Control Document - rbkc.gov.uk

Basement Impact Assessment

at 40 Adam and Eve Mews, London W8 6UJ

for Duffy Associates

Reference: 15872/BIA/Rev1.02

January 2017

1

Control Document

Project

40 Adam and Eve Mews, London W8 6UJ

Document Type

Basement Impact Assessment

Document Reference

15872/BIA

Document Status

Final

Date

January 2017

Prepared by

D V Tedesco MEng, PhD, ChIta

First check by

Eur Ing R B Higginson BSc, PGDip, CEng, MICE, FGS.

Second check by

N J Lambert BSc (Hons), CEnv, FGS, MIEncSc

This is not a valid document for use in the design of the project unless it is titled Final in the document status

box.

Current regulations and good practice were used in the preparation of this report. The recommendations

given in this report must be reviewed by an appropriately qualified person at the time of preparation of the

scheme design to ensure that any recommendations given remain valid in light of changes in regulation and

practice, or additional information obtained regarding the site.

Soils Limited 40 Adam and Eve Mews, London – Basement Impact Assessment

2

Commission

Soils Limited was commissioned by Duffy Associates to undertake a Basement Impact

Assessment on a building at 40 Adam and Eve Mews, London W8 6UJ. The scope of the

investigation was outlined in the Soils Limited quotation reference Q18099/NJL/1, dated

17th October 2016.

This document comprises the Basement Impact Assessment and incorporates the

results, discussion and conclusions.

Standards

The site works, soil descriptions and geotechnical laboratory testing was undertaken in

accordance with the following standards:

BS EN 1997-1:2004+A1:2013 Eurocode 7. Geotechnical design

BS EN ISO 14688-1:2002+A1:2013 - Geotechnical investigation and testing -

Identification and description

BS EN ISO 14688-2:2004+A1:2013 - Geotechnical investigation and testing -

Principles for a classification

The geotechnical laboratory testing was performed by GEO Site & Testing Services Ltd

(GSTL) in accordance with the methods given in BS 1377:1990 Parts 1 to 8 and their

UKAS accredited test methods.

For the preparation of this report, the relevant BS code of practice was adopted for the

geotechnical laboratory testing technical specifications, in the absence of the relevant

Eurocode specifications (ref: ISO TS 17892).

The chemical analyses were undertaken by QTS Environmental Limited in accordance

with their UKAS and MCERTS accredited test methods or their documented in-house

testing procedures. This investigation did not comprise an environmental audit of the site

or its environs.

Trial hole is a generic term used to describe a method of direct investigation. The term

trial pit, borehole or window sample borehole implies the specific technique used to

produce a trial hole.


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Contents

Section 1 Introduction ....................................................................................... 1

1.1 Objective of Investigation ..................................................................................... 1

1.2 Limitations and Disclaimers .................................................................................. 1

Section 2 Site Context ....................................................................................... 2

2.1 Location ................................................................................................................ 2

2.2 Site Details ............................................................................................................ 2

2.3 Proposed Development ........................................................................................ 2

2.4 Topography ........................................................................................................... 3

2.5 Published Geological Data ................................................................................... 3

2.5.1 Taplow Gravel Formation ..................................................................................... 4

2.5.2 London Clay Formation ........................................................................................ 4

2.6 Unpublished Geology ........................................................................................... 4

2.7 Hydrogeology ........................................................................................................ 5

2.8 Hydrology .............................................................................................................. 5

2.9 Unpublished Groundwater Records ..................................................................... 6

2.10 Flood Risk ............................................................................................................. 6

2.11 Underground Infrastructure .................................................................................. 6

Section 3 Screening ............................................................................................ 7

3.1 Introduction ........................................................................................................... 7

3.2 Surface Flow and Flooding Screening Assessment ............................................. 7

3.3 Subterranean (Groundwater) Screening Assessment .......................................... 8

3.4 Stability Screening Assessment ........................................................................... 9

3.5 Summary ............................................................................................................ 11

Section 4 Intrusive Investigation .................................................................... 13

4.1 Ground Conditions .............................................................................................. 13

4.1.1 Made Ground ...................................................................................................... 14

4.1.2 Taplow Gravel Formation ................................................................................... 14

4.1.3 London Clay Formation ...................................................................................... 14

4.1.4 Roots .................................................................................................................. 14

4.1.5 Groundwater ....................................................................................................... 14

Section 5 Discussion of Geotechnical In-Situ and Laboratory Testing ...... 16

5.1 Atterberg Limit Tests .......................................................................................... 16

5.2 Particle Size Distribution Tests ........................................................................... 16

5.3 Sulphate and pH Tests ....................................................................................... 16


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Section 6 Foundation Design .......................................................................... 17

6.1 Allowable Bearing Values ................................................................................... 17

6.2 Subsurface Concrete .......................................................................................... 17

Section 7 Basement Impact Assessment – Land/Slope Stability ................ 18

7.1 Introduction ......................................................................................................... 18

7.1.1 Heave Movements .............................................................................................. 18

7.1.2 Underpin Settlement ........................................................................................... 18

7.2 Ground Movement arising from Basement Excavation ...................................... 18

7.2.1 Short Term Heave .............................................................................................. 19

7.2.2 Long Term Ground Movement ........................................................................... 20

7.2.3 Settlement Due To Workmanship ....................................................................... 21

7.3 Ground Movement Due To Underpin Wall Deflection ........................................ 21

Section 8 Damage Category Assessment ...................................................... 22

8.1 Introduction ......................................................................................................... 22

8.2 Summary of Ground Movements and Evaluation of Relative Deflection ........... 22

8.3 Cumulative Effects .............................................................................................. 23

Section 9 Conclusions and Recommendations ............................................. 24

9.1 General Considerations ...................................................................................... 24

9.2 On the Expected Damage Category ................................................................... 24

List of Figures

Figure 1 – Site Location Map ........................................................................................... 27

Figure 2 – Aerial Photograph ........................................................................................... 28

Figure 3 – Boreholes Locations ....................................................................................... 29

Figure 4 – Scenarios ........................................................................................................ 30

Figure 5 – Undrained Heave ............................................................................................ 31

Figure 6 – Drained Movements ........................................................................................ 32

Figure 7 – Deflection at 18 Adam and Eve Mews ............................................................ 33



Figure 10 – Damage Category ......................................................................................... 36

Figure 11 – NFIS, Flooding from Rivers and the Sea ...................................................... 37

Figure 12 – NFIS, Flooding from Reservoirs ................................................................... 38


