Numerical study of random material properties within a soil specimen

Abstract

Material properties derived from laboratory soil tests often assume that the property is uniform throughout the specimen. For some exceptional soils, this may hold true, but for many others it is obviously false. We have been performing cyclic and irregular torsional simple shear (TOSS) tests on hollow cylinder samples for decades and were intrigued by the idea of how to model inherently non-uniform specimens. As an added corollary, we wanted to understand the influence of imperfections (voids, inclusions) on the measured stress-strain behaviour in these tests. This paper examines two general classes of problems: (a) uniform specimens with inclusions of voids or imperfections, and (b) non-uniform specimens with random distributions of material properties within the specimen. Finite element modelling was performed on a TOSS specimen (ID = 4cm, OD = 6cm, L = 14cm) using a set of over 500 different elastoplastic material properties within the specimen. Various distributions (Normal, Log-normal, Bimodal) of stiffness and strength properties were examined. The results were examined as torque vs. twist curves since those values are typically measured in the TOSS laboratory test before being converted (with assumptions of uniformity) to shear stress-shear strain hysteresis. The Bimodal distributions were used to represent soils with distinct hard and soft zones. Additionally, distributions with some degree of spatial correlation were also examined.