Compression and shear strength characteristics of compacted loess at high suctions

Compression and shear behavior of a loosely compacted loess is investigated via two series of saturated and unsaturated direct shear tests. The vapor transfer mechanism is used to modify a shear box device for control of suction at high range. In spite of significant volumetric strain upon wetting (up to 14%), suction-induced volumetric shrinkage is less than 2% for all suction levels considered because of the as-compacted moisture content at the dry side of the optimum value. During shearing, all unsaturated tests dried to high suctions indicate a strain-softening mode of failure associated with noticeable dilation. There is a continuous increase in peak strength with suction but at a reduced rate, which cannot be captured by the improved Bishop's effective stress model, as it underestimates the contribution of high suctions by approaching zero degree of saturation. Unsaturated tests at high suctions also show an increased rate of dilation with suction for both values of net stress, which cannot be predicted well by the classical stress-dilatancy models. It is shown that for suction values beyond 8 MPa, dilation angle increases by 2 degrees and 6 degrees per 100 MPa increase of suction under net stress of 50 and 200 kPa, respectively.