Effectiveness of horizontal drains in improving slope stability: a case ...

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2 FIELD STUDY2.1 Background of PutrajayaPutrajaya is a planned city, located 25 km south ofKuala Lumpur and 20 km north of the KualaLumpur International Airport (KLIA) that serves asthe federal administrative centre of Malaysia. Thecity sits on 4,931 hectares of land within theMultimedia Super Corridor. Its development startedin 1995 and is targeted to complete by 2015. Oncefully developed, it is estimated to have a populationof around 320,000 with 64,000 housing units.2.2 The Putrajaya landslideA landslide occurred at a slope adjacent to 3 blockscontaining 334 government quarters at Precinct 9,Putrajaya, at around 4:30am on the 22 nd March2007. The panaromic view of the landslide in Figure1, reproduced from the report that appeared inthe New Straits Times on 23 rd March 2001, providesgraphic illustration of the landslide, the location ofthe site and an indication of the distance from theslope to the adjacent apartment buildings.Figure 1. Panaromic view of the slope failure, (New StraitsTimes, 23 rd March 2007).There was no injury or fatality, but 27 cars were totallyor partially buried by the debris material. Allresidents of the 3 blocks, i.e. about 1,200 people,were immediately evacuated. It was therefore crucialto stabilize the affected slope prior to allowingthe residents to reoccupy the buildingFigure 2. Digital Terrain Model (DTM) of the landslide area.2.4 Geology of the siteFrom the field surface mapping, the study area isunderlain by graphitic quartz mica schist from KajangFormation (see Figure 3). The outcrops whichcould be noted along the cut slopes in the study areahave experienced intense weathering processes withweathering grade range from Grade III to VI. HoweverGrade II material could still be noted at certainareas. Foliation planes were clearly formed andcould be seen on the cut slopes. The orientation isquite consistent, i.e. striking approximately N-S anddipping towards W-EW with average dips of 35 o to40 o .Study Area2.3 Site description of landslide areaThe site is best described by the Digital TerrainModel in Figure 2. The landslide is located on thewestern face of a ridge that has its crest close to theupper edge of the slide scarp. A road leading to a 36million litre water tank lies on this crest. The watertank is partially buried on a part of the hill that hasbeen cut to prepare a flat site for it. The distance ofthe tank to the edge of the ridge where the slide occurredis about 25m.Kenny HillFormationKajangFormationFigure 3. Geology map of study area754
The outcrops (Grade II to IV) are dark grey toblack in color with clear foliation planes. For GradeV-VI outcrops, the color is reddish brown to brownishred due to existence of laterite which is rich inferrous oxides.The results from petrography of the core samplesat depths of 9 - 20 m, reveal that the rocks in thisarea consists of interbeded sandstone, shale and actinoliteschist.2.5 Soils investigation and instrumentationThe first soils investigation and instrumentationmonitoring package commenced in March and wascompleted in April 2007. 17 boreholes were sunkby rotary drilling with foam and water as flushingmedium (see BH1-BH17 in Fig. 4). When rock wasencountered, rotary drilling was carried out with diamond-studdedbit to obtain rock core.Seven standpipe piezometers were installed (seeSP1-SP7 in Fig. 4) to obtain the piezometric waterlevel readings, so that the performance of the dewateringscheme using horizontal drains could bemonitored. The standpipe piezometer, or sometimescalled a Casagrande piezometer, consists of a filtertip joined to a PVC riser pipe. After the filter tipand riser pipe were installed in the borehole, a sandfilter zone was tremied into place around the filtertip. The top of the filter zone is sealed with bentoniteto isolate the pore water pressure at the tip.The riser pipe was terminated above ground levelwith a vented cap. Readings were manually takenonce to three times a day using a water level indicator.Five vertical inclinometers were also installed(see IC1-IC5 in Fig. 4) to detect zones of movementand determine whether the slope is responding to theimmediate remedial measures. Inclinometer casingswere installed permanently in boreholes. A portabletraversing probe and a readout unit were used tomeasure and record the movement at 0.5 m depth interval.2.6 Horizontal drains installationBased on field data, preliminary slope stabilityanalysis were carried out which indicated that theprobable cause of failure was a rise in ground watertable within the slope. As a result, deep horizontaldrains up to 24 m, instead of the normal 6-12 mlength drains, were immediately installed at or nearthe toes of the intact slopes.A total of 40 horizontal drains ranging from 8 to24 m in lengths were installed from 28 th March to 1 stApril 2007 (Fig. 5). They were constructed by drilling100 mm diameter holes at an upward incline ofabout 5 degrees from the base of the slope and theninserting a 75 mm nominal diameter class D perforatedPVC pipe wrapped in geotextile into the drilledholes. The total drilling length was 842 m while thelength of pipe installed was 784 m.Thirty-seven drains were installed at the toe belowthe lowest berm, while another three were installedjust above the first berm. Figure 5 shows thelayout of the horizontal drains installed. It was, surprisingly,observed that the yield of the drains,measured in ml/s, greatly varies from one to another.Figure 4. Locations of boreholes and instrumentationFigure 5. Horizontal drains locations, and resistivity and seismicsurvey lines755
Page 5: Soon after installation in 2007, al

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