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Figure 13 – NFIS, Flooding from Surface Water ............................................................. 39

Figure 14 – EA, Groundwater Protection Zones .............................................................. 40

Figure 15 – EA, Flooding from Rivers and the Sea ......................................................... 41

Figure 16 – EA, Flood Warning Areas ............................................................................. 42

Figure 17 – SFRA, Critical Drainage Areas ..................................................................... 43

Figure 18 – SFRA, Flood Area 1 ...................................................................................... 44

Figure 19 – SFRA, Flooding from Surface Water ............................................................ 45

Figure 20 – SFRA, Flooding from Sewers ....................................................................... 46

Figure 21 – SFRA, Flooding from Groundwater .............................................................. 47

List of Tables

Table 3.1 Surface Flow and Flooding Screening ............................................................... 7

Table 3.2 Subterranean (Groundwater) Screening ............................................................ 8

Table 3.3 Stability Screening ........................................................................................... 10

Table 3.4 Surface Flow and Flooding .............................................................................. 12

Table 3.5 Subterranean (Groundwater Flow) ................................................................. 12

Table 3.6 Ground Movement (Land stability) ................................................................... 12

Table 4.1 – Investigatory Depths of Trial-holes ............................................................... 13

Table 4.2 – Ground Conditions ........................................................................................ 14

Table 7.1 – Soil Parameters ............................................................................................ 19

Table 8.1 – Classification Of Visible Damage To Walls ................................................... 22

Table 8.2 - Expected Damage Category ......................................................................... 23

List of Appendices

Field Work

Appendix A.1 Engineers Logs

Geotechnical In-Situ and Laboratory Testing

Appendix B.1 Classification

Appendix B.2 Interpretation

Appendix B.3 Geotechnical In-Situ and Laboratory Results

References

Information Provided by the Client


1

Section 1 Introduction

1.1 Objective of Investigation


Assessment (BIA). The objective of this investigation was to establish the impact and risk

of the proposed basement at 40 Adam and Eve Mews, London W8 6UJ on the local

environment.

The report provides details on the ground and groundwater conditions onsite and

presents calculations to determine the potential impact of the proposed development on

neighbouring properties. In addition, the report provides a qualitative risk assessment of

the potential impacts the proposed development might have on groundwater levels,

surface water flows and flooding.

It is recognised that any Basement Impact Assessment is a live document and that

further detailed assessments will be ongoing, if appropriate, as the design and

construction progresses.

1.2 Limitations and Disclaimers


Assessment to supply a risk based impact assessment with regard to hydrology,

hydrogeology and land stability.

Soils Limited disclaims any responsibility to the Client and others in respect of any

matters outside the scope of the above.

The report is personal and confidential to the Client and Soils Limited accept no

responsibility of whatever nature to third parties to whom this report, or any part thereof,

is made known. Any such party relies on the report wholly at its own risk.

The Client may not assign the benefit of the report or any part to any third party without

the written consent of Soils Limited.

The ground is a product of continuing natural and artificial processes. As a result, the

ground will exhibit a variety of characteristics that vary from place to place across a site,

and also with time. Whilst a ground investigation will mitigate to a greater or lesser

degree against the resulting risk from variation, the risks cannot be eliminated.

Current regulations and good practice were used in the preparation of this report. An

appropriately qualified person must review the recommendations given in this report at

the time of preparation of the scheme to ensure that any recommendations given remain

valid in light of changes in regulation and practice, or additional information obtained

regarding the site.


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Section 2 Site Context

2.1 Location

The site was located at 40 Adam and Eve Mews, London W8 6UJ. The approximate O.S

Land Ranger Grid Reference was TQ 25414 79343. It falls within the boundaries of the

Royal Borough of Kensington and Chelsea.

The site location plan is given in Figure 1.

2.2 Site Details

The site comprised a terraced two storey residential building. Further adjoining

residential properties of similar geometry surrounded the building to the north, east and

south boundaries and were respectively identified as 42, 18 and 38 Adam and Eve

Mews. The main façade, to the west boundary, faced a further building, identified as 36

Adam and Eve Mews which was located at a distance of about 5.80m to the west.

Some wisteria from a very small flowerbed was found creeping up the front façade, while

a few further shrubs were similarly found to the front of the neighbouring buildings. No

other soft landscaping was observed adjacent to the building, but apparently mature

trees were noted within the backyards of the properties to the west, at a distance of more

than 15m.

The property was connected to the public sewer and a public surface drainage system

was found.

The local site topography was generally flat and level. The wider topography gently

sloped down towards the River Thames in a north to south/south-east direction.

Given the information provided by the Client, it was understood that the adjoining

property at 42 Adam and Eve Mews already had a basement. No details were provided

by the Client about the depth to the basement foundations, but it was likely they were

similar to the proposed formation depth for 40 Adam and Eve Mews. Assuming that there

is an existing basement it can be considered that the proposed development will not

provide a relevant increase in the differential foundation depths with the building at 42

Adam and Eve Mews. No further basements were identified at close distance from the

development site.

2.3 Proposed Development

The proposed redevelopment is to comprise the construction of a basement to be used

as media room, playroom and shower. Natural light was provided by means of lightwells

in structural glass located to the front and back of the house but within the footprint of the

building. In order to avoid the ingress of water from the existing sewers, the basement

drainage system related to the presence of the shower must be provided with positively


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pumping devices (PPDs) and anti-flood valves will be fitted to contrast the risk of flooding. The proposed basement will be constructed within the footprint of the existing building, with no increase in the ratio between paved and unpaved areas, as reported on drawings provided by the Client and appended to the report. It must be pointed out that the area of Adam and Eve Mews was almost entirely paved and impermeable. The construction of the proposed basement, therefore, will not result in changes in the amount and quality of surface water flows discharged to the ground, public drainage system, neighbouring properties and water features. The top of the finished basement floor level was proposed at a depth of about 2.90m bgl, while the total maximum depth of the excavation was expected to be at about 3.80m bgl, as reported within the drawings and documents provided by the Client. This depth was used within the calculations. The proposed development is likely to have effects on the wisteria shrub to the front of the building (west boundary). NHBC Standards consider wisteria shrubs as potentially damaging and precautions against the effects of this shrub must be taken. Precautions must be taken independently from the nature of the soils encountered on site. In compiling this report reliance was placed on the structural drawings no. 1321.S01-C, 1618.S02-A, 1618.SO3-C, 1618.SO4-B, 1618.S05-B, 1618.S06 to 1618.S07, 1618.S08-A to 1618.S12-A, all dated December 2016, prepared by Duffy Associates, on temporary works drawings no. 1321.TW01 and 1321.TW02, dated December 2016, and on drawings no. 1321.S01 and 1321.S02, dated 2013, prepared by Duffy Associates, and on the architectural drawings no. 1239 PL010 and 1239 P011, dated October 2016 and prepared by Shape Architecture. All the drawings were provided by the Client. Any change or deviation from the scheme outlined in the drawing could invalidate the recommendations presented within this report. Soils Limited must be notified about any such changes. The proposed development layout as provided by the client are included in Appendix D. 2.4 Topography The local site topography was generally flat and level. The wider topography gently sloped down towards the River Thames on a north to south/south-east direction. No relevant slopes were noted in the area of the proposed development and no reprofiling of the site was required. 2.5 Published Geological Data The 1:50,000 BGS map showed the site to be located on bedrock of the London Clay Formation with overlying superficial geology represented by the Taplow Gravel Formation.


