Role of critical shear stress in erosion, drainage, and mechanical behavior of soils

In geotechnical engineering, fine particle detachment and their subsequent transport is of important concern in studies related to failure of earth embankments and clogging of filters. The critical shear stress of soils, corresponding to the onset of particle detachment, is an important parameter in this regard. This parameter is examined in detail in this study. First, experimental methods are presented to evaluate critical shear stress at which internal erosion and surface erosion of soils begin. Experimental results indicated significant differences between surface erosion and internal erosion. Second, since critical shear stress is analogous to cohesion of soil at macroscale, a mathematical model is presented relating cohesion and critical shear stress. Experimental results enabled validation of the mathematical model. A fractal formulation is proposed to upscale interparticle bond strengths (represented by critical shear stress) to cohesion. Third, the role of critical shear stress in filtration and drainage behavior of soils is addressed. An analytical solution to the filter clogging problem is presented using method of characteristics. The mathematical formulation uses the critical velocities of the pore fluid as indicators of particle detachability in both base soils and filters. The role of pore fluid composition in soil filter clogging is studied by evaluating its effect on the erodibility of base soils, size of the migrating particle, and on the likelihood of particle deposition. Results show that increasing salt concentration of the pore fluid reduces erodibility of the base soils and increases the likelihood of deposition. Finally, a rational approach to design soil filters based on pore fluid composition parameters and on performance criteria is presented.