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i) Professor, School of Civil and Environmental Engineering, Nanyang Technological University, Singapore (chrahardjo＠ntu.edu.sg).ii) Project O‹cer, ditto.iii) Associate Professor, ditto.iv) Deputy Director and Professional Engineer, Building and Infrastructure Department, Housing and Development Board, Singapore.v) Engineer, ditto.

The manuscript for this paper was received for review on March 18, 2010; approved on February 7, 2011.Written discussions on this paper should be submitted before January 1, 2012 to the Japanese Geotechnical Society, 4-38-2, Sengoku,Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month.

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SOILS AND FOUNDATIONS Vol. 51, No. 3, 437–447, June 2011Japanese Geotechnical Society

PERFORMANCE OF HORIZONTAL DRAINS IN RESIDUAL SOIL SLOPES

H. RAHARDJOi), V. A. SANTOSOii), E. C. LEONGiii), Y. S. NGiv) and C. J. HUAv)

ABSTRACT

In tropical and subtropical regions, shallow landslides often occur in residual soil slopes. Short-duration, high-in-tensity rainfall will increase the pore-water pressure. As a result, the shear strength of the soil in the slopes decreasesand the stability of the slopes is aŠected. In this study, horizontal drains were installed in a residual soil slope in Singa-pore in order to improve the stability of the slope. The slope was instrumented with tensiometers and piezometers to in-vestigate the eŠectiveness of the horizontal drains as a slope stabilization method against rainfall-induced slopefailures. The variations in water table elevation and matric suction in the slope due to rainfall events were monitored.In addition, numerical analyses of the seepage into the slope brought about by the rainfall were carried out, and theresults showed a reasonably good agreement with the data obtained from ˆeld measurements. The ˆeld measurementresults indicated that horizontal drains were indeed eŠective for lowering the water table and for increasing the stabilityof the investigated slope. Therefore, horizontal drains are considered to be a useful and economical method for im-proving the stability of residual soil slopes against rainfall.

Key words: horizontal drain, pore-water pressure, rainfall-induced slope failure, residual soil (IGC: E7/H9/K1)

INTRODUCTION

Rainfall-induced slope failures are common problemsin tropical and subtropical regions where the residual soilhas a deep groundwater table. Short-duration, high-in-tensity rainfall will increase the pore-water pressure andresult in a decrease in the shear strength of the soil, even-tually aŠecting the slope stability. Slope failures due tofrequent and heavy rainfalls are commonly due to shal-low slides, as have been observed, for example, in HongKong (Brand, 1992), Malaysia (Liew et al., 2004), andSingapore (Pitts, 1985; Tan et al., 1987; Toll et al., 1999;Chatterjea, 1989; Lim et al., 1996; Rahardjo et al., 2001).Singapore is located in a tropical region where heavy rain-falls and high temperatures are conducive to rapid in situchemical and mechanical weathering, which results indeep residual soil proˆles. Due to these climatic condi-tions and geological features, slope instabilities are com-mon in this region. To protect slopes against the possibili-ty of rainfall-induced failures, preventive measures arenecessary to ensure the safety of nearby buildings and/orpublic facilities.

The application of horizontal drains for the loweringof groundwater levels is recognized as the most economi-cal method available. In practice, however, these drainsmust still be regularly maintained and frequently

replaced. Horizontal drains have been used extensivelyfor the stabilization of slopes in most countries aroundthe world, including Australia (Snowy Mountains Hydro-Electric Authority, 1983), Austria (Veder and Lackner,1984, 1985), Brazil (Costa Nunes, 1985), France (Pilotand Schluck, 1969; Cartier and Virollet, 1980; Amar etal., 1973), Great Britain (Hutchinson, 1977; Robinson,1967), the United States (Smith and StaŠord, 1957;Royster, 1977, 1980; Smith, 1980), and Hong Kong(Craig and Gray, 1985; McNicholl et al., 1986).

