ABSTRACT
The determination of strength parameters for stability analysis is the most important job in landslide slope stability evaluation. In actual practice in Japan, strength parameters have been almost always determined by an empirical method named as “reverse calculation method”. In this method, strength parameters are back calculated based on equilibrium conditions of sliding earth mass. In the first place, the apparent cohesion (c) is assumed as c0 ≒ d (kN/m2) (d:thickness of sliding mass(m)) and then the angle of shear resistance (ϕ) is obtained by substituting the value of c in the stability equation and assuming the current safety factor F0 = 1.0. Although criticisms have frequently been made to this conventional method, it is still widely used in practice. The authors proposed a method for determining strength parameters for stability calculation rationally, and also proposed a practical method in which the strength parameters for design purpose are given by combining the conventional reverse calculation method with the strength parameters obtained by laboratory shear test (Mitachi et al. 1996 and 1999). Landslide slope stability calculation by using this method requires a shear test apparatus by which the strength parameters corresponding to peak, fully softened and residual states can be evaluated. The present authors have newly developed a high-precision automatic cyclic direct shear test apparatus using a digital servomotor for vertical force loading. This paper presents the results of cyclic direct shear tests on several clay samples using the new apparatus and comparisons with the results obtained from other test methods by ring shear test apparatus and also by “one way cyclic” test using the new direct shear apparatus. Examples of calculating design strength parameters by the authors’ method (Mitachi et al., 1999) using the test results obtained by new testing apparatus on the specimens sampled from the slip surface of actual landslide sites are also presented.
