or lined they may actually encourage captured water to soak into the ground. They are very prone to clogging and thus should be inspected regularly with a view to undertaking complete replacement should the indications be that water is no longer infiltrating into the drain. In some instances where drains up to 1.0m deep are employed there is merit in using a geotextile wrapped coarser filter material in the top 300mm of the drain. This can be easily removed and replaced thereby negating the need to reconstruct the entire drain. Feedback from some of the HA Managing Agent Contractors (MACs) indicates a preference towards using open lined ditches to intercept surface run-off.

4.2.3 Embankment drainage blankets and sand drains This type of drainage is normally employed to alleviate porewater pressures during construction and therefore its continued effectiveness long term is less critical. As it is buried within and below the base of embankments access to it for maintenance purposes is generally not possible without major reconstruction and thus the importance of appropriate design is crucial.

4.2.4 Filter and fin drains The efficiency of filter drains can be seriously impaired by the formation of a silt crust, with attendant vegetation growth at the top of the filter material or by the accumulation of trapped silt in the lower layers. The surface defect can be detected easily by inspection at ground level, but the deeper accumulation can generally only be confirmed by excavation, usually by means of trial pitting. It is probable that, unless there is an obvious case for a localised defect, a length of filter drain will show a consistent defect. As with deep drains the replacement of the filter media, by either new or cleaned existing material will often prove to be a large enough job to warrant a special scheme. Where piped filter drains have been employed maximum use of gully, catchpit and inspection chambers should be made for any emptying and cleansing operations and for general inspection procedures to check that the filter drains are operating properly.

4.2.5 Open ditch and surface water channels

4.2.5.1 Concrete Channels

Maintenance of concrete channels is generally confined to sweeping. Sediment washed from the carriageway and adjacent unpaved area is carried along the channel towards the outlet. At various points there is potential for the sediment to be deposited in the channel. This can occur where there is a change in longitudinal gradient; as the gradient reduces the flow velocity is reduced and consequently there is less energy to transport the sediment. This material can build up into significant volumes thereby reducing the channel capacity. Conventional cleaning is by mechanical sweeping.

Debris can build up in the outlet gratings and may not be removed by conventional sweeping. Sediment may also accumulate in the catchpits of the outlet chambers; this can only be removed by accessing the chamber by removing the grating and using a conventional gulley sucker or vacuum

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tanker. A further issue with regards to maintenance of these drains which often gets overlooked is that the soil adjacent to the edge of the channel can sometimes be softened and eroded often leading to the formation of a slight step. This step can act as a barrier to the surface run off and actually hold the water out and force it to sink into the ground under the channel, thus negating the channel’s value. Maintenance of the soil edges around the channels is therefore essential to ensure it continued effective operation.

4.2.5.2 Grass channels

The maintenance of grass channels is more onerous. The grass requires cutting 2 to 3 times during the summer and autumn and the carriageway side channel edge must be trimmed to ensure that a grassed ridge does not develop. Grass channels are prone to damage when over run by a vehicle, particularly if the grass is short and the channel wet. Ridges and ruts that may form as a result can be flattened by repositioning the topsoil and turf into the rut. Vehicular over run trials conducted at TRL (Escamareia and Todd, 2006) found that HGVs under braking tend to cause the most severe damage and that long grass tends to result in less rutting.

The presence of litter in grassed channels was observed to have a detrimental effect on the grass growth with grass dying when covered by large pieces of litter, such as cardboard sheet and vehicle debris. Damaged areas of grass channel can be readily repaired using cut turf. Where a polymer grid has been used to reinforce the grass channel, care needs to be taken when mowing to ensure that the grid has been correctly located below the grass surface. Instances of the grid being placed on top of the channel occurred on the A120 resulting in damage to both the polymer grid and the mowing machinery.

4.2.6 Vertical and horizontal drains

4.2.6.1 Vertical Drains

Vertical drains are normally installed to alleviate porewater pressures during construction and therefore their continued effectiveness long term is often less critical. If their continued effectiveness is however required for long term embankment stability and they are suspected of no longer functioning, flushing is not normally a practical option and a complete reinstatement is required.

4.2.6.2 Horizontal Drains

Published information found relating to post-construction maintenance has been found in literature from both Hong Kong and North America. In Hong Kong, details of maintenance procedures are set out in WBTC No 10/91, whilst in North America, Caltrans provides details of maintenance procedures.