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2.5.1 Taplow Gravel Formation

The rivers of the south-east of England, including the River Thames and its

tributaries, have been subject to at least three changes of level since Pleistocene

times. One result has been the formation of a complex series of River Terrace

Gravels. These terraces represent ancient floodplain deposits that became isolated

as the river cut downwards to lower levels. The Taplow Gravel Formation is found

at an elevation that approximates to the present floodplain gravel

The composition of the River Terrace Gravel varies greatly, depending on the

source material available in the river’s catchment. Deposits generally consist of

sands and gravels of roughly bedded flint or chert gravels commonly in a matrix of

silts and clays.

2.5.2 London Clay Formation

The London Clay Formation comprises stiff grey fissured clay, weathering to brown

near surface. Concretions of argillaceous limestone in nodular form (Claystones)

occur throughout the formation. Crystals of gypsum (Selenite) are often found

within the weathered part of the London Clay, and precautions against sulphate

attack to concrete are sometimes required.

The upper boundary member of the London Clay Formation is known as the

Claygate Member and marks the transition between the deep water, predominantly

clay environment and succeeding shallow-water, sand environment of the Bagshot

Formation.

The lower boundary is generally marked by a thin bed of well-rounded flint gravel

and/or a glauconitic horizon. The formation overlies the Harwich Formation or

where the Harwich Formation is absent the Lambeth Group.

In the north London area, the upper part of the London Clay has been disturbed by

periglacial processes and may contain pockets of sand and gravel.

2.6 Unpublished Geology

Published information was taken from boreholes available in a radius of about 300m

from the site.

Soils expressly described as Made Ground were identified within some of the boreholes,

generally observed from ground level to depths ranging from 1.50m to 2.50m bgl. The

Made Ground was variable in lithology and composition, typically comprising drift, clay to

gravel with brick, ash, concrete, rubble, clinker and coal fragments.

Soils of the Taplow Gravel Formation were observed within the boreholes to depths

ranging between 1.50m and 4.50m bgl and typically comprised sand and gravel in

variable proportions with clay and silt.


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Soils from London Clay Formation were observed from below the Made Ground and/or

Taplow Gravel Formation to a depth of about 60.00m bgl. A weathered layer was usually

found at the top of the stratum, comprising firm becoming stiff to very stiff and fissured

brown to brown orange to brown grey mottled clay with occasional selenite crystals and

orange brown stained fissures. The soil of the unweathered London Clay comprised stiff

to very stiff and closely fissured dark grey to dark brown silty clay with occasional light

brown to brown fine sand and silt partings and rare fine gypsum.

2.7 Hydrogeology

The Environment Agency has produced an aquifer designation system consistent with

the requirements of the Water Framework Directive. The designations have been set for

superficial and bedrock geology and are based on the importance of aquifers for potable

water supply and their role in supporting water bodies and wetland ecosystems.

The London groundwater model was generally split into three aquifers, the Upper,

Intermediate and Lower Aquifer. The Upper Aquifer was confined to the River Terrace

Deposits, overlying the London Clay Formation, which acts as an aquiclude. The

Intermediate Aquifer was generally associated with granular layers within the Lambeth

Group. The Lower Aquifer was principally associated with the Chalk, but can include the

Thanet Sand Formation.

Information presented by the Environment Agency classifies the Taplow Gravel

Formation as a Secondary ‘A’ aquifer and the London Clay Formation bedrock as

unproductive strata.

The Taplow Gravel Formation comprised permeable layers capable of supporting water

supplies at a local rather than strategic scale, and in some cases forming an important

source of base flow to rivers.

Any water infiltrating the London Clay Formation will generally tend to flow vertically

downwards at a very slow rate. Due to the predominantly cohesive nature of the soils,

the groundwater flow rate is anticipated to be very slow. Published permeability data for

the London Clay Formation indicates the horizontal permeability to generally range

between 10-10 m/s and 10-8 m/s (~5 mm/year to ~5 mm/day), with an even lower vertical

permeability.

2.8 Hydrology

The nearest surface water feature was the Round Pond in Hyde Park, recorded at

~950m to the north-east of the site.

The site was recorded at an elevation of approximately 10m AOD, and the Round Pond

was at approximately 24 m AOD.


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According the Lost Rivers of London (Barton, 1992) the site is not affected by any

historic rivers. The nearest historic river to the site is Counter’s Creek, at more than

500m to the west, flowing in a south-easterly direction.

2.9 Unpublished Groundwater Records

Groundwater was generally not reported in the unpublished data. Where observed, it

was found at depths ranging from 55.00m to 60.00m bgl. The regional water table in this

location is about 20m to 30m below ordnance datum (Environment Agency, 2016). It

must be pointed out that the groundwater levels from unpublished geology may describe

conditions that differ from the results of the site investigation.

2.10 Flood Risk

The National Flood Information Service shows the site to be at no risk for flooding from

rivers and the sea and from reservoirs. A high risk of flooding from surface water was

identified.

According to the Environmental Agency, the site did not fall within a Groundwater Source

Protection Zone. The site does not lie within a flood risk zone from rivers or the sea or in

a Flood Warning Area/Flood Alert Area.

The Strategic Flood Risk Assessment (SFRA) for the Royal Borough of Kensington and

Chelsea excluded the site to be located within any of the Critical Drainage Areas (CDAs),

but it falls within the Environmental Agency Flood Risk Zone 1. The SFRA identified the

site to be at moderate risk from surface water and groundwater flooding. A risk of

flooding from sewers was also noted.

The maps related to the mentioned flood risk assessment are presented in Figure 11 to

Figure 21.

