Determination of Shear Strength for Design of Cut Slopes in Partly Weathered Rockand Saprolite

摘要

Investigation of three active landslides located in western North Carolina;Balsam Gap II landslide located along US 23/74 in Haywood County, Boone landslide located along US 321 in Watagua County, and Waterville landslide located along I-40 in Haywood County. At the Balsam Gap II and Boone landslides, borings were made to obtain soil samples, install slope indicators and piezometers, and to perform pressuremeter tests. Surveys were made to determine the surface topography, and test pits were excavated to inspect the shear surface which was located in the sparolite just below the interface with the overlying colluvium. The Waterville landslide had already been investigated by the NCDOT and LAW Engineering so that only a boring was performed to obtain undisturbed samples for laboratory testing. At NCSU, the undisturbed samples were tested in direct shear, ring shear, and consolidation tests. At the NCDOT laboratory, the undisturbed samples were tested in consolidated undrained triaxial tests. Back analyses show that at failure the pore pressure ratios, Ru, vary from 0.2 to 0.49 while piezometers measured zero pore pressure. Rainfall causes pore pressures to rise quickly to the failure values. Laboratory measured permeabilities and observations made in the test pits indicate rainfall infiltration caused a perched water table in the colluvium at the Boone landslide while it caused an artesian condition in the saprolite at the Balsam Gap II landslide.