Anisotropy and cyclic loading characteristics of a stiff Bolders Bank glacial till at Cowden

¹ University of Bristol, Department of Civil Engineering, Bristol, United Kingdom
² ARUP, London, United Kingdom
³ Imperial College London, Department of Civil and Environmental Engineering, London, United Kingdom

^* Corresponding author: tingfa.liu@bristol.ac.uk

Abstract

Glacial tills are widespread across the world, common in northern Europe and found under the North and Baltic Seas. Their geological and geotechnical characterisation is important to the geotechnical design of a wide range of onshore and offshore structures. This paper reports outcomes from coordinated monotonic small-strain stress probing, cyclic and larger strain triaxial and hollow cylinder apparatus (HCA) testing on a natural low-to-medium plasticity, apparent highly over consolidated, stiff glacial clay till. Monotonic HCA and triaxial experiments investigated the till’s stiffness and shear strength anisotropy from its limited linear elastic range up to ultimate failure, showing that stiffnesses are higher in the horizontal direction than in the vertical and that higher undrained shear strengths develop under “passive” horizontal loading than “active” vertical loading. The cyclic triaxial tests revealed the impact of cycling on effective stress paths and strain development, and cyclic stiffness degradation. Stable, metastable and unstable patterns of behaviour are identified, along with different cyclic failure modes, which are related to the till’s static yielding behaviour. The experiment results provide the basis for developing, testing and calibrating monotonic and cyclic constitutive models and establishing simplified design procedures for piles installed in comparable tills to support offshore energy and other structures.