2.11 Underground Infrastructure

Underground infrastructures were observed at about 200m to the north-east of the site

and were represented by the High Street Kensington Station.


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Section 3 Screening

3.1 Introduction

Soils Limited has adopted a screening process to meet the requirements of the Royal

Borough of Kensington and Chelsea (RBKC) Basement Planning Policy and Final

Basements Supplementary Planning Document (SPD) to identify potential risks to the

ground, groundwater/surface water, land stability, adjacent properties and infrastructure.

The assessment is undertaken in the form of tabulated questions, setting out relevant

considerations for conditions in the borough. Where simple answers may be provided

without further analysis, these are provided.

3.2 Surface Flow and Flooding Screening Assessment

The response to the Surface Flow and Flood Screening Assessment is given in Table

3.1.

Table 3.1 Surface Flow and Flooding Screening

Question Response Action Required

1. Is the site within a Critical Drainage

Area?

No

RBKC SFRA shows the site to be located off the

CDAs

None

2. As part of the proposed site drainage,

will surface water flows (e.g. volume of

rainfall and peak run-off) be materially

changed from the existing route?

No

The area surrounding the site was known to be

predominantly paved and impermeable. The

proposed development will not change the ratio

between paved and unpaved areas and/or the

existing slopes. As a consequence, surface water

flows will not be materially changed from the

existing routes.

None

3. Will the proposed basement

development result in a change in the

proportion of hard surfaced / paved

areas?

No

The proposed basement will be constructed within

the footprint of the existing building and no

changes will be applied to the ratio between paved

and unpaved areas.

None


development result in changes to the

profile of the inflows (instantaneous and

long term) of surface water being

received by adjacent properties or

downstream watercourses?

No

No changes in the infiltration rates or infiltration

profiles will take place as the area is fully paved and

impermeable and the proposed basement will be

constructed within the footprint of the existing

building..

None

5. Will the proposed basement result in

changes to the quality of surface water

being received by adjacent properties or

downstream watercourses?

No

No changes are expected because of the proposed

development as it will take place within the

None


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footprint of the existing building and does not

change the status quo of water quality.

6. Is the site in an area known to be at

risk from surface water flooding or is it

at risk from flooding because the

proposed basement is below the static

water level of a nearby surface water

feature?

Yes

The site falls within an area at risk of surface water

flooding.

Dewatering of the

basement to be

provided.

3.3 Subterranean (Groundwater) Screening Assessment

The response to the Subterranean (Groundwater) Screening Assessment is given in

Table 3.2.

Table 3.2 Subterranean (Groundwater) Screening


1a. Is the site located

directly above an

aquifer?

Yes

The Taplow Gravel Formation

was classified as a Secondary

“A” Aquifer. The London Clay

Formation was classified as an

Unproductive Stratum.

Dewatering of the basement premises may be needed.

Groundwater flow should not be adversely affected, as

flow around and below the basement will not be

interrupted, but an assessment of potential cumulative

effects on groundwater levels must be carried out due

to the presence of an existing adjoining basement.

1b. Will the proposed

basement extend

beneath the water table

surface?

No

No groundwater was

encountered to the depth of

3.80m bgl at the time of the site

investigation. Historical data

report groundwater depths of

10.00m to 20.00m bgl

None

2. Is the site in an area of

groundwater flooding

risk?

Yes Premises to be sealed up to prevent groundwater

ingress. Sump and pumps to be provided with

appropriate design and redundancy, considering the

risk for potential electric black-outs during flooding

events. Definition of a maintenance programme

recommended.


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3. Is the site within 100

m of a watercourse, well

(used/ disused) or

potential spring line?

No

The closest water feature was

the Round Pond at Hyde Park,

located at about 950m to the

north-east of the site. The

Counter’s Creek, as reported by

Barton’s Lost Rivers of London,

was identified at more than

500m to the west. The River

Thames was located at about

2400m to the south/south-east.

None

4. Will the proposed

basement development

result in a change in the

proportion of hard

surfaced / paved areas?

No

The proposed basement will be

constructed within the footprint

of the existing building. The area

surrounding the site was

identified as a predominantly

paved, impermeable area and no

changes will be caused by the

proposed development.

None

5. As part of the site

drainage, will more

surface water (e.g.

rainfall and run-off) than

at present be discharged

to the ground (e.g. via

soakaways and/or

SUDS)?

No

Soils Limited was not informed

about the proposed site

drainage, but no changes are

expected as the area

surrounding the site was fully

paved and impermeable.

Drainage strategy not in the brief of the present

report.

6. Is the lowest point of

the proposed excavation

(allowing for any

drainage and foundation

space under the

basement floor) close to

or lower than, the mean

water level in any local

pond or spring line?

No

The closest water feature with

an elevation lower than the one

existing on site was the River

Thames, located at more than

700m to the south/south-east

None

3.4 Stability Screening Assessment

The response to the Stability Screening Assessment is given in Table 3.3.


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Table 3.3 Stability Screening


1. Does the existing site include slopes,

natural or manmade, greater than about

1 in 8?

No

No relevant slopes were noted.

None

2. Will the proposed re-profiling of

landscaping at the site change slopes at

the property boundary to more than 1

in 8?

No

No reprofiling was part of the

proposed development.

None

3. Does the development neighbour

land, including railway cuttings and the

like, with a slope greater than 1 in 8?

No

No slopes were found in the

neighbouring land.

None

4. Is the site within a wider hillside

setting in which the general slope is

greater than 1 in 8?

No

No relevant slopes in the

surrounding settings.

None

5. Is the London Clay the shallowest

strata at the site?

No

Superficial layers of Taplow Gravel

Formation were both expected and

observed

None

6. Will any trees be felled as part of the

proposed development and / or are any

works proposed within any tree

protection zones where trees are to be

retained?

Yes

A wisteria shrub to the front of the

site will be affected by the proposed

construction.

NHBC Standards classify wisteria

shrubs as potentially damaging.

Precautions must be taken even

if the development should take

place within the granular soils of

the Taplow Formation due to

the potential presence of clayey

lenses and the usual low

thickness of the Taplow Gravel

Formation, directly overlying the

overconsolidated soils of the

London Clay Formation and, in

particular, its weathered upper

layer.

7. Is there a history of seasonal shrink-

swell subsidence in the local area and /

or evidence of such effects at the site?

No

No cracks were noted on the

existing walls and the predominantly

granular nature of the soils of the

Taplow Gravel Formation should

exclude shrink/swell phenomena.

None


11


8. Is the site within 100 m of a

watercourse or potential spring line?

