POREWATER PRESSURE UNDER AXISYMMETRIC UNDRAINED LOADING
The porewater pressure changes in soils are due to the changes in mean total and deviatoric stresses. Skempton (1954) proposed the following equation to determine the porewater pressure under axisymmetric conditions:
where Ds3 is the increase in lateral principal stress, Ds1 2 Ds3 is the deviatoric stress increase, B is a coeffi cient indicating the level of saturation, and A is an excess porewater pressure coeffi cient. The A coeffi cient is due to the deviatoric stress. The coeffi cient B is 1 for saturated soils and 0 for dry soils. However, B is not directly correlated with saturation except at high values of saturation (S . 90%). At failure,
where Duq is the change in excess porewater pressure resulting from changes in deviatoric (shear) stresses. Experimental results of Af presented by Skempton (1954) are shown in Table 10.5. The coeffi cient A was found to be dependent on the overconsolidation ratio (OCR). A typical variation of Af with OCR. Equation is very useful in determining whether a soil is saturated in an axisymmetric test. Let us manipulate Equation (10.50) by dividing both sides by Ds3, resulting in
During isotropic consolidation, Ds3 5 Ds1 and Equation. becomes
If a soil is saturated, then B 5 1 and Du 5 Ds3. That is, if we increase the consolidation stress or confi ning pressure by Ds3, the instantaneous excess porewater pressure change must be equal to the increase of confi ning pressure. Equation (10.53) then provides a basis to evaluate the level of saturation of a soil sample in an axisymmetric test. The coeffi cients A and B are referred to as Skempton’s porewater pressure coeffi cients.
Variation of OCR with Af .
THE ESSENTIAL POINTS ARE:
1. Under an axisymmetric loading condition, the porewater pressure can be predicted using Skempton’s porewater pressure coeffi cients, A and B.
2. For a saturated soil, B 5 1.