Structural (fabric) changes along shear planes in clayey soils

The purpose of this study is to investigate structural (fabric) changes occurring along shear planes under the maximum principal stresses. In the field, shear planes can be encountered as fault and landslide failure planes as well as failure planes in foundation soils caused by excessive loading due to an engineering structure. In this study, the relationship between the soil structure changes and the maximum principal stress (direction and magnitude) was studied by means of direct shear tests with different shear-rates and strains on artificially prepared soil samples under a constant normal pressure. An artificial soil block sample was prepared by using a natural clay soil mixed with muscovite sands at 1/3 proportion, and preconsolidated under 0.02 kgf/cm2. 10 consolidated-undrained (CU) and 10 consolidated-drained (CD) direct shear tests were performed on subsamples obtained from the block sample under different shear-rates and strains, and a constant normal pressure in order to form soil structures of different shear strains. Thin-sections prepared from the samples sheared at different shear-rates and strains were investigated first under polarized microscope then on computer using appropriate programs. Orientations of particles (mainly muscovite), pores and other constituents under the different strains were measured with respect to the horizontal, and representative photographs were taken for the conditions examined. The measurements of soil particle (mainly mica) and pore orientations were presented using rose diagrams and the effects of the maximum principal stress on the soil structures formed at different shear strains were determined. Key words: Soil fabric, Shear strain, Thin-section, Orientation II