Excess pore pressure and in situ measurement of shear strength gain in clays

study compares predicted and field measured excess pore pressures in a soft, saturated clay subjected to an embankment loading. It compares the increases in soil strength due to consolidation as measured with in situ vane shear, unconfined compression, and triaxial test methods. The predicted pore pressures were obtained from the equation: Au-B Aa3+A Adj - Acr3 The A and B parameters in the equation were determined from triaxial pore pressure measurements. The principal incremental stresses Ao"| andAcr^ were obtained using the Finite Element and Elastic Theory methods of stress analysis. The influence of the loading rate, soil consolidation, and permeability on pore pressure development were also investigated. Field pore pressures were measured with hydraulic and pneumatic type piezometers. The results indicate that there is reasonably close agreement between the predicted and field measured pore pressures using stresses determined by the Finite Element method. The predicted pore pressures determined ty the Elastic Theory method were considerably higher than the measured pore pressures. The equation used for predicting pore pressures assumes no pore pressure dissipation during construction. This assumption was shown unvalid for soils in this area because there was a continuous dissipation of pore pressure during embankment construction. The in situ vane shear test values were nearly of the same magnitude as those obtained by the conventional unconfined compression and triaxial test methods, except for soft to very soft clays where the vane shear test results were considerably higher. For soft to very soft clays the triaxial and the vane test results were in close agreement but the samples in triaxial tests had to be initially back pressure saturated and then completely consolidated under the in sicu overburden pressure. The increase in soil strength due to consolidation was not constant for all soils. Some clays gained more strength than the others for equal consolidation pressures. Quick clays lost strength during the initial stages of soil consolidation. With the exception of soft to very soft clays most clays developed about 70 percent of their ultimate strength gain during the initial 40-50 percent soil consolidation. In soft clays most of the strength gain took place during the final stages of soil consolidation.