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

Soon after installation in 2007, all 40 horizontaldrains, except for 2, were able to discharge water.The total discharge rate measured from all drains, atthat time, was 652.19 ml/s. However, 71% of theyield comes from 5 drains (listed in the Table 1)close to the northern side of the failure area. Flowrates measured from the other 35 drains range from0-18.02 ml/s, with 31 of the drains yielding less than10 ml/s.These observations concurs with Fernandez-Rubio and Singh (1983) which discovered yield ofhorizontal drains from dry to very high flow rates,with the maximum discharge located in the areas offault zones or slope failures.The flow rates measured after 3 years were muchless than immediately after installation. Only theflow rates from 4 horizontal drains were able to bemeasured. The total yield was only 114.56 ml/s,17.5% of the yield immediately after installation.Although the flow rates from horizontal drainshas much reduced after 3 years, the peizometric waterlevel measured on 8 th March 2011 at BH5/SP2, 4years after the drains were installed, was 5.25 m,about the same level it used to be, immediately afterthe installation of the drains, in April 2007.Five inclinometers were installed to monitor lateralmovement of the slope. Except for BH2/IC2which is located at mid height next to the landslide,there is very little movement of the rest of the slopesince the inclinometers were installed soon after collapse.Figure 6 plots the maximum movement detectedat 2.5m below the ground surface at this inclinometer.It can be seen that there’s about 8mmmovement during the horizontal drains installationworks, after which the inclinometer cease to showany.The above observations suggested that horizontaldrains is an effective method to lower down groundwater table and improving slope stability in the shortand medium term. Further studies are, however, requiredto determine its effectiveness in the longterm.3.2 Results from geophysical surveysThe topography model for the seismic line S1 issimplified in Figure 7. Three seismic boundarieswere detected. The first topmost layer is characterizedby the lowest p-wave velocity with values rangingfrom 260 m/s to a maximum value of about 500m/s. These values represent the range of seismic velocitiesof the inhomogeneous near surface soil materials.The second layer is particularly exhibiting lowervelocities of 650 to 1300 m/s. On the basis of theirreflection configurations, this layer could beassociated with highly weathered rock materials orstiff/hard sediment.The third layer is interpreted as the less weatheredrock mass. The computed seismic velocities rangefrom 1926 to 4350 m/s. Theoretically, the low valueof seismic velocity is associated with highlyweathered bedrock, whereas the higher valuerepresents relatively less weathered bedrock. Hence,the third seismic unit could be interpreted as highlyand less weathered fresh Schist.Figure 7. Seismic survey result for line S1Figure 8 shows the 200 m resistivity measurementswith 5 m electrode spacing of survey line R1 (Fig.5). The top layer of the pseudo section is interpretedas a dry layer of silt and clay with low resistivity inthe range of 10 to 1,000 Ωm. The relatively high resistivityin the range of 5,000-30,000 Ωm observedin second layer comprises of slightly weathered tofresh schist. Rock Quality Designation (RQD) of70%-90% at a depth of 11.5m from BH1 substantiatesthis.Figure 8. Resistivity survey result for line R1757