Effect of pore water pressure on tensile strength

Direct tensile testing with measurements of the pore water suction was used to investigate the relationship between tensile strength and suction. The tests were conducted on a till and a clayey soil, both homogenized. A close r view is focused on the relationship between material strain and the devel opment of suction. Beyond, the factor chi of the effective stress equation for unsaturated soils by Bishop (1959), which was calculated by the data of tensile strength and corresponding matric suction is compared to the volum etric chi of the tested soil specimens. It could be shown, that the pore water pressure changes with strain. Theref ore, not the initial suction of a soil is relevant for its failure but the actual one that can be measured in the failure zone at the moment of fractu re. In addition the application of the volumetric chi in the effective stre ss equation compared to the chi derived from tensile testing leads to an 1. 6 to 2.8 fold overestimation of the contribution of matric suction to soil tensile strength.