Residual Shear Strength Determination of Overconsolidated Nespelem Clay Available to Purchase

Peaking operations at Grand Coulee Powerplant, Washington, could result in future downstream river fluctuations of up to 12 m (38 ft) within a 24-h period and trigger dangerous landslides. Much of the riverbank consists of old landslide deposits in varved, overconsolidated clays and silts. The residual shear strength of the clay was determined from drained, repeated (reversing) direct shear tests on slickensided or precut sliding surfaces under three normal loads. Two displacement rates were initially used. The residual strength, measured after large cumulative displacements, was used in slope stability analyses. This paper describes problems encountered during testing, the test procedure adopted, the fully automated direct shear apparatus and data recording system, results to date, and planned future research. In one of the downstream riverbank areas, a residual shear strength range of 10 to 13 deg was confirmed by a back analysis of a recent slide.