In Hong Kong, GEO suggests that maintenance is undertaken on horizontal bored drains six months after installation and then once every year. Chan (1987) examines the use of a multi purpose valve, installed in horizontal bored drains, which can be used to flush the drains. The valve is closed for a few days to allow an accumulation of water, and is then opened in order to flush the drains. Care must however be taken not to destabilise the slope by preventing drain flow. In addition to using water flow to flush the drains, the GEO states that a thin brush or rake can be used to clean dry soil from the drain invert (Martin and Siu, 1996).

In North America, Caltrans recommends an ongoing inspection programme starting immediately after installation, complete with a cleaning schedule if a reduction in discharge is noted. Cleaning should be accomplished using a high pressure water pump, complete with a self-propelling jet inserted into the entire length of the drain (Long, 1980). At Cloverdale, (Smith, 1980) cleaning of horizontal bored drains constructed in 1941 was undertaken 15 years after installation. Whilst this is much longer than

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the recommendations detailed by Caltrans, the flushing event saw an improvement in the drain flow rate from 697m3/day up to 1077m3/day (Smith, 1980).

In the UK, the only available data on drain maintenance is related to Albion Lower Tip in South Wales. Internal cleaning of the pipes in the horizontal bored drains was undertaken in 1998, almost 10 years after their installation. This operation resulted in a 300% increase in flows from the drains. Over the subsequent months however, flows gradually returned to their ‘pre-cleaning’ levels.

4.2.7 General

In recognition of the difficulty in undertaking routine maintenance of earthworks and in particular earthworks drainage, the Highways Agency have published HD 41/03 entitled “Maintenance of Highway Geotechnical Assets” in 2003, a document providing best practice guidance for the inspection and maintenance of highway geotechnical assets for trunk roads in England. The document advises on an effective approach to the management of geotechnical assets, linked with the introduction (in England) of a data management system and of mandatory requirements for inspection activities and maintenance works. The document stipulates that inspections of geotechnical assets shall be undertaken annually and five yearly (detailed inspection) to meet the requirements of the HA’s “Routine and Winter Service Code”, with the intention that the inspections will thus provide for identification of maintenance needs. The Code states that “the inspection and reporting of the condition of the highway Geotechnical Assets as required by the standard, provides an assessment of the likely risk of defects occurring or deteriorating with time. The identification of such risk and thus the provision of maintenance prior to defects occurring, as opposed to remediation afterwards, provides a more cost effective and proactive approach to maintenance”.

4.3 Interaction with highway drainage systems

The standard HD33 (DMRB 4.2.3: Surface and subsurface drainage systems for highways), Clause 5.1, states “It is essential that existing land drainage and runoff from existing catchments be taken into account in the design of highway drainage.” Failure to take due consideration of external water flows could potentially result in flooding of the highway drains leading degradation of the highway pavement and the instability of earthworks.

A review of flooding incidents around the highway network (Todd, 2004) established that 65 per cent were associated with the performance of drainage systems and 35 per cent were due to high water levels in adjacent rivers or streams. Subsequent research to develop techniques to minimise the potential for flood conditions resulted in the publication of Advice Notes HA 106 (DMRB 4.2.1: Drainage of runoff from natural catchments) and HA107 (DMRB 4.2.7: Design of outfall and culvert details). These documents should be used in conjunction with the recommendations for earthwork drainage and the use of cut-off and intercepting drains provided in the Standard HD33 (DMRB 4.2.3).

HA 106 (DMRB 4.2.1) presents guidance on how to deal with surface water runoff from natural catchments draining towards a highway, in order to limit the frequency and severity of flooding incidents caused by runoff beyond the highway boundary. It is aimed at increasing the resilience of the highway to extreme weather conditions. Advice is given on different approaches to the collection of runoff, the estimation of the volume of runoff and the design of ditches. A flow chart describes the methodology for the identification of an external catchment through to the design of a satisfactory a drainage solution.

HA 107 (DMRB 4.2.7) gives guidance on detailing outfall structure to highway drainage systems and the design of culverts. Advice on the hydraulic design of culverts is given in Chapters 5 and 6 of CIRIA Report 168, Culvert Design Guide; the Advice Note relates the guidance in this report specifically to highway applications.

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The only reference to drainage from external sources within the Specification for Highway Works (MCHW 1) and the Notes for Guidance (MCHW 2) is found in Clause 511 and relates to land drains severed by highway construction and the remedial works that should be undertaken. Highway works that effect external watercourses are covered in Clause 606.

