The back-analysis of shear modulus reduction with strain in weathered mudstone beneath an instrumented embankment

Full-scale earthworks trials were undertaken in advance of the construction of the High Speed 2 (HS2) railway between London and the West Midlands. They provided in-situ measurements of the ground response due to loading and unloading from the overlying earthworks. Results from the trials were used by HS2 Ltd to refine the design parameters, make cost savings and eliminate the need for engineering interventions to control ground movement. However, the instrumented trials also provided more fundamental information about the engineering properties of the ground. This is particularly useful in difficult ground such as weathered mudstone.This paper presents results from back-analyses of in-situ measurements at an embankment trial, 14 km north of Banbury, to measure the stiffness of the underlying, weathered mudstone.Geophysical shear wave velocity measurements were undertaken to determine the in-situ shear modulus at very small strain. These showed that the shear modulus of the ground profile increased linearly with depth in the clay, consistent with empirical equations. During well-defined construction loading stages, measurements were obtained from extensometers installed beneath the embankment. These were used to measure deformation and strains within the underlying ground profile and calculate the in-situ shear modulus. The shear modulus was normalised to compare with empirical equations for shear modulus reduction with strain. The results showed that the gradient of the normalised, non-linear stiffness of the clays was 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.