INTERPRETATION OF THE SHEAR STRENGTH OF SOILS
In this book, we will interpret the shear strength of soils based on their capacity to dilate. Dense sands and overconsolidated clays (OCR . 2) tend to show peak shear stresses and expand (positive dilation angle), while loose sands and normally consolidated and lightly overconsolidated clays do not show peak shear stresses except at very low normal effective stresses and tend to compress (negative dilation angle). In our interpretation of shear strength, we will describe soils as dilating soils when they exhibit peak shear stresses at a . 0, and nondilating soils when they exhibit no peak shear stress and attain a maximum shear stress at a 5 0. However, a nondilating soil does not mean that it does not change volume (expand or contract) during shearing. The terms dilating and nondilating refer only to particular stress states (peak and critical) during soil deformation.
Contribution of sliding friction, dilatancy,
crushing, and rearrangement of particles on
the peak shear strength of soils
angle). In our interpretation of shear strength, we will describe soils as dilating soils when they exhibit peak shear stresses at a . 0, and nondilating soils when they exhibit no peak shear stress and attain a maximum shear stress at a 5 0. However, a nondilating soil does not mean that it does not change volume (expand or contract) during shearing. The terms dilating and nondilating refer only to particular stress states (peak and critical) during soil deformation.
The peak shear strength of a soil is provided by a combination of the shearing resistance due to sliding (Coulomb’s frictional sliding), dilatancy effects, crushing, and rearrangement of particles (Figure).
At low normal effective stresses, rearrangement of soil particles and dilatancy are more readily facilitated than at high normal effective stresses. At high normal effective stresses, particle crushing signifi cantly infl uences the shearing resistance. However, it is diffi cult to determine the amount of the shear strength contributed by crushing and the arrangement of particles from soil test results. In this textbook, we will take a simple approach. We will assume that the shear strength of an uncemented soil is a combination of shearing resistance due to frictional sliding of particles and dilatancy. That is, we are combing the shearing resistances due to crushing, rearrangement of particles, and dilatancy into one
The peak effective friction angle for a dilating soil according to Coulomb’s model is
Test results (Bolton, 1986) show that for plane strain tests,
We will continue to use Equation (10.24), but in practice you can make the adjustment [Equation ] suggested by Bolton (1986). Typical values of f9cs, f9p, and f9r for soils. The peak dilatancy angle, ap, generally has values ranging from 0° to 15°.
We will drop the term effective in describing friction angle and accept it by default, such that effective critical state friction angle becomes critical state friction angle, f9cs, and effective peak friction angle becomes peak friction angle, f9p. Table 10.4 provides a summary of the equations for each soil model at peak and at critical state. For soils that exhibit residual shear strength, replace f9cs by f9r in the critical state column . The residual shear strength is very important in the analysis and design of slopes in heavily overconsolidated clays and previously failed slopes.
THE ESSENTIAL POINTS ARE:
1. The friction angle at the critical state, f9cs, is a fundamental soil parameter.
2. The friction angle at peak shear stress for dilating soils, f9p, is not a fundamental soil parameter but depends on the capacity of the soil to dilate.