The eŠectiveness of a horizontal drainage system isgoverned by several factors, such as drain type, location,number, length, and spacing (Kenney et al., 1977; Non-veiller, 1981; Lau and Kenney, 1984; Nakamura, 1988;Martin et al., 1994; Prellwitz, 1978), and the importanceof the soil properties and the slope geometry as the con-trolling parameters cannot be disregarded. The two-dimensional ˆnite element modeling of horizontal drainshas been conducted by Choi (1974, 1977), Nonveiller(1981), and Rahardjo et al. (2003), while the three-dimen-sional ˆnite element modeling of such drains has beencarried out by Choi (1977, 1983) and Nonveiller (1981).The eŠects of rainfall on residual soil slopes have beenstudied by Chipp et al. (1982), Sweeney (1982), Pitts(1985), Krahn et al. (1989), Fredlund and Rahardjo(1993), Lim et al. (1996), Rahardjo et al. (1998), Ng et al.

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Fig. 1. Cross section of the instrumented slope

Fig. 2. Deviator stress and changes in water volume versus axial straincurves for the sedimentary Jurong Formation residual soil

438 RAHARDJO ET AL.

(2003), Li et al. (2005), and Ng et al. (2008) through pore-water pressure measurements using tensiometers andpiezometers.

The application of unsaturated soil mechanics for solv-ing seepage and slope stability problems involving resid-ual soil slopes with horizontal drains has not been fullyexplored in previous studies. Negative pore-water pres-sure, as a crucial part of the stability of residual soilslopes, needs to be maintained in slopes under varying cli-matic conditions and taken into account in the design. Onthe other hand, rainwater inˆltrating the slope surfacecontributes to the rise in the groundwater table and to theincrease in pore-water pressure. Therefore, it is im-portant to install horizontal drains near the toe of theslope to lower the groundwater table (Rahardjo et al.,2003), and consequently, to lower the pore-water pres-sure. The main focus of this study is to evaluate the roleof horizontal drains in increasing the stability of residualsoil slopes during heavy rainfall events through measure-ments of the variations in matric suction in the slope andnumerical analyses.

FIELD STUDY

The geology of Singapore can be classiˆed into threemain formations: (a) the sedimentary rock of the JurongFormation in the west, (b) the igneous rock of the BukitTimah granite in the northwest and center, and (c) thesemi-hardened alluvium or Old Alluvium in the east(PWD, 1976). Residual soil from granitic and sedimenta-ry rock occupies about two-thirds of Singapore Island.The slope studied in this paper consists of residual soilfrom the Jurong Formation. The Jurong Formationresidual soil covers about one-third of the total land areaof Singapore. It consists of grey to black interbeddedmudstone and sandstone, or a reddish sandstone andmudstone conglomerate. Sediments in the form of shaleand conglomerates have been found in these areas as well(PWD, 1976). As a result of tectonic movements, the for-mation has been severely folded and faulted in the past.

Site DescriptionThe sedimentary rock of the Jurong Formation resid-

ual soil slope on Havelock Road has a slope height of13.24 m and a slope angle of 249(Fig. 1). There is aretaining wall, 1.1 m in height, at the toe of the slope.The slope surface is very grassy and there are some treeson the right- and left-hand sides of the observed slope.The investigated area was carefully chosen so that the in-strumentation would not be aŠected by the presence oftrees. There is no previous history of slope failure in thisarea, including during the period of monitoring. Never-theless, signiˆcantly high groundwater levels were detect-ed during heavy rainfall through the piezometers installedat the site. Therefore, rectiˆcation work using horizontaldrains was implemented to increase the factor of safety ofthe slope due to its proximity to a residential area.

