Pore Pressure Measurement in Saturated Stiff Soils

Errors associated with the measurement of pore water pressures in saturated cemented soils during undrained triaxial shear are presented. Methods of modifying standard testing equipment to minimize these errors are described and experimentally evaluated. Replacing the null indicator with a pressure transducer of low compressibility and minimizing the volume of water in the pore water lines will reduce the error in measuring pore pressures from over 15% to less than 3%. Volume changes due to leakage and operation of the drainage valves can significantly alter the measured pore pressures. However, the volume changes can be minimized by enclosing the test specimens in thick rubber membranes and by using no-volume change ball type valves. In undrained triaxial compression at low consolidation pressures cemented soils create a film of water between the sample and membrane, and the lateral effective stress becomes zero.