No

The closest water feature was the

Round Pond at Hyde Park, located

at about 950m to the north-east of

the site. The Counter’s Creek, as

reported by Barton’s Lost Rivers of

London, was identified at more than

500m to the west. The River

Thames was located at about 2400m

to the south/south-east.

None

9. Is the site within an area of previously

worked ground?

No

The site investigation identified

Made Ground, but no infilled pits

and quarries were noted

None

Investigation regarding the

presence of soil gas not in the

brief of the present report.

10. Is the site within 5m of highway or

pedestrian right of way?

Yes

The proposed development will take

place immediately along Adam and

Eve Mews.

The proposed basement is not

to be extended below Adam and

Eve Mews and therefore it is

suggested that the impact on this

access road is likely to be

minimal. However, the

excavations must be carried out

in order to ensure that traffic

and construction activity does

not harm pedestrian, cycle,

vehicular and road safety.


significantly increase the differential

depth of foundations relative to

neighbouring properties?

Yes A basement already exists under

the property at 42 Adam and

Eve Mews, so no Basement

Impact Assessment is required

with regards to this property.

Assessments must be carried out

for the other surrounding

properties, with particular

regards to the propertied at 18,

36and 38 Adam and Eve Mews

12. Is the site over (or within the

exclusion zone of) any tunnels, e.g.

railway lines?

No

No tunnels expected in the area of

interest.

None

3.5 Summary

Based on the screening exercise, further stages of the basement impact assessment are

required. A summary of the basement impact assessment requirements has been

provided in Table 3.4, Table 3.5 and Table 3.6.


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Table 3.4 Surface Flow and Flooding

Item Description

6 Dewatering of the basement to be provided.

Table 3.5 Subterranean (Groundwater Flow)

Item Description

1a Dewatering of the basement premises may be needed. Groundwater flow should not

be adversely affected, as flow around and below the basement will not be interrupted,

but an assessment of potential cumulative effects on groundwater levels must be

carried out due to the presence of an existing adjoining basement.

2 Premises to be sealed up to prevent groundwater ingress. Sump and pumps to be

provided with appropriate design and redundancy, considering the risk for potential

electric black-outs during flooding events. Definition of a maintenance programme

recommended.

Table 3.6 Ground Movement (Land stability)

Item Description

6 NHBC Standards classify wisteria shrubs as potentially damaging. Precautions must be

taken even if the development should take place within the granular soils of the

Taplow Formation due to the potential presence of clayey lenses and the usual low

thickness of the Taplow Gravel Formation, directly overlying the overconsolidated

soils of the London Clay Formation and, in particular, its weathered upper layer.

10 The proposed basement is not to be extended below Adam and Eve Mews and

therefore it is suggested that the impact on this access road is likely to be minimal.

However, the excavations must be carried out in order to ensure that traffic and

construction activity does not harm pedestrian, cycle, vehicular and road safety.

11 A basement already exists under the property at 42 Adam and Eve Mews, so no

Basement Impact Assessment is required with regards to this property. Assessments

must be carried out for the other surrounding properties, with particular regards to

the propertied at 18, 36and 38 Adam and Eve Mews


13

Section 4 Intrusive Investigation

4.1 Ground Conditions

Two hand held windowless sampler boreholes (WS1 and WS2) were drilled on site, at

locations selected by Soils Limited and agreed with the Client, where access could be

gained and no live services were identified, within the footprint of the proposed

redevelopment.

On 26th October and 14th November 2016 two starter pits were hand dug through the

existing ground floor slab: the intrusive investigation was stopped because asbestos

containing materials were encountered. On both the occasions, the Client was informed

and remedial actions were required in order to carry on with the proposed investigation.

The two proposed boreholes were finally drilled on 12th December 2016. The depth

investigated ranged from 2.00m (WS2) to 2.40m bgl (WS1). It was impossible to reach

greater depths because of the presence of obstructions and of the relative density of the

soils encountered.

Dynamic probing was to be undertaken prior and adjacent to the respective boreholes,

but the available headroom inside the building made it impossible to undertake the tests.

The Client was informed of the risk at the quotation stage.

One standpipe monitoring well was installed within borehole WS1 to a depth of 2.50m

bgl (below ground level) to allow long-term groundwater levels monitoring. Groundwater

readings were then carried out on one occasion, as agreed with the Client.

One trial pits for foundation exposure test was carried out by the Client and the results

provided in Appendix D.

The borehole locations are outlined in Figure 3.

Table 4.1 outlines the depths of each trial-hole.

Table 4.1 – Investigatory Depths of Trial-holes

Trial-hole

(WS)

Final Depth

(m bgl)

Trial-hole

(WS)

Final Depth

(m bgl)

WS1 2.40 WS2 2.00

Ground conditions encountered are presented below; detailed information including logs

are presented in Appendix A:

Made Ground (MG)

Taplow Gravel Formation (TPGR)


14

Table 4.2 summarises the ground conditions encountered.

Table 4.2 – Ground Conditions

Stratum Epoch Depth Range (m bgl) Thickness (m) Description

Top Bottom

MG Recent GL 0.80 - 1.00 0.90 Fine to coarse brick and concrete with

ACM (removed by specialised personnel).

TPGR Palaeogene 0.80 – 1.00 2.001 – 2.401 Not proven2 Stiff orange brown slightly gravelly sandy

CLAY to clayey gravelly SAND.

Note: 1 Final depth of trial hole. 2 Base of strata not encountered.

4.1.1 Made Ground

Made Ground was encountered in both of the two trial holes from ground level to a

depth of 1.00 m bgl and comprised fine to coarse brick and concrete with asbestos

containing materials. The materials were removed by a specialist contractor.

4.1.2 Taplow Gravel Formation

Soils described as Taplow Gravel Formation were found in both the two trial holes

directly beneath the Made Ground to the full depths of the investigation and

comprised stiff orange brown slightly gravelly sandy CLAY to clayey gravelly

SAND. Sand was fine to coarse. Gravel within the clayey beds was frequent fine to

medium rounded to sub-rounded and fine to medium angular to sub-angular within

the sandy beds.

4.1.3 London Clay Formation

The soils of the London Clay Formation were not encountered in the test holes.

4.1.4 Roots

No roots were observed at the time of the site investigation. The depth to roots

and/or of desiccation may vary from that found during the investigation. The client

is responsible for establishing the depth to roots and/or of desiccation on a plot by

plot basis prior to the construction of foundations. Supplied site surveys may not

include substantial shrubs or bushes and is also unlikely to have data or any trees,

bushes or shrubs removed prior to or following the site survey.