Five boreholes were drilled at the Havelock Road site.Four boreholes were drilled in the monitored area and

used for the installation of inclinometers and Casagrandepiezometers after sampling and SPTs. The ˆfth boreholewas used to obtain undisturbed samples for laboratorytesting. The soil observed in the borehole investigationaround mid-slope consisted of ˆrm to hard yellowishbrown, reddish brown, and white sandy silt with somemica ‰akes and weathered rock fragments. The soil of theslope was classiˆed as sandy silt, based on the Uniˆed SoilClassiˆcation System (USCS), with a unit weight of 17.5kN/m3, an eŠective cohesion of 5 kN/m2, an eŠectivefriction angle of 269, and an angle indicating the rate ofincrease in shear strength relative to the matric suction,the qb angle, of 159. The shear strength parameters weredetermined from multistage consolidated drained triaxialtests at a constant net conˆning pressure and diŠerentmatric suctions (25, 50, and 100 kPa) for the sedimentaryJurong Formation residual soil (Fig. 2). The shearstrength envelope in Fig. 3 illustrates the decrease in q?from 269, before the air-entry value of the soil (i.e., 15kPa), to 159, beyond the air-entry value, as the matric

439

Fig. 3. Shear strength of the sedimentary Jurong Formation residualsoil with respect to matric suction

Fig. 4. Geotechnical parameters from the boreholes located at mid-slope

Fig. 5. Layout of the instrumented slope

439PERFORMANCE OF HORIZONTAL DRAINS

suction increased from 0 to 100 kPa. The geotechnicalparameters of the residual soil from the borehole locatedat mid-slope are given in Fig. 4. The test results indicatethat the water content, the speciˆc gravity, the liquidlimit, and the plastic limit of the soil were 38–40z, 2.65,61z, and 38.8z, respectively.

Site Instrumentation and Data CollectionManual monitoring of the measuring devices in the

slope was carried out from the end of May 2008 to themiddle of April 2009 in order to study the response of theresidual soil slope to rainfall. The layout of the in-strumentation at the Havelock Road site is presented inFig. 5. The measuring devices installed in the slope con-sist of jet-ˆll tensiometers, piezometers, and inclinome-ters to provide pore-water pressure, groundwater leveldata, and lateral deformation, respectively. A total ofnine tensiometers and three piezometers were installed inthree rows, as illustrated in Fig. 5. The lateral deforma-tions of the slope were found to be insigniˆcant, i.e., lessthan 10 mm, due to the stiŠness of the residual soil, andtherefore, will not be discussed in this paper.

The nine tensiometers were installed in groups of three,placed at depths of 0.47, 1.04, and 1.98 m below groundlevel and located at the upper, middle, and lower sections

of the slope, respectively. Tensiometers were installed inthe slope to provide direct measurements of the matricsuction or the negative pore-water pressure in the soil,particularly during and immediately following rainfallevents.

Three Casagrande piezometers were installed for thecontinuous monitoring of the groundwater table eleva-tion. The piezometers were installed within the area of thehorizontal drains. The installation depths of the piezome-ters at the upper, middle, and lower sections of the slopeswere 14, 10, and 10 m, respectively, with reference to theground surface level. The piezometer tubes were protect-ed by a lockable PVC pipe, 300 mm in diameter and 500mm in height, with a cap to prevent vandalism. TheCasagrande piezometers were intensively monitored us-ing an electric dip meter to measure the variation ingroundwater levels and to check the drain response. Ingeneral, measurements were conducted two to three timesper week during dry and wet periods. At the same time,pore-water pressure readings were also taken from thetensiometers to monitor their response to the heavy rain-fall events. However, there was no monitoring of the sur-face runoŠ from the slope.

Rainfall data was collected from a rainfall gauge in-stalled at the Jalan Kukoh site, around 2 km from theHavelock Road site. An online monitoring system was es-tablished at Jalan Kukoh to record data in real time. Therain gauge was connected to a data logger so that the rain-fall data in real time could be retrieved from a securedwebsite and used in the seepage analyses.

