Shear Behaviour of Pre-Existing Shear Zones Under Fast Loading

摘要

Shear zones formed by drained slow rates of displacement, in the ring shear, were submitted to fast shearing and the following behaviour was identified: 1. an immediate increase in shear strength; 2. a further increase in shear strength up to a maximum; 3. followed by a decrease in shear strength with displacement; and 4. a perturbation of the clay particle alignment in the shear zone. The minimum shear strength during fast shear can be higher, equal to or lower than the slow residual strength - the positive, the neutral or the negative rate effect, respectively. The neutral rate effect was identified in soils formed by granular (silt and sand) particles, and the positive rate effect in high plasticity clay soils. The causes for the negative rate effect are discussed and a possible mechanism is put forward based on free water access into the shear zone sucked by the vacuum generated by the tendency to increase the critical void ratio with the rate of displacement.