Abstract: The geotechnical investigations indicate that the soil at the failure plane of a landslide consisted of gravelly silty clay. The shear strength of the gravelly silty clay soil has proved to be strongly influenced by the content of the coarsegrained gravels. Further, only direct shear tests were performed on soil samples from the failure plane and triaxial shear test were rarely conducted. In the conventional engineering practice, portions of the coarsegrained soil were removed to meet the maximum grained size requirements in the laboratory direct shear tests. Therefore, the tested shear strength from the laboratory direct shear tests are usually deviated from the actual site conditions. Triaxial shear tests were conducted on remolded samples based on the grain size distribution of undisturbed samples from the landslide shear plane. Same triaxial tests were also performed on the remolded sample with coarsegrained soil removed to investigate the influence of coarsegrained soil on the shear strength. Test results show that: with the gradually reduce coarse particle size, the tested cohesion of the gravelly silty clay samples first increases then decreases; The internal friction angle, the trend shows an decrease with reduction of the coarsegrained soils.

Key words: gravelly silty clay, coarse particles, grain sizes, cohesion, internal friction angle