Horizontal Drainage DesignOne row of horizontal drains, 12–18 m in length and

with a 59inclination, was installed near the toe of theslope to lower the groundwater level. Thirty-three PVCpipes, 100 mm in diameter and containing perforations inthe upper half, were wrapped in a geo-ˆlter fabric and in-

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Table 1. Groundwater movement monitored in the slope

DateDepth of groundwater table from the slope surface (m)

At the crest of the slope In the middle of the slope At the toe of the slope

16 January 2007 4.63 1.23 1.36 Before the horizontal drain installation

31 May 2008 6.14 3.26 0.95 During the horizontal drain installation

9 October 2008 5.68 5.09 1.77 After the horizontal drain installationAt the end of a heavy rainfall

31 January 2009 11.26 8.25 2.34 After the horizontal drain installationDuring the dry period

Fig. 6. Average pore-water pressure readings at various depths nearthe crest of the Havelock Road slope (31 May 2008 to 25 April2009)

440 RAHARDJO ET AL.

stalled in the 100-m wide stretch of slope at a lateral spac-ing of 3 m. The geo-ˆlter wrapping was necessary to pre-vent clogging and to ensure the long-term performance ofthe horizontal drains. Rahardjo et al. (2003) have sug-gested that horizontal drainage systems be placed as lowas possible in a slope in order to lower the groundwatertable. In this case, there was a low retaining wall at thetoe of the slope and the horizontal drains were installed30 cm from the top of the retaining wall.

Monitoring ResultsA set of data from an eleven-month period was com-

piled from the instrumentation at this site. Data ongroundwater level movements were available back to Au-gust 2006. The horizontal drains were installed from 12May to 7 June 2008, starting from the right-hand side ofthe slope and moving to the left-hand side. The horizon-tal drains below the instrumented area were installed on30–31 May 2008. Following the installation of thehorizontal drains, tensiometer readings were immediatelytaken on 31 May 2008 and continuously monitoredtogether with the piezometric levels throughout theeleven-month period following the installation of thedrains.

Piezometric monitoring, before and after the installa-tion of the drains, indicated that there was a drawdownof the groundwater table due to the installation of thedrains (Table 1). It was observed that the horizontaldrains could lower the groundwater table in the range of 1to 4.6, 0.2 to 6.3, and 0.1 to 1.9 m on the crest, the mid-dle, and the toe of the slope, respectively, from the initial-ly high groundwater table between May 2008 (prior to theconstruction of the drains) and April 2009. Flow ratemeasurements of the horizontal drains were immediatelyperformed after their installation to ensure that thehorizontal drains worked as designed. It was observedthat some of the drains were discharging water and thatthe ‰ow increased due to rainfall. The ‰ow rate of thewater monitored two weeks after the drain installationwas about 1.1–1.7×10－7 m/s. A regular schedule ofmaintenance by hydroblasting was recommended to pre-vent clogging along the horizontal drains, as clogging canaŠect the drainage performance. Hydroblasting involvesthe removal of sediment by a low-pressure, high-volumewater jet, since high-pressure water jets will damage pipesand add water into the ground.

Figures 6 to 8 present the dynamics of the changes inpore-water pressure in response to the heavy rainfall thatfell over the eleven months monitored by the tensiometersat diŠerent depths. Near the crest and the middle of theslope, highly negative pore-water pressure reached up to－80 kPa due to evaporation near the ground surfaceduring the dry period, as seen in Fig. 6. During rainfall,the pore-water pressure rose instantly at shallow depths.Measurements from the days when an immediate increasein pore-water pressure occurred indicate that the tensiom-eters responded well to the heavy rainfall events. General-ly, at a depth of 0.47 m below ground level, the measuredpore-water pressure in the slope approached zero due torainfall. Figure 6 shows that at depths of 0.54 and 1.21 mbelow the ground level, positive pore-water pressure upto 10 kPa developed in the crest of the slope after rain-fall. Figure 7 shows that at depths of 0.58 and 1.23 m be-low the ground level, positive pore-water pressure up to20 kPa developed in the middle of the slope after rainfall.Meanwhile, at depths of 0.47 and 1.04 m below theground surface, near the toe of the slope ( see Fig. 8),there was an indication that the tensiometer positionswere so close to the horizontal drain elevations that thepore-water pressure tended to be positive for the ˆrstseven months. The negative pore-water pressure of lessthan －20 kPa at a depth of 1.98 m, near the toe of the

