Performance of Repaired Slope Using a GEONET or GEOPIPE Drain to Lower Ground-Water Table

Abstract

A 70 m long by 5 m high slope with gradient of 1(V):2(H) was cut into a medium-stiff residual soil of undrained shear strength better than 60 kPa, with drained strength parameters of about c' = 10 kPa, and ϕ' = 22°, to form the bank for an effluent pond used for irrigation of a racetrack turfing. Both drained and undrained slope stability analysis indicates stable slopes under reasonable ground-water (GW) levels expected in the cut slope. However, after a period of intense rainfall during construction, the slope suffered a shallow slip of about 1 m to 1.5 m depth over a 30m stretch of the slope length with a vertical scarp near the top of the cut slope. This paper examines the causes of slope failure, and the strategy adopted for a permanent repair of the slope by providing internal geosynthetic drains beneath the re-compacted slope, using either a GEONET or closely spaced geo-pipe inclusions in the slope. For design, the GEONET or geo-pipe drains used must have adequate factored transmissivity to conduct expected heavy rainfall infiltration water safely out of the slope mass. Under a steady-state very heavy rainfall condition of 150 mm/h on the racetrack, it is demonstrated by the Finite Element Method (FEM) analysis, that GEONET must be provided to at least as far back as the mid-depth of the slope (about 4 m depth) to produce sufficient GW lowering to give stable slopes. The construction method of the slope repair to avoid further failure is described briefly, and the performance of the sub-soil drains in enhancing slope stability is demonstrated in the field project.