Where trees are mentioned in the text this means existing trees, substantial bushes

or shrubs, recently removed trees (approximately 20 years to full recovery on

cohesive soils) and those planned as part of the site landscaping).

4.1.5 Groundwater

Groundwater was not encountered at the time of the intrusive investigation.

Equilibrium conditions may only be conclusively established if a series of

observations are made via groundwater monitoring wells. One groundwater

monitoring well was installed into borehole WS1 to the depth of 2.50 m bgl.


15

One visit was done on site for the monitoring of groundwater on 13th January 2017,

when groundwater levels should be rising towards their annual maximum (i.e.

highest) elevation which typically occurs around March, but the monitoring well was

found dry to its full depth of 2.50m bgl.


16

Section 5 Discussion of Geotechnical In-Situ and Laboratory Testing

5.1 Atterberg Limit Tests

Atterberg Limit tests were performed on three sample from the Taplow Gravel Formation.

The results were classified in accordance with BRE Digest 240 and NHBC Standards

Chapter 4.2.

The soils of the Taplow Gravel Formation were classified as non-plastic to medium

volume change potential in accordance with both BRE Digest 240 and NHBC Standards

Chapter 4.2.

A full interpretation of the Atterberg Limit tests is outlined in Table B.2.1, Appendix B.2

and the laboratory report in Appendix B.3.

5.2 Particle Size Distribution Tests

Particle Size Distribution (PSD) tests were performed on five samples from the Taplow

Gravel Formation.

PSD classified three samples from the granular beds of the Taplow Gravel Formation as

having no volume change potential in accordance with both BRE Digest 240 and NHBC

Standards Chapter 4.2. The results from grading analysis confirmed that two out of the

five samples tested had a volume change potential in accordance with both BRE Digest

240 and NHBC Standards Chapter 4.2. Note that a cohesive soil is only classified as

having a volume change potential if it is also plastic and an Atterberg Limit test can be

conducted on the strata.

A full interpretation of the PSD tests are outlined in Table B.2.2, Appendix B.2 and the

laboratory report in Appendix B.3.

5.3 Sulphate and pH Tests

One sample was taken from the Made Ground (WS2:0.50m bgl) for water soluble

sulphate (2:1) and pH testing in accordance with Building Research Establishment

Special Digest 1, 2005, ‘Concrete in Aggressive Ground’.

The tests recorded water soluble sulphate at 776mg/l with a pH value of 8.8.

The significance of the sulphate and pH Test results are discussed in Section 6.2 and

the laboratory report in Appendix B.3.


17

Section 6 Foundation Design

6.1 Allowable Bearing Values

The available headroom within the premises made it impossible to investigate the soils at

and below the expected basement formation level.

The Taplow Gravel Formation usually presents a thickness of a few metres, Therefore it

was expected that the proposed basement foundations will be placed within or

immediately above the soils of the weathered layer of the London Clay Formation.

Considering the data available from the literature, an undrained shear strength (cu) of

about 50kPa has been published for the soils of the London Clay Formation at the top of

the stratum. An angle of shearing resistance (∅’) of about 22º can be considered under

drained conditions.

Assuming a strip foundation, 5.00m x 0.75m in plan, the recommended net allowable

bearing pressure for the London Clay Formation at basement formation level is 110kPa.

Given the removal of about 3.80m of soils above the basement formation level, a gross

bearing pressure of 185kPa can be considered. The evaluation of the expected

settlements was reported in detail in Section 7.

The above bearing pressure values were conservatively assumed due to the

impossibility to carry out the investigation using a larger rig. If greater values for the

bearing capacity are needed, it is strongly recommended to carry out further testing on

the roadside, where more powerful rigs can be conveniently located. However, it is

usually recommended that a maximum bearing capacity of 150kPa should not be

exceeded for the soils of the London Clay Formation at basement formation level.

6.2 Subsurface Concrete

Sulphate concentration measured in 2:1 water/soil extracts fell into Class DS-2 of the

BRE Special Digest 1 2005, ‘Concrete in Aggressive Ground’. Table C2 of the Digest

indicated ACEC (Aggressive Chemical Environment for Concrete) site classifications of

AC-1s. The pH of the soil tested was 8.8. The classification given was determined using

the mobile groundwater case, as groundwater was encountered. The laboratory results

are presented in Appendix B.3.

Concrete to be placed in contact with soil or groundwater must be designed in

accordance with the recommendations of Building Research Establishment Special

Digest 1 2005, ‘Concrete in Aggressive Ground’ taking into account any possible

exposure of potentially pyrite bearing natural ground and the pH of the soils.


18

Section 7 Basement Impact Assessment – Land/Slope Stability

7.1 Introduction

This section provides calculations to determine ground movements that may result from

the construction of the additional basement level and to assess how these may affect

adjacent structures.

Movements are likely to occur through the following mechanisms:

7.1.1 Heave Movements

The excavation will unload the Taplow Gravel Formation and the London Clay

Formation and will cause a degree of heave during excavation and possibly post

construction, and/or settlement after construction.

7.1.2 Underpin Settlement

Construction of underpins can lead to settlement. With good workmanship, this

would be expected to be limited to 5mm of settlement per underpin ‘lift’. Settlement

may potentially also occur where foundation loads are transferred to deeper,

previously unload, soils.

Underpinning settlement only affects shared foundations, such as foundations to

party walls. This was applied to the Ground Movements Assessment and Damage

Category Assessment for the properties at 18 and 38 Adam and Eve Mews.

Where foundations are not shared, or the properties linked, workmanship will not

affect the adjoining structure. This was applied to the GMA and DCA for the

property at 36 Adam and Eve Mews.

7.2 Ground Movement arising from Basement Excavation

The soils at formation level will be subject to stress relief during excavation, as up to

about 3.80m of overburden is to be removed under the house. This is likely to give rise to

a degree of heave over the short term and potential heave or settlement over the longer

term as structural loads are reapplied.

A ground movement assessment has been undertaken using OASYS Limited PDISP

(Pressure induced DISPlacement analysis) analysis software. PDISP assumes that the

ground behaves as an elastic material under loading, with movements calculated based

on the applied loads and the soil stiffness (Eu and E’) for each stratum input by the user.

PDISP assumes perfectly flexible loaded areas and as such tends to overestimate

movements in the centre of loaded areas and underestimate movements around the

perimeters. Notwithstanding this, the structure has not been modelled as an evenly

loaded flexible raft and loads from underpins around the perimeter have been accounted

for and modelled in the analysis. The calculated movements are therefore, not

considered to be underestimated.