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Fig. 7. Average pore-water pressure readings at various depths in themiddle of the Havelock Road slope (31 May 2008 to 25 April 2009)

Fig. 8. Average pore-water pressure readings at various depths nearthe toe of the Havelock Road slope (31 May 2008 to 25 April 2009)

Fig. 9. Pore-water pressure proˆles near the crest of the HavelockRoad slope during rainfall (21 August to 11 September 2008)

Fig. 10. Pore-water pressure proˆles in the middle of the HavelockRoad slope during rainfall (21 August to 11 September 2008)

Fig. 11. Pore-water pressure proˆles near the toe of the HavelockRoad slope during rainfall (21 August to 11 September 2008)

441PERFORMANCE OF HORIZONTAL DRAINS

slope, was due to the fact that the tensiometer positionwas actually higher than the groundwater table.However, the negative pore-water pressure near theground surface could be as high as －40 kPa due toevaporation during the dry period from January toFebruary 2009. As soon as the rain fell, the pore-waterpressure at depths of 0.47 and 1.04 m below the groundlevel became positive as the drains discharged water dur-ing the rainfall, but the pore-water pressure at the depthof 1.98 m remained negative.

Figures 9 to 11 show the pore-water pressure proˆlesmeasured by the tensiometers near the crest, the middle,and the toe of the slope on 21 August 2008. It can be seenin Figs. 9 and 10 that the pore-water pressure near thecrest and the middle of the slope can be as low as －60kPa and －50 kPa near the ground surface, respectively,during the dry period, as measured on 21 August 2008,and tend to be more negative at deeper depths. The maxi-mum changes in pore-water pressure occurred near theground surface, and the magnitude of these changes

decreased with depth. The pore-water pressure near theground surface was the ˆrst to be aŠected by the heavyrainfall, followed by the pressure at greater depths. Ex-tended rainfalls caused a signiˆcant change in the nega-tive pore-water pressure towards the positive pore-water

442

Fig. 12. Geometric and boundary conditions of the Havelock Roadslope

Fig. 13. Rainfall data and normalized ‰ow rate of the horizontaldrains (starting on 21 August 2008)

442 RAHARDJO ET AL.

pressure as observed by the tensiometers at each depth.Figure 11 shows the apparent saturation that occurred

at depths of 0.47 and 1.04 m due to rainwater inˆltrationnear the toe of the slope, indicating the presence of water‰ow in the horizontal drainage system, which was slightlybelow the tensiometer set at a depth of 0.47 m. The ten-siometer equipped at a depth of 1.98 m remained unsatu-rated, since its position was above the groundwater level.

NUMERICAL STUDIES

The SEEP/W ˆnite element code (Geoslope Interna-tional Pte Ltd, 2004a) was used to simulate the unsaturat-ed groundwater ‰ow and to determine the variation inpore-water pressure with time. Meanwhile, the slope sta-bility analysis was conducted using SLOPE/W (GeoslopeInternational Pte Ltd, 2004b).

Slope ModellingA homogeneous slope, rising at 249to a height of 13.24

m, was used as the model for the numerical analyses. Theanalyses were performed as two-dimensional plane strainproblems with 4-noded quadrilateral elements. The slopeconˆguration, horizontal drain location, ˆnite elementmesh conˆguration, and boundary conditions are shownin Fig. 12.

The boundary of the slope model was set at three timesthe height of the slope. The data from each step weresaved over the transient seepage analyses. In order to a-void excessive accumulation of rainwater on the slopesurface, the non-ponding condition was selected. On theground surface, surface runoŠ would occur because in-crements in pore-water pressure were prevented and themaximum computed pore-water pressure was limited tozero.

