The Structure and Formative Process of Shear Surfaces of Kaolin Clays during Cyclic Direct Shear Test

The determination of strength parameters for stability analysis is the most important job in evaluation of landslide slope stability. Mitachi et al (1999) 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. Calculation of landslide slope stability 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.Residual strength parameters can be obtained from a series of cyclic direct shear test. However, the shear mechanism of the soil specimen which is subjected reciprocal shear forces in cyclic direct shear test is different from that occurring along the shear surfaces. This paper describes the effect of reciprocal shear forces on the clay particle orientation along the shear surface based on the observation by using scanning electron microscope. For this purpose, three series of cyclic direct shear test were conducted by using reconstituted kaolin clay. The tests are conducted under consolidated constant pressure condition using normal type and precut specimen, and consolidated constant volume condition. Based on the test results, completely oriented clay particle structures were observed after two cycles of cyclic shear under the condition of consolidated con-stant pressure test using normal type and pre-cut specimen.