Torsional ring shear tests on residual strength of expansive soils with different swelling potentials

The correlation between residual strength and compositional characteristics of expansive soils with different swelling potentials is investigated. The torsional ring shear tests of Denver expansive soil with high swelling potential and Jingmen expansive soils with medium and low swelling potentials are carried out by using the modified torsional ring shear system, and the results are compared with those of the reversal direct shear tests proposed by the Chinese standard for geotechnical testing method (GB/T 50123−2019). The empirical correlations for the drained residual shear strength suggested by Mitchell and Soga (2005), Stark et al. (2013) and Lehmann et al. (2021) are assessed by the measured results. The conclusions are drawn as follows: (1) The residual friction angles of Denver expansive soil with high swelling potential and Jingmen expansive soils with medium and low swelling potentials measured by the torsional ring shear tests are 11.3°, 12.3° and 23.7°, 1.7°~3.3° lower than those measured by the reversal direct shear tests and have good accuracy, and are not stress-dependent, which means more suitable for application. (2) Mitchell and Soga (2005) and Stark et al. (2013) overestimate the residual friction angles of expansive soils, while Lehmann et al. (2021) underestimate them. (3) Although the qualitative relationship between the residual strength and the compositional characteristics of expansive soils is perfectly clear, the above empirical correlations are capable to predict the residual friction angles of expansive soils with normal compositions (Jingmen expansive soil with low swelling potentials) correctly. However, it is difficult to predict the residual friction angles of expansive soils with complicated compositions (Denver expansive soil with high swelling potential) quantitatively and accurately. In this case, the torsional ring shear tests are necessary.