Cost effective stabilization of clay slopes and failures using plate piles

Progressive, seasonal soil slips occurring on expansive clay slopes at a large, developed, landscaped commercial site, and in a slide-prone residential subdivision in Northern California, prompted a progressive shift from conventional slope reconstruction methods to a more innovative approach to slope stabilization. These sites have been plagued with predominantly translational soil slips on the order of 0.3m to 1m deep, that generally initiate after prolonged rainfall events, especially those in excess of 50mm/day. Based loosely on limited experimentation by researchers on similar shallow clay slips, Kleinfelder personnel expanded on their idea of installing 88.9mm-square plastic pins to designing 6.35mm (1/4 inch) thick galvanized steel, 63.5mm by 63.5mm (2.5-inch) angle iron with a 0.3m (12-inch) by 0.6m (24-inch) flange plate pile, approximately 2.1m long. Kleinfelder's (patent pending) technique utilizes plate piles laid out in a 1.2m-centred offset grid pattern. This stabilization method is being used to provide stabilization on un-failed and some existing failed slopes. Preliminary costs are on the order of $5 (U.S.) per square foot (9.290x10 -2 m) of slope face, which is six to ten times less than the cost of conventional slope repairs. Two case studies will be presented illustrating the design, use and placement techniques of the plate pile system. The commercial site case study is characterized by over 2.4 hectares of 2H:1V cut and fill slopes comprised of highly plastic clay/claystone, up to approximately 27m high. Conventional grading techniques and compaction efforts were utilized during construction in 1996-1997 to construct the slopes. Since that time, vegetation has had little success in providing deep-rooted retention of soils and prevention of shrinkage cracks and shallow soil slippage. Since completion of grading, conventional mitigation of slips has included removal of clay soils and reconstruction with select import fill (low-expansion granular material) and geogrid reinforcement or removal and replacement with rip-rap. The residential subdivision case study predominantly includes natural slopes composed of expansive soil over expansive claystone bedrock. This study also includes experimental "test" slopes created to test the lateral load of the plate piles and various plate pile patterns.