Question

1) reasons why unsaturated soil mechanics is needed in the analysis of seepage and shear strength of unsaturated soil, and 2) how to evaluate the seepage and shear strength of unsaturated soil

Answer 1

While studying fundamentals of geotechnical Engineering, focuse will be on saturated soil mechanics. Interfacial physics, physicochemical mechanics are not considered while studying saturated soils. Due to which estimated shear strength of saturated soil in lab is completely different with the soil with realistic shear strength of soil. Therefore, focus must be there on unsaturated soils to estimate shear strength more accurately. There for the better estimate of the soil strength, seepage and shear strength test are conducted on unsaturated soils.

Coming to evaluation of seepage, seepage on unsaturated soils is almost quite similar to the saturated soils. that is constant head permeability test which generally done on saturated soils can be implemented on unsaturated soils also. Darcy law is valid in case of unsaturated soil, while considering Darcys permeability coefficient K as a function of water content. While conducting the experiment, a provision is taken so that water content of the unsaturated soil should be less than saturated water content. generally soil mass will contain pores these pores filled with water and air. Generally is replace with other liquid so that water content of unsaturated soil will be less than that of the saturated during the experiments. The remaining procedure is same as that of the saturated soils which is dealt in Basic geotechnical Engineering.

Share strength test for unsaturated soil is little different from that of the saturated soils , but not completely different , but the thing is there would be some modifications to the conventional method of testings. For example unsaturated direct shear test contains HAE disk, constant pore water pressure, air pressure, etc . which are not provided in case of saturated direct share test of soil.