Effects of Wetting on the Shear Strength of Plastic Silty Sands

This paper presents the results of intensive laboratory testing program investigating the effects of wetting on the drained shear strength and deformation characteristics of silty sands. The materials used were clean medium to fine grained Nile River bed sand and plastic Nile silt. The sand was mixed with 4%, 8%, 12%, 16%, 25% and 40% by dry weight of the Nile silt. The test specimens were prepared at two relative densities 30% and 79.4% and drained direct shear tests were performed on air dry and wetted "submerged" specimens. Results from the tests, i.e. shear stress versus displacement and volume change versus displacement, were analyzed and discussed. The dry specimens showed compression and then dilation during shearing. The volume change behavior of wetted silty sands depends on the silt content and the relative density. With silt content up to 25%, the wetted loose silty sands compressed during shear whereas the wetted dense mixtures showed compression and then dilation during shear. The wetted specimens with fines content of 40% showed compression only during shear. The wetted specimen with (sand + 40% fines) behaved like fine grained soil. Significant drop in shear strength caused by wetting has been measured for all tested specimens. The drop in shear strength was greater for the dense specimens compared to the corresponding loose ones. The drop showed general trend of increase with fines content.