For the purpose of design, the abiding principle is that surface runoff that would naturally drain towards the highway should be intercepted before it can enter the highway drainage system. Where this is not possible due to topographic or spatial limitations the flow may be directed into the highway drainage system but as far downstream from the edge drain as can practically be achieved. It is important that standing water should not occur adjacent to the pavement construction except in the most extreme conditions when it may become unavoidable.

The current emphasis in slope and highway drainage is to use sustainable urban drainage (SUDS) techniques with careful thought on how water can be treated to improve quality and attenuated to reduce peak flows. As part of this approach the use of permeable pavements and pavement reservoirs to achieve better water control is the subject of separate HA research by TRL.

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5 Survey of practitioners A questionnaire was devised primarily aimed at HA Managing Agent Contractors (MACs) to assess for each drainage technique the following factors:

• the frequency of usage,

• the design principle used,

• any problems found with design, installation, or maintenance,

• a broad assessment of effectiveness,

• the required maintenance intervals.

Initially the questionnaire was sent out to four of the HA MACs to assess whether any refinements of the questionnaire were necessary before its wider circulation. The most detailed reply was received from Area 8 and a copy of this is given in Appendix A. Considerable use had been made of counterfort drains and rock ribs in this Area primarily to lower the groundwater table in the newly constructed cutting slopes of the A14. This technique was assessed as having good effectiveness; the need for any future maintenance was not known. Area 8 had also employed drainage blankets in the A14 and A421 embankment constructions and these were considered to be very effective. Open ditch and surface water channels had also been employed at three schemes with better success when at the top of the slope than when mid-slope.

The other three MACs only returned limited information and a summary of their responses is as follow:-

• Area 7 identified occasional usage of slope drains (e.g. herringbone drains) in the 1980s at one remediation scheme. These were assessed as having good effectiveness and having needed no maintenance over the more than 10 year period in service. This particular MAC was unable to identify any other specific slope drainage measures in recent years. It is not clear whether this absence of data is because of the topography and geology in the area, lack of funding for preventative drainage measures, or other reasons.

• Area 11 identified the use of slope drains (e.g. herringbone drains) on three highway cuttings although no further details were available. Open ditch and surface water channels had been used at more than thirty sites; these were employed at both cuttings and embankments at both toe and crest locations. Filter drains had also been employed at more than 24 sites, once again these were a mix of embankment or cutting and toe or crest locations. Information on design principles, problems, effectiveness, and maintenance intervals were not forthcoming.

• Area 5 identified two cutting slopes where slope repair had been carried out employing counterfort drains running from crest to toe. The design of the counterfort drainage system had been undertaken using the charts published by Bromhead (1992). These drains had been installed in 2005 and 2006 and no problems had been encountered in their design and installation. Drain performance was assessed as good. No maintenance is currently planned.

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6 Selection of Drainage Measures The implementation of drainage measures has two main objectives, (i) to prevent or minimise the potential for ground water reaching the slope, and (ii) the removal of both groundwater and surface runoff from the slope and the toe area. Gilbert (2007) recommended that ground investigation for the design of slope drainage systems should include:-

• geomorphological mapping,

• soil permeability assessment,

• ground water table monitoring,

• ground water response monitoring.

Gilbert also emphasised the importance of combining seepage and slope stability analysis at the design stage.

Run-off from adjacent catchments and future engineering works may affect the in-service performance of a slope drainage system, and the occurrence of such events should be considered at the design stage.

Drainage trenches installed behind or below the slope crest to intercept surface and ground water should have an impermeable membrane below the invert and on the downhill side to minimise the likelihood of collected water from re-entering the soil. The absence of a membrane may result in an increase in groundwater, so increasing the potential for failure of the slope.

Drainage measures (such as filter drains) often have a working life of only about 15 to 20 years. Positive drainage measures such as channels have a much longer life. Where possible, sustainability issues should be considered in the advice on the choice of materials for the different drainage techniques, bearing in mind the required 60 year design life of slopes.

Aspects of hydrology relating to catchment assessment, catchment runoff estimation, annual rainfall and extreme events, etc. are not addressed in this report. Such factors however must be considered when designing drainage measures. Guidance on these issues is given in HA216 (DMRB 11.3.10).

Some guidance on appropriate typical drainage measures for use in different situations is provided in Table 6.15. The likely relative cost of each technique is indicated as being low, medium, or high; this crude ranking takes no account of issues involved in disposing of the water collected. The remedial measures suggested in Table 6.1 should not be considered in isolation; it is probable that in many situations a combination of measures will be employed to provide an optimal solution.