Nodal ‰ux Q, equal to zero, was applied along the sidesof the slope above the groundwater table and along thebottom of the slope geometry to simulate the no-‰owzone. The boundary along the sides of the slope geometrybelow the groundwater table was assigned with the corre-sponding total head, hw, of each side. The desired rainfallintensity and its duration were applied to the surface ofthe slope as ‰ux boundary q. Thin-layer ˆnite elements,with a high permeability of 2×10－4 m/s, were employedto model the horizontal drains. The boundary conditionsalong the horizontal drains were set to Q＝0 and reviewedby the maximum pressure. During the iteration process inSEEP/W, the pore-water pressure along the drain wouldbe adjusted back to zero if any of the nodes had positivepressure.

A seepage analysis was performed on the slope for aheavy rainfall event on 21 August 2008 and the resultswere then compared with the results from the manualmonitoring of pore-water pressure in the slope. The totalamount of rainfall on 21 August 2008 was 296.8 mm witha maximum rainfall intensity of 92.4 mm/hr. The rainfallpattern of 21 August 2008, as shown in Fig. 13, wasselected for the numerical analysis because this rainfallresulted in a high increase in pore-water pressure during

the wet season. The resulting pore-water pressure distri-bution was calculated using SEEP/W and then employedto compute the factor of safety of the slope at varioustime steps in SLOPE/W using Bishop's simpliˆedmethod of slices.

Soil Properties of Investigated SlopeFigures 14 and 15 show the soil-water characteristic

curves (SWCC) and the permeability function for thesedimentary Jurong Formation residual soil at the Have-lock Road site. The drying SWCC data for the soil onHavelock Road was best-ˆtted by the Fredlund and Xingequation (1994) from

uw＝C(c)us

{ln «e＋Øua－uw

a »n

$}m (1)

where uw is the volumetric water content, us is the saturat-ed volumetric water content, C(c) is the correction fac-tor, (ua-uw) is the matric suction (kPa), and e is the natur-al number (2.71828 . . .). Leong and Rahardjo (1997)suggested that C(c)＝1. Fitting parameters a＝44.2508, n＝2.37, and m＝2.1 were related to the air-entry value(AEV) of the soil (kPa), the slope of the SWCC, and theresidual water content, respectively.

The wetting SWCC was used in the numerical analysessince the curve represents increasing water content due to

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Fig. 14. Soil-water characteristic curves

Fig. 15 Permeability function

Fig. 16. Pore-water pressure contours (kPa) obtained from data meas-ured in the ˆeld before rainfall

Fig. 17. Comparison of pore-water pressure contours (kPa) obtainedfrom numerical analyses and pore-water pressure data measured inthe ˆeld before rainfall

443PERFORMANCE OF HORIZONTAL DRAINS

rainfall or adsorption process. The wetting SWCC of thesoil was estimated using the simpliˆed model by Feng andFredlund (1999). The model assumes that the boundarywetting curve and the boundary drying curve are parallelwhen the soil suction is plotted on a logarithmic scale. Asa result, only one point on the boundary wetting curve isrequired to calibrate the model. Parameter d controls theslope of the curve in the Feng and Fredlund (1999) equa-tion and can be set to the same value for both boundarydrying and wetting curves as

w(c)＝wubw＋ccd

bw＋cd (2)

Curve-ˆtting parameter bw, for the boundary wettingcurve, can be calculated as follows:

bw＝(w1－c)cd

1

wu－u1(3)

where wu is the water content on the boundary dryingcurve at zero soil suction, b, c, and d are curve-ˆttingparameters, c1 and w1 are the soil suction and the gravi-metric water content of the additional point on theboundary wetting curve, respectively, and c and d are thecurve-ˆtting parameters obtained by ˆtting the boundary

drying curve. The curve-ˆtting SWCC parameters of thesoil used in the seepage analyses were bw＝6.57, c＝2.37,and d＝2.1. The measured saturated permeability, ksat, ofthe sandy silt of the Havelock Road slope was 2.1×10－7

m/s. The saturated permeability was determined by therising head ˆeld permeability test method (Hvorslev,1951). The wetting SWCC and the saturated coe‹cient ofthe permeability tests were incorporated into a statisticalmodel to indirectly predict the permeability function.

