Particle-scale mechanisms controlling the response of granular and clayey geomaterials at very small strains

Student thesis: Doctoral Thesis

Abstract

Soil stiffness at very small strains is a fundamental parameter for a wide range of geotechnical applications. The correct evaluation of the soil stiffness parameters at very small strains is essential for the realistic prediction of ground deformations occurring around geotechnical structures under operational conditions (serviceability limit state design) and is used to derive the stiffness degradation curve with increasing strain.The aim of this research is to explore the behaviour of granular and clayey geomaterials at very small strains in cases where the macroscopic response observed experimentally cannot be easily interpreted. In these cases, existing models commonly adopted for the evaluation of the small strain stiffness fail to capture the soil response and may in turn lead to erroneous estimations of ground deformations.Two examples were analysed in this research. Firstly, the stiffness at very small strains of unsaturated granular materials was investigated. Despite a number of recent studies confirming the dependency of the shear modulus at very small strains on suction and degree of saturation, no existing models are able to capture the different trends of variation observed experimentally. Since this dependency has to be accounted for in the design of infrastructure interacting with the atmosphere, there is scope to investigate the effect of these two variables further. Secondly, the macroscopic response at very small strains of saturated clayey geomaterials was explored.Studies on the one-dimensional compression of saturated non-active clays demonstrated how the processes occurring at the particle scale may significantly affect the response observed at the macroscale. Since the evaluation of soil responses in terms of the soil stiffness is generally based on the assumption that soil can be treated as a continuum medium, an attempt was made to take into account the mechanisms occurring at the microscale and their effect on the macroscopic response observed at very small strains.The thesis goals were achieved by carrying out two separate experimental investigations on unsaturated well-graded sand specimens, and on kaolin clay specimens saturated with different pore-fluids. The shear modulus at very small strains, G0, was inferred from the measurement of the velocity of propagation of shear waves through the specimens using the bender element technique.The interpretation of the experimental results was based on the analysis of the micro-mechanisms underlying the macroscopic response.For the case of unsaturated sand, a microscale-based model relating the small strain stiffness with the suction-induced intergranular stress was derived by analysing the stiffness at the contact between sand particles in the presence of water menisci and in the bulk water. The model was successfully validated against the experimental data, and was able to capture the different trends of variation of G0 along a drying or a wetting path observed in the literature. For the case of saturated clay, a DEM model with newly-designed contact laws (accounting for the mechanical and electro-chemical interactions occurring between clay particles) was first introduced. The DEM model was able to reproduce basic aspects of the macroscopic compression behaviour of kaolin clay specimens at a qualitative level. Then, the results of the DEM simulations and the quantitative analysis of the stiffness of the different particle-to-particle interactions were successfully used to elucidate the microscopic mechanisms affecting the velocity of propagation of shear waves, in turn related to the small strain stiffness.
Date of Award20 Apr 2018
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorAlessandro Tarantino (Supervisor) & Phillippe Sentenac (Supervisor)

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