Stiffness of geomaterials at very small strains

Experiments have been conducted in the triaxial apparatus to observe the behaviour at very small strains of three widely different natural materials: Bothkennar clay, London clay, and a high-porosity Chalk. Local strain measurements made using LVDTs were calibrated by a Fabry–Pérot laser interferometer. A displacement accuracy as high as ±0·027 μm was obtained, leading to an axial strain accuracy of the order of ±10⁻⁴%. Special care was taken to ensure that the rates of creep reduced to acceptably small levels before loading commenced. At very small strain levels the observed stress–strain behaviour appeared to be linear for all three materials. The extent of this linear behaviour was similar (0·002–0·003%) for all three materials, despite the maximum stiffness (E_max) ranging over two orders of magnitude. The initial influence of stress change and strain level on secant modulus was relatively small, with stiffnesses at 0·01% strain being of the order of 80–95% of E_max. Effects of loading path were observed, giving different rates of stiffness degradation as a function of strain, but recent stress history effects did not occur even though stress excursions went well beyond the range normally thought to induce plastic strains. Stiffnesses measured in the triaxial apparatus at very small strain levels were found to be similar to those obtained using field geophysical techniques, despite assumptions of isotropy, and the effects of sampling. The implications of these observations for practice are discussed.