19

The proposed development gives rise to a net unloading of the underlying strata both

during construction and over the long term. The excavation will unload the soils under

the house by approximately 75kPa, assuming that the ground conditions encountered

during the site investigation are laterally continuous across the extent of the proposed

new basement. These values assume a typical bulk unit weight of 19kN/m3 for the soils

of the Made Ground and a unit weight of 20kN/m3 for the soils of the Taplow Gravel

Formation. The combined effects of both the immediate undrained unloading and the

long-term drained recovery of pore pressures have been analysed.

Considering the results of the site investigation and the absence of information from

dynamic probing, an estimation of the mechanical characteristics of the soils

encountered was carried out using data available from the literature. The stiffness

parameters reported in Table 7.1 were adopted for the soils involved in the analyses.

Table 7.1 – Soil Parameters

Stratum Top of Stratum

(m bgl)

Young’s Modulus (MPa) Poisson’s Ratio

Drained Undrained Drained Undrained

Made Ground1 0.00 5.00 5.00 0.25 0.50

Taplow Gravel Formation1 1.00 12.00 12.00 0.30 -

London Clay Formation2 3.80 40 + 4z 50 + 5z 0.25 0.50

Note: 1 Based upon CIRIA C580, Chapter 5. 2 From CIRIA SP200.

Two critical sections were identified in the neighbouring adjoining properties at 18 and 38

Adam and Eve Mews and one in the detached property at 36 Adam and Eve Mews.

No Ground Movements Assessment and Damage Category Assessment were

developed with regards to the property at 42 Adam and Eve Mews, as the proposed

development will not increase the differential depth between adjacent foundations due to

the presence of an already existing basement.

The critical sections at 18 and 38 Adam and Eve Mews were respectively identified at a

distance of about 8.30m and 8.90m from the outer face of the underpinning and, as

already stated, no basement was present under the adjoining buildings.

The building at 36 Adam and Eve Mews, as already discussed, was detached from the

proposed development and located at the distance of 5.80m from the outer face of the

underpinning.

The mentioned scenarios were reported in Figure 4.

7.2.1 Short Term Heave

Calculated short term heave considered the increase of stiffness after the

unloading of clayey soils and the evaluation of heave. The huge amount of data


20

available from the literature allowed the adoption of appropriate values for the

Youngs Modulus under both drained and undrained conditions. In the absence of

similar conditions, Simons and Menzies (2000) suggested to divide the calculated

heave by a factor equal to three in order to take into account the increase in

stiffness along a path of unloading and reloading. Poisson’s Ratio was considered

equal to 0.50 for cohesive soils under undrained conditions.

Maximum short term heave is predicted to be of the order of -4.10mm, occurring

towards the centre of the footprint of the building (negative values indicate an

upwards movement). The movement decreases towards the underpins located

along the boundary lengths of the basement. Heave was noted to occur within

these areas up to a maximum of -2.00mm due to the net increase of surcharged

load. The movement decreases with negligible movement at 6m from the boundary

underpins and towards the front of the property. A contour plot showing the

variation of short term movements across the entire basement footprint is

presented in Figure 5.

Minimum short term heave under the footprint of the detached building at 36 Adam

and Eve Mews predicted to be of the order of -0.10mm, occurring under the wall

facing the proposed development.

7.2.2 Long Term Ground Movement

Long term movements depend on the almost contemporary development of the

increase of heave (negative settlements) in the long term due to the reduction in

stiffness of the soils and the development of (positive) settlements due to the

construction of the basement and the application of the loads from the upper

structure to greater depths. Those movements develop contemporary and cannot

be distinguished, but an evaluation of the long term heave, as developing alone,

was also reported for completeness on the graphs in Figure 7, Figure 8 and Figure

9. It must be pointed out that long term heave and other long term movements

develop contemporary and on the same time scale because of the methods

adopted for the excavation and construction sequences used in the construction of

basements under existing buildings.

In the case of the building at 40 Adam and Eve Mews, the maximum long term

ground movements, therefore, are obtained by adding the long term heave

(estimated to a maximum of -7.60mm) to the settlements induced by the application

of the construction loads and are of the order of 8.10mm, occurring under the front

underpinning foundation. The movement decreases to an average of 5.00mm at

the outer faces of the underpinning movement at 5.0m from the excavation. A

contour plot showing the variation of long term movements across the entire

basement footprint is presented in Figure 6.

Maximum long term ground movements under the footprint of the building at 36

Adam and Eve Mews were calculated to be less than 0.50mm under the wall facing

the proposed development.


21

7.2.3 Settlement Due To Workmanship

The heave/settlement assessment undertaken within PDISP assumes perfect

workmanship in the underpin construction and does not allow for settlement of the

dry pack between existing footings and the new concrete. With good construction

practice, these would be expected to not exceed 5.00mm (assuming 5.00mm per

underpin lift). This value will be applied to the overall ground movement and

corresponding impact assessment to give a worst case damage category for the

adjacent party wall properties.

Underpinning settlement only affects shared foundations, such as foundations to

party walls. This was applied to the Ground Movements Assessment and Damage

Category Assessment for the properties at 18 and 38 Adam and Eve Mews.

Where foundations are not shared, or the properties linked, workmanship will not

affect the adjoining structure. This was applied to the GMA and DCA for the

property at 36 Adam and Eve Mews.

7.3 Ground Movement Due To Underpin Wall Deflection

The retained height of the underpin wall is to be a maximum of some 3.80m beneath the

ground floor. Assuming 300mm thick concrete underpins for Wall A (east boundary,

scenario at 18 Adam and Eve Mews) and Wall B (south boundary, scenario at 38 Adam

and Eve Mews), the presence of permanent propping at the toe and at the top of the

walls and taking no account of a liner wall for long-term movements, deflections of the

underpins would be expected to be less than 2mm. Calculations were carried out using

the commercial software Wallap for the properties adjoining 40 Adam and Eve Mews. It

is assumed that underpins are propped in the temporary condition as described within

the temporary works drawings in order to prevent lateral movement.

A different configuration was assumed for Wall C (west boundary, scenario at 36 Adam

and Eve Mews), where no permanent propping was considered at the top of the wall due

to the presence of lightwells in structural glass. Wall thickness was therefore increased to

350mm. The property at 36 Adam and Eve Mews was located at a distance of 5.80m

from the underpinning.