Seepage Analysis ResultsThe pore-water pressure and the total head contours

were generated using Surfer (Golden Software, Inc.,1997), based on the data measured in the ˆeld by the ten-siometers before and at the end of a heavy rainfall. Thepore-water pressure and the total head contours obtainedfrom the ˆeld measurements before and at the end of theheavy rainfall are presented in Figs. 16, 18, 20, and 22. Acomparison of the pore-water pressure contours obtainedfrom the numerical analyses and the pore-water pressureobtained from the ˆeld before and at the end of the heavyrainfall is shown in Figs. 17 and 21. A comparison of thetotal head contours obtained from the numerical analysesand the total head contours based on the measured data

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Fig. 18. Total head contours (m) obtained from data measured in theˆeld before rainfall

Fig. 19. Comparison of total head contours obtained from numericalanalyses and total head measured in the ˆeld before rainfall

Fig. 20. Pore-water pressure contours (kPa) obtained from data meas-ured in the ˆeld at the end of rainfall

Fig. 21. Comparison of pore-water pressure contours (kPa) obtainedfrom numerical analyses and pore-water pressure data measured inthe ˆeld at the end of rainfall

Fig. 22. Total head contours (m) obtained from data measured in theˆeld at the end of rainfall

Fig. 23. Comparison of total head contours (m) obtained from nu-merical analyses and total head measured in the ˆeld at the end ofrainfall

444 RAHARDJO ET AL.

obtained from the ˆeld before and at the end of the heavyrainfall are shown in Figs. 19 and 23.

Generally, it can be seen that the pore-water pressureand the total head contours obtained from the ˆeld mea-surements and the numerical analysis results before andat the end of the heavy rainfall are in reasonably goodagreement. Figures 16 and 17 show that there was thepresence of signiˆcant matric suction near the groundsurface during the dry period, which could aŠect the sta-

bility of the slope. Moisture migration occurred withinthe slope where water moved upward in the slope, asevaporation, and also across the slope, as presented inFigs. 18 and 19. Meanwhile, decreasing matric suctionnear the ground surface due to rainfall is shown in Figs.20 and 21. The total head distributions in Figs. 22 and 23illustrate that water was ‰owing along and percolating

445

Fig. 24. Comparison of pore-water pressure proˆles obtained fromnumerical analyses and pore-water pressure data measured in theˆeld near the crest the of slope from the beginning (t＝0) until theend (t＝21 days) of rainfall

Fig. 25. Comparison of pore-water pressure proˆles obtained fromnumerical analyses and pore-water pressure data measured in theˆeld in middle of the slope from the beginning (t＝0) until the end (t＝21 days) of rainfall

Fig. 26. Comparison of pore-water pressure proˆles obtained fromnumerical analyses and pore-water pressure data measured in theˆeld near the toe of the slope from the beginning (t＝0) until theend (t＝21 days) of rainfall

445PERFORMANCE OF HORIZONTAL DRAINS

downward into the slope during the wet period.Comparisons of the pore-water pressure proˆles ob-

tained from the numerical analyses and the ˆeld measure-ments near the crest, the middle, and the toe of the slopeare presented in Figs. 24 to 26, respectively. It can be seenfrom Figs. 24 and 25 that the numerical analyses wereable to simulate the process in the ˆeld with a reasonablygood agreement. The pore-water pressure proˆles ob-tained from the numerical analyses near the crest and inthe middle of the slope were very close to the data meas-ured in the ˆeld.

