In situ shear modulus reduction with strain in stifissured clays and weathered mudstones

The nonlinear stress–strain behaviour of stiff clays and weak rocks at small and medium strains may be a critical consid-eration in the design of geotechnical structures. Empirical methods have been developed for estimating the maximum shear modulus and the normalised shear modulus reduction with strain of fine-grained soils. These are usually expressed as func-tions of the void ratio (or specific volume) and average effective (confining) stress, based on results from laboratory tests. However, the fidelity of these equations has not been widely evaluated in situ. This paper describes the use of in situ measurements from an instrumented embankment to calculate the operational in situ shear modulus of the underlying stiff clays and weathered mudstones at medium and large strains. It is shown that the shear modulus at very small strain of the weathered clays increased linearly with depth, consistent with empirical equations. The gradient of the normalised, nonlinear stiffnesses of the clays were comparable with those measured in laboratory tests of fine-grained soils, at a range of strains. However, the values for the reference strain, where the maximum shear modulus reduces by 50%, were lower than was predicted by the empirical equations.