Many problems with the stability of highway slopes and flooding of the carriageway arise as a result of defective drainage systems. Efficient and well maintained drainage systems are important to the continued operation of the adjacent highway, and a small improvement to a drainage system can lead to a significant improvement in embankment stability. The consequences of poor construction or lack of maintenance can be very costly in terms of remedial works and disruption to traffic.

6.1 Selection of drainage measures

In general, application of the drainage measures listed in Table 6.1 will be successful in most situations. Instances will occur however, where the typical remedial measures identified in the Table may be inappropriate, and therefore these measures should not be applied unilaterally. A full appreciation should be made of both “cause” and “effect” and of all factors likely to affect the operation of the drainage measure.

5 An assessment of approximate costs of various drainage remedial measures was carried out by Halcrow in 2002. This information is included in Appendix B although costs are normally very site specific and the data should therefore be used with caution.

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Over-consolidated clay embankments for example are prone to shallow failures (see Section 2). High suctions existing within the clay pores cause free water to be sucked into the superficial layers thereby softening the clay. In this situation the installation of an interceptor/filter drain (as suggested in Table 6.1) is unlikely to be effective because of the impermeable nature of the clay. In addition there is a risk that runoff collected in the ditch may enter the slope through the ditch sides and invert. The second option identified in Table 6.1, of increasing the slope vegetation, may be more appropriate for reducing the potential for shallow failures; this is confirmed by its recommended use to alleviate water infiltration. For extreme cases it could be further argued that the infiltration results from excessive surface water which could be controlled by using surface water channels at top or mid-slope locations.

The conditions specific to each site should also be carefully considered, before referring to Table 6.1, as they may govern the suitability of typical solutions. For example, an over-consolidated clay embankment near Junction 29 of the M25 (see Section 2.1.5) showed some evidence of water ponding in the clayey sand overlying the more impermeable clay. This is likely to feed water towards the side slopes causing both shallow and deep instabilities and therefore in this instance, one of the techniques for alleviating “high pore pressure within the slope” given in Table 6.1 would be the preferred solution.

The use of vegetation is recommended for the provision of sustainable drainage measures. It is important however to select the type of vegetation to achieve the desired objective; engineering judgement may be required. For example, grass is very effective in binding the slope surface together to combat softening and degenerative weathering. In general, a more open vegetation cover results in higher runoff volumes and a more rapid runoff response, whilst shrubs and trees result in more attenuation. Vegetation will lose water through transpiration and generate a reduction in pore pressures. Where shallow slip failures are likely to exist, the dowelling effect of live willow poles can stabilise the slope, and the ability of the willow to reduce the pore pressures in the slope will also promote stabilisation. The former provides an immediate solution whereas the latter is a slower process, but in the long term both mechanisms may be equally effective. Grass turfs used to line surface water channels will attenuate flows and thereby reduce the likelihood of surcharging the drain under storm conditions.

6.2 Impact of climate change

The increasing impact of climate change and the probability of summer storms of increased intensity mean that the design of effective slope drainage systems is now more important than ever. For example the change in precipitation patterns was especially evident in the summer of 2007, which was the wettest since records began in the UK and resulted in severe flooding. Climate change would however, normally be expected to produce hotter summers, albeit with intense storms, thus the weather in 2007 was totally unexpected and difficulties in its prediction were highlighted.

The increased frequency of extreme rainfall events means that a more innovative approach may be needed in the design of slope drainage systems. Greater emphasis must now be placed on designing for storm water and surface run-off, to avoid mud and debris slides, and to prevent water infiltration into clay slopes after the development of shrinkage cracks in prolonged hot spells. The following recommendations may be of particular benefit:

• An increased use of surface water channels6 not only at the top and bottom of slopes but also in critical cases at mid-slope locations. Maintenance of such channels is relatively straightforward in so far as any build-up of silt and detritus is visible and can readily be removed. The state of subsurface drains, after storm conditions, is not so easily assessed and dealt with.

6 Adequate outlets and sufficient catchments or balancing ponds are required when using channels to deal with the rapid collection of water.

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• Adoption of a more proactive bioengineering approach in designing vegetation cover to either attenuate or promote surface water flow off the slope according to site requirements. Both root reinforcement and foliage cover (to prevent the surface heating up) may limit the extent of shrinkage cracking in the summer; the use of live willow poles for this purpose can provide additional slope stability by dowelling action.

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Table 6.1. Appropriate drainage measures for different situations

Potential or actual drainage issue Appropriate slope drainage measure

Instability

Potential for shallow failures (where the maximum depth of the rupture surface does not exceed 2m)

Increase vegetation (L).

