Determination of drained shear strenght parameters in fine grained soils

Shear strength parameters of soils can be divided as drainedand undrained conditions. Drained parameters are proper to be used for the conditions, where pore water can drain out of the soil easily, under external loading. There is no change in pore water pressure due to external loading. Drained parameters are valid for coarse grained soils and fine grained soils if the rate of the load is slow enough not to generate excess pore pressure in the system. On the other hand, undrained condition occurs when the pore water cannot drain out of the soil. The rate of the load is much quicker in undrained loading than the rate at which pore water can drain out.In undrained conditions, a large part of the load is carried by the pore water, as a result pore pressures increase. For long term stability analysis in fine grained soils, drained parameters can be used accepting the pore water can drain out of the soil. For dynamic conditions, if the rate of the loading is fast enough as in the earthquakes, even coarse grained soils can experience undrained loading. Direct shear test is one of the most commonly used laboratory tests to determine the shear strength of soils. In direct shear test, the test sample is placed in a rigid box which is divided horizontally into two halves. After the sample is consolidated by a constant vertical compressive force, a horizontal force is applied to the upper half of the box. The maximum shear stress can be obtained by plotting shear stress values versus horizontal displacements. The tests are run several times for various vertical stresses and shear strength parameters are determined by using related plots. It is not possible to prevent completely the drainage of the water out of the sample in available shear box test systems. To ensure the drained condition, the rate of the shearing should be selected as slow as not to allow the occurrence of excess pore water pressure. This results a faster rate for coarse grained soils and much slower rate for fine grained soils with low permeability. Some empirical correlations, which use consolidation characteristics of the soils, are available in the literature, regarding the rate of the shearing. Keywords: Direct shear test, shearing rate, drained condition, silty soil