The measurement and interpretation of small strain stiffness of soils
The measurement of soil stiffness at very small strains (Gmax) has been carried out under both dynamic and continuous loading in the triaxial apparatus up to high stresses. A new system of LVDTs has been used to measure axial strain locally during continuous loading while dynamic stiffnesses were measured using bender elements. Using the two methods good agreement was found between the stiffnesses at 0.0001% strain for two different materials. Bender elements measure the propagation time of shear waves through a soil sample so that the value of Gnax can be determined. The design of the bender element configuration in the triaxial apparatus has been improved so that the reduction in the electrical noise resulted in a high quality of received signal. Bender elements were also incorporated into high pressure triaxial cells and were used to test very stiff soils for the first time. Theoretical studies and dynamic finite element analyses are presented which have been carried out to develop more objective criteria for the determination of Gmax. Several techniques have been proposed for simple and direct measurement of the arrival time of the shear wave to an accuracy of ±1%. The thesis presents the results of bender element tests examining the variation of Gmax for sands, which is then related to the stiffness at larger strains determined under continuous loading using the new system of LVDTs. Three sands with very different geological origins were tested over a wide range of stresses allowing a general framework for stiffnesses to be established. The interpretation of the results is based on the correct normalisation of the data by which means unique relationships have been derived for each soil. The framework demonstrates that the confining stress and volumetric state relative to the normal compression line are the principal controlling factors of stiffness as they would be for clays. However, the framework distinguishes that for sands the means of arriving at its initial volume stress state are also important, in particular whether this is by overconsolidation or compaction. The thesis also presents results of bender element tests examining the anisotropy of Gmax of both natural and reconstituted clays. The work investigates both the stress induced and inherent components of anisotropy related to an axi-symmetric stress state or cross anisotropy of the soil fabric. The results show that the stress induced anisotropy of soils is generally small for an axi-symmetric stress state while significant inherent anisotropy was found in natural London clay. The same degree of inherent anisotropy has been reproduced by anisotropic straining of reconstituted London clay suggesting that the inherent anisotropy is a variable factor related to the strain history. The rate of change was, however, found to be very small so that. when an isotropic stress regime was applied the Gmax anisotropy persisted long after the plastic strain increments became isotropic. Finally, finite element analyses were carried out using the 3 SKH model (Stallebrass, 1990b) to investigate the influence of Giax on calculated ground movements. The analyses consisted of parametric studies of the stress-strain behaviour of a triaxial sample, the load deflection behaviour of a foundation and the ground movements around a tunnel. Although constrained by the limitations of the model the analyses offer an insight into the behaviour which can be also important for a general understanding of the behaviour of real soils. The studies highlighted that the assessment of the importance of Gmax on the stress strain behaviour should consider all the relevant factors which control the non linearity of soil's response, in particular the recent stress history of the soil and the geometry of the boundary value problem.