Selecting the rate of loading for drained stress path triaxial tests

Despite certain restrictions on its validity, the use of a triaxial apparatus is still a commonly used laboratory test method to determine soil behaviour. Solutions were obtained many years ago for evaluating mean excess pore pressure values in soils subject to continuous rates of loading. They have been used to predict mean excess pore pressures in consolidation tests with constant rates of loading. Such solutions may be suitable for predicting excess pore pressures in soil specimens, subject to continuous loading, in drained triaxial stress path tests. This note proposes the use of these solutions to select the rate of loading for continuously loaded, drained stress path triaxial tests. The analytical solutions of the equations in the known mathematical model may be used to predict the average excess pore pressure in a triaxial sample, subjected to time-dependent loading, at any time during a continuous triaxial stress path. This requires an estimate of the imposed excess pore pressure. A calculation method is given for this pressure, together with a proposed method for predicting deviation from the required effective stress path. As an illustrative example, a prediction of the effective stress path, during a typical drained triaxial stress path test, is presented.