Small Strain Stiffness of Fine Grained Soils

Dynamic measurements of very small strain stiffness (c 0.001% and less) were carried out both in the laboratory, using piezoceramic bender elements of the type developed at the Norwegian Ceotechnical Institute and at the University College of North Wales, and in situ, using surface wave techniques. For the purpose of the research described in this dissertation, the piezoceramic bender elements were incorporated in two computer controlled hydraulic stress path cells so that it was possible to measure the very small strain stiffness of the soil at' any stage of a triaxial stress path test and compare it directly to shear nioduli obtained at larger strain levels. The electronics required to run the bender elements proved to be simple and inexpensive, especially if compared to those required for other dynamic testing techniques. Also the interpretation of the tests was much simpler and more direct than in other dynamic testing techniques. Some preliminary work based on the numerical analysis of the signals at the bender elements was carried out to improve the definition of the arrival of the shear wave. The dependence of the very small strain stiffness on factors such as stress state and history, as described by mean effective stress, deviatoric stress, voids ratio and overconsolidation ratio, was investigated for fine grained soils of different plasticity (P1 11-41). The dependence of the very small strain stiffness of fine grained soils on stress state and history could be conveniently expressed as: __ -A fr-TR Pr 1°rJ where Pr is a reference pressure introduced to render the relationship between shear modulus and mean effective stress non—dimensional and A, n and ni are material properties. The values of the parameters A, n and m obtained for the four fine grained soils used in the present work compared well with published data. In particular the multiplier A was found to decrease and the exponents n and m to increase with increasing plasticity. The experimental results also seemed to indicate that the very small strain stiffness of fine grained soils is not significantly affected by an anisotropic state of confinement at least for stress states relatively far from failure. Provided that the stress state and history in situ were taken into account the values of very small strain stiffness obtained from laboratory tests compared well with those obtained from the field dynamic surveys. The laboratory values of very small strain stiffness were found to be about 20% smaller than the in situ values. At larger strain levels the dependence of stiffness on stress state and history could still be expressed using a power law like the one used for the very small strain stiffness but this time the coefficients A, n and m depended on strain level. In particular coefficient A was found to decrease and the exponent of mean effective stress to increase with increasing strain level. At low strain amplitudes the exponent n approaches values similar to those obtained at very small strains while at larger strains the exponent approaches unity, indicating that the stress strain response is then dominated by frictional behaviour. The dependence of stiffness on overconsolidation ratio was found to be less pronounced at very small strains than at relatively larger strains.