Title Soil-particle and pore orientations during drained and undrained shear of a cohesive sandy silt-clay soil

Author1. Hasan Cetin2. Mehmet Soylemez

Introduction Shearing changes the structure (microstructural units) by first breaking down the particles and (or) aggregate assemblages and then rearranging them. The final structure of a soil deposit depends on both the initial structure of that soil and on the changes that have occurred to the initial structure (Lambe 1958). Therefore, the final structure of a soil deposit reflects all facets of the soil composition, stress history, present state, and environment (Mitchell 1993). Particles in movement tend to have a significant preferred orientation in space (Knopf and Ingerson 1938). In general, during shear, depending on the magnitude of the displacement, the soil composition, and its consistency (Morgenstern and Tchalenko 1967), pores, particles and other constituents within the shear zone may align with their long axes parallel to the shear stress acting in the shear plane (Sloane and Nowatzki 1967; Pusch 1970; Crampton 1974; Douglas et al. 1983; Mitchell 1993), whereas outside of this zone they tend to align with their long axes normal to the major principal stress direction (Morgenstren and Tchalenko 1967; Tchalenko 1968; Morgenstern 1969; Garga 1973).

Thin-section analysis Average angles with respect to the horizontal and percentages of the orientation measurements between 0 and 5 versus shear displacements. It is clear that the orientation pattern before shearing and application of the 98 kPa normal consolidation pressure is nearly random, with an average angle to the horizontal of 42 for all three zones. Only 3% of the measurements showed a preferred orientation between 0 and 5 which is probably a result of the 20 kPa pressure applied for about 1 year in the metal box to simulate an overburden pressure during the making of the artificial block soil sample in the laboratory. The voids are large, generally interconnected, and randomly oriented. The orientation pattern before shearing but after application of the 98 kPa normal consolidation pressure is similar to the orientation pattern before shearing and application of the normal consolidation pressure described previously.

Consolidated undrained shear tests The percentages of the orientation measurements between 0 and 5 increase to 11%, 9% and 8% within the center, intermediate and outer zones, respectively. Also, measurements showing a preferred orientation between 0 and 5 increase to 32%, 27% and 20# within the center, intermediate and outer zones, respectively. This indicates that the preferred orientation increases towards the failure plane, meaning that the particles, pores and other constituents tend to become parallel to the shear plane as shearing continues. The percentage of measurements showing a preferred orientation between 0 and 5 in the center zone stays about the same, whereas it continues to increase in the other two zones. This indicates that further shearing after failure does not cause considerable additional preferred orientation near the failure plane.

Consolidated drained shear tests The preferred orientation increases towards the failure plane, indicating that particles tend to become parallel to the failure plane as the shearing continues