Install shallow slope drains* (L).

Potential for deep failures Install drainage (deep trench drains, counterfort drains, or rock ribs) running down the slope (H).

Install horizontal bored drainage (H).

Surface water run-off

Run-off from external catchment (i.e. adjacent land/highway) at top of slope

Install interceptor filter or fin drains – lined below and on the downslope side (M).

Install surface water channel (M).

Flooding at toe of slope from adjacent land/highway

Install filter or fin drains (M).

Install open ditch (L) or surface water channel (M).

Significant run-off from slope surface Install surface water channel at toe or mid-slope (M).

Install filter or fin drains at toe (M).

Install drainage (deep trench drains, counterfort drains, or rock ribs) running down the slope (H).

Ground water (excess pore pressure)

High pore pressure at toe of slope Install filter or fin drains (M).

High pore pressure within slope Install drainage (deep trench drains, counterfort drains, or rock ribs) running down the slope (H).


Increase vegetation (L).

Install horizontal or vertical bored drainage to replace malfunctioning drainage blanket (H) – embankment only.

Install electro-osmosis system (H) – usually embankment situation.

Water infiltration

Softening/weathering of surficial layers Increase vegetation (L).

Water seepage

Poor condition of any existing slope drainage

Implement maintenance/ replacement (L to M).

Install additional drainage measures (L to H).

Poor condition of highway drainage Implement maintenance/ replacement (L to M).

Seepage through permeable layer or from spring


Install interceptor filter drain (M).

* Soil dependent - may not be effective in over-consolidated clay slopes.

Relative cost rating: L Low M Medium H High.

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7 Conclusions The stability of highway cuttings and embankments is critically dependent on the magnitude and distribution of pore water pressures within the materials (soils, rock and earthworks fill) forming the slopes. Pore pressures arise from the presence of water within the slopes, and may be introduced via natural infiltration from runoff and/or groundwater flow and also possibly through artificial recharge. In the case of infiltration, rainfall will enter earthwork slopes and the underlying soil through the surface in quantities which depend on the duration and intensity of the rainfall, the soil type, the topography, and the vegetative cover; for a vegetated slope, some water will be removed by transpiration. Groundwater flow into a slope will depend on the local hydrogeological regime. Artificial recharge can arise from deliberate sources such as soakaways or accidental sources such as leakage from ‘closed’ or blocked drainage systems.

The failure of highway slopes owing to adverse pore pressures can have significant impact on the operation of the highway network and has implications for the safety of highway users. The recognition of the risks associated with slope failures and the mitigation of such risks should be identified during the design and construction phases of new infrastructure or improvement schemes. Where the security of slopes is to be provided by means of drainage, either in part or as a whole, designers should identify all factors that may affect the efficacy of the proposed drainage and take this into account.

The importance of ground investigation in the design of slope drainage systems has been emphasised. Investigations should include geomorphological mapping, permeability testing, and monitoring of ground water levels and responses. Both seepage and slope stability analysis should be considered at the design stage together with the effects of climate change, as the increasing frequency of extreme rainfall events requires greater emphasis to be placed on a consideration of storm water and surface run-off. This is to avoid in-service problems with erosion, mud and debris slides and to minimise the infiltration of water into clay slopes after the development of shrinkage cracks in prolonged hot spells.

Robust, sustainable and cost-efficient drainage measures are of critical importance for ensuring the long-term stability of highway slopes. Comprehensive information for consideration in design is presented in the earlier chapters of this report. It has been shown that where practical the use of surface water channels and bioengineering designs offer significant advantages in relation to reduced construction costs, ease of inspection and general maintenance providing for increased sustainability.

A survey of HA’s practitioners yielded little information in relation to inspection/maintenance routines currently being undertaken on HA assets. The surveys appear to indicate that a reactive approach to problems is routine and that there is little to no planned maintenance of drainage systems. It is considered that a proactive approach to inspection/maintenance as advocated by DMRB HD 41/03 should be adopted to provide for early identification of problems and timely and cost effective implementation of remediation.

8 Recommendations for further research • The continued satisfactory functioning of drainage designed and installed to slopes is

essential to maintaining their stability and security long-term. Whilst a 60 year design standard is normally applicable to highway earthworks including slopes, the current study found virtually no information readily available to designers or those responsible for maintenance on the longevity/reasonably expected design life for the various forms of drainage normally adopted to provide for slope stability. Neither is there any comprehensive documentation detailing the factors likely to affect drainage longevity, which should be taken into account in design and require prior investigative studies. It is considered that the collation of details and preparation of a guidance note on factors affecting drainage longevity, design and management considerations would be beneficial. The study should include details of industry experience of typical service life for the various forms of drainage used in slopes

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to allow consideration of whole-life costing, maintenance and planned replacement requirements.

