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Results of torsional ring shear tests on cohesive soils reveal that the 
drained residual strength is related to the type of clay mineral and quantity of clay-size particles. The liquid limit is used as an indicator of clay mineralogy, and the clay-size fraction indicates quantity of particles smaller than 0.002 mm. Therefore, increasing the liquid limit and clay-size fraction decreases the drained residual strength. The ring shear tests also reveal that the drained residual failure envelope is nonlinear, and the nonlinearity is significant for cohesive soils with a clay-size fraction greater than 50% and a liquid limit between 60% and 220%. This nonlin- earity should be incorporated into stability analyses. An empirical correlation for residual friction angle is described that is a function of liquid limit, clay-size fraction, and effective normal stress. Previous residual strength correlations are based on only one soil index property and provide a residual friction angle that is independent of effective normal stress. For slope stability analyses, it is recommended that the residual strength be modeled using the entire nonlinear residual strength envelope or a residual friction angle that corresponds to the average effective normal stress on the slip surface. 
Journal of Geotechnical and Geoenvironmental Engineering, Vol 120, No 5, 1994
University of Ilinois, USA
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