Determination of the Shear Strength Parameters of Two Unsaturated Colluvium Soils using the direct Shear Test

The shear strength of two colluvium soils, situated at the Serra Geral Formation scarp in Southern Brazil, was determined with conventional and suction-controlled direct shear tests. One investigated colluvium soil was originated from the weathering of a volcanic rock (Sample RO) and the other, possibly from the degradation of intertrapp sandstone layers (Sample AV). A study of matric suction influence on the shear strength of undisturbed samples of both soils is presented in this paper. Suction-controlled direct shear tests showed that shear strength envelopes are linear for matric suctions between 0 and 150 kPa. Further direct shear tests, carried out with air-dried samples, showed that those shear strength envelopes tend to become bilinear for higher suctions. In addition, the experimental data indicated that for both soils the internal friction angle seems to increase with matric suction and values are higher than corresponding values.