Experimental Study On Strength Of The Clay Soil Based On The Different Direct Shear Rate

Earth fissures result from bend and shear force. Shear fractures are affected by the shear strength of the soil, and shear strength of the soil is related to shear rate. On the other hand, compared with the usual construction loading, the changes stress caused by groundwater withdrawal in soil are slower. Therefore, it has great meanings to study the effect of shear rate on shear strength of soil when the mechanism of earth fissures due to groundwater exploitation is involved.The relationship of shear displacement and shear stress and the shear strength are studied for clays with normal and overconsolidated state. The tests are carried on the direct shear appatatus. The used shear rates were 0.02,0.1,0.5,0.8 and 2.4 mm/min. The shear rate has clear effects on the curves of shear displacement and shear stress. It also has clear impacts on shear strength parameters, such as the angle of internal friction, cohesion, residual friction angle, and residual cohesion.For normally consolidated soils, the shear stress versus shear displacement curve can be represented by a hyperbolic. The initial stiffness coefficient increases with increasing shear rate, and the peak shear stress decreases with the increasing shear rate. As far as the strength parameters are concerned, the internal friction angle decreases with increasing shear rate. The greater the density of soils, the more obviously the angle of internal friction is affected by shear rate. The shape of the failure surface of direct shear tests of soil samples is also affected by the shear rate. The greater the shear rate, the smoother the shear plane. The smoother the shear surface, the smaller the friction coefficient during sliding, the smaller the corresponding peak shear stress, and vice verse.For overconsolidated clays with overconsolidation ratio of 2 and 3, shear rate has less effect on the initial stiffness coefficient shear, and the peak stress decreases with increasing shear rate. Shear rate has impacts on the shape of shear displacement-shear stress and the strength parameters. With various shear rate, the internal friction angle, residual internal friction angle, and residual cohesion change randomly with in a narrow range. However, cohesion increases with increasing shear rate. The dilatancy of overconsolidated soil is affected by shear rate. Under certain normal vertical stress, the greater the shear rate, the smaller the vertical displacement, showing a clear negative dilation.With considering the non-thickness Goodman interface unit and hyperbolic shear displacement-shear stress relationship, a formula for tangential shear stiffness coefficient is deduced on the basis of shear rate. This formula is helpful for finite element numerical simulation.