Drilled shafts for slope stabilization in expansive soils

摘要

Residual soils derived from the clay shales in Texas and other areas of the United States have been used in the construction of levees and earth dams built 50 to 70 years ago. With the state of the art of technology available at that time, these slopes were designed using shear strength values close to their residual shear strength, derived often from undrained shear strength tests to simulate rapid drawdown conditions, which was assumed to be the worst case scenario. Long-term stability was assumed to be related o the residual shear strength being mobilized. Though this had provided reasonable performance over the years, minor surface failures have occurred frequently and have become a part of the routine maintenance. Such shallow slides have been traditionally repaired with lime stabilization. Larger slides however involve substantial reconstruction of the slope and lime stabilization creates a dissimilar joint between the original and the new construction, resulting in more failures around the stabilized area. In an effort to provide stability against sliding using the original material of the slope, with or without additional stabilization, drilled shafts were recently used in expansive clays. Some preliminary studies have shown that the drilled shafts add to the stability of the repaired slopes. Laboratory tests indicate that first-time slope failures in slopes can occur at stress levels higher than the residual shear strength. Drilled shafts placed either intentionally or by other design constraints on the slope appear to add to the long-term stability and prevent first-time failures occurring long after the slopes are constructed. Further research is needed to establish that installation of drilled shafts can retard or stop further shear once the failure has been mobilized. This paper discusses observations on levees in Texas where drilled shafts have been used or proposed for the purpose of slope stabilization.