The numerical analysis results did not quite agree withthe data measured near the toe of the slope (Fig. 26). Inthe numerical analyses, the initial pore-water pressureproˆle near the toe of the slope did not match the ˆelddata that had a negative pore-water pressure of about－15 kPa at a depth of 1.98 m from the ground surface.In the ˆeld, the groundwater table was initially lowerthan the horizontal drain position at a distance of 0.4 L(L＝horizontal drain length) from the toe of the slope,while the groundwater table beyond that point was higher

than the position of the horizontal drain. In the numeri-cal analyses, the initial proˆle of the pore-water pressurewas obtained by applying a small uniform inˆltration rateto the slope model for a long duration prior to applyingthe actual rainfall. As a result of establishing the initialconditions for the numerical analyses, the groundwatertable dropped to the drain level in the upper part of theslope, but rose near the toe of the slope. However, it ap-pears that in the numerical analyses the pore-water pres-sure near the toe of the slope built up faster than the ac-tual pore-water pressure in the ˆeld which remained nega-tive, causing the discrepancies.

Figure 13 shows the ‰ow rate of the horizontal drainsfrom the numerical analysis results normalized by thespacing of the horizontal drains. It can be seen that the‰ow rate of the horizontal drains reached the maximumrate at 4.2×10－7 m/s and was in the range of 1.6–4.2×10－7 m/s during the rainfall event.

Slope Stability Analysis Using SLOPE/WIn the analyses, the sandy silt layers were modeled with

and without horizontal drains having a unit weight of17.5 kN/m3 and measured shear strength parameters,namely, an eŠective cohesion of 5 kN/m2, an eŠectivefriction angle of 269, and a qb angle of 159. By importingthe pore-water pressure distribution resulting fromSEEP/W, the factors of safety were computed for theslope model with and without horizontal drains inSLOPE/W using Bishop's simpliˆed method of slices.Variations in the factor of safety, with respect to timeduring heavy rainfall, are shown in Fig. 27.

Before the heavy rainfall started, the factor of safetyfor the Jurong Formation residual soil slope withouthorizontal drains was computed to be 1.27. The factor ofsafety decreased gradually during the rainfall until itreached 1.25 after 21 days of heavy rainfall. The factor ofsafety subsequently decreased until a minimum value of1.19 was computed at an elapsed time of 125 days. It is in-teresting to note that the initial factor of safety, 1.27, wasconsidered close to the recommended minimum factor of

446

Fig. 27. Changes in factor of safety of the Havelock Road slope dur-ing and after rainfall (21 August 2008)

446 RAHARDJO ET AL.

safety for slopes with a ten-year return period rainfall,i.e., 1.2 (GEO, 2007). In addition, the minimum factor ofsafety did not occur at the end of the rainfall, but at amuch later time, indicating the possibility of a delayedslope failure when the factor of safety is equal to 1.

On the other hand, the same Jurong Formation resid-ual soil slope with horizontal drains had a higher initialfactor of safety of 1.35 and decreased gradually until itreached 1.32 after 21 days of rainfall. The factor of safetycontinued to decrease to a minimum value of 1.30 83 daysafter the rainfall stopped. Water percolated downwardslowly through the Jurong Formation residual soil layerdue to its low permeability. As a result, the occurrence ofthe minimum factor of safety was delayed to sometimelater after the rainfall had ceased. Some cases of delayedslope failure that took place sometime after the rainfallhad ended could be explained by the characteristics of thevariation in the factor of safety of the Jurong Formationresidual soil slope. At all times, the factor of safety forthe slope with horizontal drains was higher than the fac-tor of safety for the slope without horizontal drains, in-dicating that horizontal drains improved the stability ofthe slope, especially during heavy rainfall events.

CONCLUSIONS

A good agreement has been found between the numeri-cal analyses and the ˆeld data for the pore-water pressurein the sedimentary Jurong Formation residual soil slopewhich indicates that appropriate modeling parametershave been selected to show the eŠectiveness of horizontaldrains in slope stabilization. One row of horizontaldrains installed near the toe of the slope has been foundto be eŠective in lowering the groundwater table sig-niˆcantly from 0.2 to 6.3 m across the slope. The long-term eŠectiveness of a drainage system has been veriˆedby the extensive monitoring of the reduction in thegroundwater level and in the drain discharge over time,especially during heavy rainfall events.

ACKNOWLEDGEMENTS

This work has been gratefully supported by the Hous-ing and Development Board and Nanyang TechnologicalUniversity, Singapore.

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