• The impact of climate change and the probability of summer storms of increased intensity mean that the design of effective slope drainage systems is now more important than ever. To facilitate improved design it is considered that an assessment of the efficacy of existing slope drainage systems within the highway network needs to be made, taking account of the increased risk of extreme rainfall events and flooding from external sources. This assessment needs to be related to geotechnical databases quantifying slope condition and risks of failure.

• DMRB HD 41/03 provides for a proactive approach to managing geotechnical assets including slopes and any associated drainage. However, it is far from clear as to whether this is being effectively implemented. Early identification of problems can be expected to provide for timely cost effective management and remediation in a planned manner compared to a reactive approach to incidents that could require closure or part closure of the highway network at short notice. It is considered that further enquires with HA’s practitioners might be used to identify best practice in the implementation of HD 41/03 and for the dissemination of that experience to others.

57



9 Acknowledgements This Task 153 was commissioned under the HA SSR Framework for Technical Consultancy 2/666 and was undertaken by Halcrow Group Ltd in collaboration with TRL Limited. The TRL work was undertaken within the Infrastructure and Environment Division. The HA client for the project was Dr T Messafer. The project managers for Halcrow and TRL were Ms L Campton and Dr D R Carder respectively.

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Appendix A. Questionnaire to practitioners

64TRL

Limited

64PPR

341

PublishedProjectR

eportV

ersion:Final

65 TRL

Lim

ited

65PP

R34

1

Publ

ishe

dPr

ojec

tRep

ort

Ver

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:Fi

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66



Appendix B. Costs of drainage remedial measures at 2002 prices

Operation Unit Cost (£/unit)

Open ditches m 10 – 30 Lined channels m 80 – 460 Piped filter drains (<3m depth) m 80 – 230 Piped filter drains (<5m depth) m 280 – 460 Drainage blankets m2 20 – 80 Bored drains m 390 – 460 Wells m 180 – 290 Drainage tunnels m 750 - 1050



Abstract For all earthwork slopes, rainfall will enter the underlying soil through the surface, in quantities which depend on the soil type, topography, and vegetation cover. Slope drainage can control the movement of surface water and also the subsurface pore water pressure in the slope. Drainage can be very effective if installed at the correct location on or within the slope. In the long term, systems need to be designed with maintenance operations in mind so that a sustainable system is installed with a design life comparable to the 60 year design life of a highway slope. The report reviews the distribution of pore water pressures within a slope measured by TRL at a number of sites on highway schemes and follows with a critical assessment of the applicability of different slope drainage techniques. The report concludes with best practice guidance in the design and maintenance of the various drainage systems. The increased use of surface water channels and a more proactive bioengineering approach are recommended in the design of slope drainage systems. These methods offer the advantages of ease of inspection, reduced maintenance costs and a sustainable technique.

TRLCrowthorne House, Nine Mile RideWokingham, Berkshire RG40 3GAUnited Kingdom

T: +44 (0) 1344 773131F: +44 (0) 1344 770356E: [email protected]: www.trl.co.uk

ISSN 0968-4093

Price code: 3X

Published by IHSWilloughby Road, BracknellBerkshire RG12 8FBUnited Kingdom

T: +44 (0) 1344 328038F: +44 (0) 1344 328005E: [email protected]: www.uk.ihs.com PP

R3

41

Drainage of earthworks slopes

For all earthwork slopes, rainfall will enter the underlying soil through the surface, in quantities which depend on the soil type, topography, and vegetation cover. Slope drainage can control the movement of surface water and also the subsurface pore water pressure in the slope. Drainage can be very effective if installed at the correct location on or within the slope. In the long term, systems need to be designed with maintenance operations in mind so that a sustainable system is installed with a design life comparable to the 60 year design life of a highway slope.

The report reviews the distribution of pore water pressures within a slope measured by TRL at a number of sites on highway schemes and follows with a critical assessment of the applicability of different slope drainage techniques. The report concludes with best practice guidance in the design and maintenance of the various drainage systems. The increased use of surface water channels and a more proactive bioengineering approach are recommended in the design of slope drainage systems. These methods offer the advantages of ease of inspection, reduced maintenance costs and a sustainable technique.

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