Limited information was available from the literature with regards to the propagation of

movements at a distance from the excavation and the wall stem, especially with regards

to horizontal movements. An estimation was then done considering the information

provided by CIRIA C580 in Section 2.5.2, Figure 2.8, and the wall assumed as a

cantilevered secant pile wall. Horizontal movements were estimated to be zero at a

distance of 1.5 times the wall depth, corresponding to 5.70m. As a consequence,

horizontal movements at the distance of 5.80m were considered as equal to zero.


22

Section 8 Damage Category Assessment

8.1 Introduction

These ground movements were considered for assessing the expected potential damage

category that the construction of a new basement was expected to induce to the

neighbouring properties. The assessment was carried out considering the method

described in CIRIA Special Publication 200 (Burland et al., 2001) and CIRIA C580 (Gaba

et al., 2003), based upon the method proposed by Burland et al. (2001) and taking into

account the works by Burland and Wroth (1974) and Boscardin and Cording (1989).

The general categories of damage entity were summarised in Table 8.1.

Table 8.1 – Classification Of Visible Damage To Walls

Category Description

0 (Negligible) Negligible – hairline cracks

1 (Very slight) Fine cracks that can easily be treated during normal decoration (crack width <1mm)

2 (Slight) Cracks easily filled, redecoration probably required. Some repointing may be required

externally (crack width <5mm)

3 (Moderate) The cracks require some opening up and can be patched by a mason. Recurrent cracks can

be masked by suitable linings. Repointing of external brickwork and possibly a small

amount of brickwork to be replaced (crack width 5 to 15mm or a number of cracks >

3mm).

4 (Severe) Extensive repair work involving breaking-out and replacing sections of walls, especially over

doors and windows (crack width 15mm to 25mm but also depends on number of cracks).

5 (Very severe) This requires a major repair involving partial or complete re-building (crack width usually

>25mm but depends on number of cracks).

The London Boroughs usually indicate that the design and construction methodology

should aim to restrict the damage category to neighbouring properties to not higher than

Category 2 damage (slight damage), as it will only imply the risk of aesthetic damages to

the buildings.

However, the Councils considers that neighbouring residential properties are particularly

sensitive to damage, where relatively minor internal damage to a person’s home can

incur cost and considerable inconvenience to repair and redecorate. The Councils

therefore will expect that a damage not higher than Category 1 (very slight) is achieved.

8.2 Summary of Ground Movements and Evaluation of Relative Deflection

The analysis of the ground movements reported in Section 7 allowed to estimate the

relative vertical and horizontal deflections for all the considered buildings and scenarios,

corresponding to an expected Damage Category of 1 (Very slight) according to the

classification by Burland (2001). The results were summarised in Table 8.2.


23

Table 8.2 - Expected Damage Category

Critical Section Horizontal

Movement

(mm)

Vertical

Deflection

(mm)

Horizontal

Strain

h (%)

Deflection

Ratio

/L (%)

Damage

Category

18 Adam and Eve Mews 2.00 5.70 0.0241 0.0687 1 (Very slight)



The results for the sections at 18 and 38 Adam and Eve Mews, as already explained,

consider the effects of good workmanship to increase the potential damage. No

workmanship error was considered for the section at 36 Adam and Eve Mews, as that

building does not share foundations with the proposed development. It must be pointed

out that generally the evaluation of vertical and horizontal movements is over-

conservative and this induces the evaluation of expected damage categories indicating a

higher risk than the real one.

8.3 Cumulative Effects

The information provided by the Client identified the presence of a basement under the

building at 42 Adam and Eve Mews. Therefore, quantitative calculations were not carried

out as the construction of the proposed basement will not increase the differential depths

between the foundations of the two buildings.

The presence of the basement, however, cannot be excluded for the development of

qualitative evaluations regarding to the construction of a new basement adjoining an

existing one, and the potential effects on the depth of groundwater table.

The proposed development was to take place within the soils of the Taplow gravel

Formation and it did not exclude the possibility for the groundwater to flow around and

below the basement.

The construction of a further basement, therefore, will increase the length of the flow

path and the potential for the increase of the upstream groundwater level exists. This

effect can increase the potential ingress of water within the basement, increasing the risk

already considered in Section 2.10. The design of the sump and pumping system,

therefore, must be carried out considering a higher total volume and flow than the one

obtainable for the construction of an isolated basement. Also, the choice of pump type

must be carried out considering the risk of a long lasting use and the potential for electric

black-outs during flooding events. A malfunctioning of the pumping device must be also

taken into account. In order to reduce the risk of malfunctioning, an adequate

maintenance programme must be prepared and it is strongly recommended to consider

an appropriate system redundancy.


24

Section 9 Conclusions and Recommendations

9.1 General Considerations

The findings of this report are informed by site investigation data and information

regarding construction methods, sequence, loading and allowable bearing capacity

provided by the client. The analysis is undertaken on the assumption of high quality

workmanship.

The analysis is based on site investigation data and published data, particularly within

the Taplow Gravel Formation, the London Clay Formation and regarding to the

groundwater depths. Soils Limited must be notified if different conditions are observed

during the construction phase.

The site investigation could not reach the proposed formation level, which is likely to be

still within the Taplow Gravel Formation or, as conservatively assumed within the report,

at the top of the London Clay Formation.

Due to the shallow investigated depth, because of the obstacles encountered and the

density of the soils of the Taplow Gravel Formation, no groundwater was encountered.

For the development of the calculations, however, it was conservatively assumed to be

at the depth of 2.50m bgl as a moderately conservative scenario and the depth of 1.00m

bgl within the worst-case scenario.

The basement design would take into account the potential of water ingress over the

lifetime of the structure and would include appropriate water proofing design as also the

presence of appropriate water sumps. The basement must also be protected against the

risk of flooding from sewers by applying positive pumping devices.

The proposed development will consider no changes in the proportion between paved

and unpaved areas, as the area of interest was predominantly impermeable. The net

effect on surface water flow or flooding can be, as a consequence, considered as

negligible.

If perched water will be observed at the time of the excavation, precautions must be

taken to allow for a safe and comfortable development. The application of pumps for

allowing the dewatering of the excavation must have a local relevance and interest the

excavation only. No dewatering must be applied to the surrounding areas to avoid

unwanted settlements of the surrounding buildings.

9.2 On the Expected Damage Category

The construction of the basement will generate ground movements due to a variety of

causes including; heave, settlement, underpin construction and underpin wall deflection

during and after excavation. Calculations indicate that these could give rise to a damage

category within ‘Category I’ (Very Slight) for the adjacent properties. The above assumes

Documents

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