Drained residual shear strength at effective normal stresses relevant to soil slope stability analyses

• New residual strength correlations were developed as a function of soil index parameters and wide range of normal stresses.
• The suitability of the new correlations in predicting the residual strength was verified.
• An expression was introduced to express the residual strength correlations for incorporation in slope stability software.

An extensive torsional ring shear testing program has been conducted to measure the drained residual shear strength of soils at the wide range of effective normal stress (10 to 700 kPa) usually mobilized in reactivated and first-time landslides. Soils, mudstones and shales of different plasticity and gradation were tested in the program. The effects of the change in nonlinearity of shear strength envelope over the utilized normal stress ranges on slope stability analyses were investigated. Using this data, new empirical residual shear strength correlations were developed as a function of soil index parameters and wide range of effective normal stresses. In essence, the correlations are presented as revised versions of those previously developed for a limited number of normal stresses utilizing the same soil index parameters. Comparisons were made with a considerable amount of back-calculated shear strength data reported in the literature for reactivated landslides as well as results predicted from existing shear strength correlations to verify the increased suitability of the new correlations for use in slope stability analyses. A numerical expression was also introduced to express the residual shear strength correlations for direct incorporation in slope stability software.