Shear Strength

Stability of many geotechnical engineering problems is associated with the measurement, analysis, and prediction of shear strength of the soils in their natural state. Geotechnical engineering problems such as bearing capacity of foundations, stability of natural & man-made soil slopes, design of pavements, and earth retaining problems require the qualitative and quantitative prediction of the strength. The shear strength of the soil may be defined as the maximum resistance along the failure plane in soils per unit cross-sectional area under the load application. It was well understood that the strength of the soil is related to stress state in the soil. The state of stress in a soil consists of certain combination of stress variables that are independent of material properties. They are called stress-state variables. Stress-state variables are required for the characterization of the stress conditions in the soil. The number of stress state variables to describe the stress state of soil depends on the degree of saturation. The effective stress is a sole stress state variable of a saturated soil which can completely define the state of stress. Thus the mechanical behavior of saturated soils is completely controlled by the effective stress. However, ascertaining the stress state variables for unsaturated soils is very difficult. Of late, two stress-state variables viz. net normal stress and matric suction have been widely accepted and used. Some of the early developments of shear strength of soils are briefly described in the following section

Some early developments

The early quantification and application of shear strength was done in the eighteenth century by a French scientist, Coulomb, in his classical work on friction model. This work was drawn on the work of Leonardo da Vinci on law of frictional materials in the sixteenth century. Coulomb described useful design models based on his work that are used even today in the design of retaining walls and other applications that are central to the geotechnical engineering.

Coulomb distinguishes soil and rock only based on its relative strength. The soil was considered to be a rigid homogeneous material similar to rock mass. This one-phase material ruptures into two different blocks at failure. Based on his experiments on solid rock samples, Coulomb hypothesized that these blocks slide on each other along the slip surfaces. The state of soil at peak stress was described in his classical work. It was suggested that both cohesion and friction need to be overcome along the slip surface during the shearing as described in the following classical friction model

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