Relationship Among Excess Porewater Pressure, Preconsolidation Ratio, and Critical State Friction Angle
Recall that Skempton (1954) proposed the A and B porewater pressure coeffi cients for the triaxial test; A is related to the shear component and B is related to the degree of saturation. We can use CSM to establish the theoretical A coeffi cient and its relationship to Ro and f'cs. Let us consider the excess porewater pressure at failure (critical state) for a saturated soil. From Equation
For the standard triaxial undrained test, 1Ds3 2f 5 0 since the cell pressure is held constant and the axial (deviator) stress is increased to bring the soil to failure.
Substitution of Equation
The shear component of the excess porewater pressures is the fi rst part of the right-hand side of Equation while the last part, 1/ 3 , is the total stress path component. A plot of Equation. The Af is dependent not only on Ro but also on f9cs. Recall that Skempton’s Af is dependent only on the Ro.
Undrained Shear Strength of Clays at the Liquid and Plastic Limits:
Wood (1990), using test results reported by Youssef et al. (1965) and Dumbleton and West (1970), showed that
where R depends on activity and varies between 30 and 100, and the subscripts PL and LL denote plastic limit and liquid limit, respectively. Wood and Wroth (1978) recommend a value of R 5 100 as reasonable for most soils (R up to 170 has been reported in the literature). The recommended value of (su)LL, culled from the published data, is 2 kPa (the test data showed variations between 0.9 and 8 kPa) and that for (su)PL is 200 kPa. Since most soils are within the plastic range, these recommended values place lower (2 kPa) and upper (200 kPa) limits on the undrained shear strength of disturbed or remolded clays.
Vertical Effective Stresses at the Liquid and Plastic Limits:
Wood (1990) used results from Skempton (1970) and recommended that
The test results showed that 1srz 2LL varies from 6 to 58 kPa. Laboratory and fi eld data also showed that the undrained shear strength is proportional to the vertical effective stress. Therefore,