Numerical modelling of field vane shear test in sensitive clay

Kundu, Pritam (2022) Numerical modelling of field vane shear test in sensitive clay. Masters thesis, Memorial University of Newfoundland.

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Abstract

The undrained shear strength of clay is one of the key parameters in geotechnical design, which can be determined from laboratory or field tests. The disturbance of soil samples during sampling, transporting, storing and trimming could influence the laboratory test result, which could be significantly high for sensitive clay because the structure of the soil might be changed. Therefore, the field vane shear test (VST) is commonly used to determine the undrained shear strength of sensitive clay. Although the test is relatively simple, the interpretation of the VST results is challenging, as the failure mechanisms are still not well understood and cannot be observed in the field. Therefore, numerical simulation might be a viable option to examine such mechanisms. Some attempts have been made previously using Lagrangian-based finite element (FE) analysis but not could model the complete process, because large shear strains develop along the thin shear plane, which causes mesh distortion and numerical issues. In the present study, VST is simulated using an Eulerian-based FE approach. In this approach, the Eulerian material (soil) flows through the fixed mesh; therefore, the simulation can be continued for a large penetration and rotation. A nonlinear strain-softening and strain-rate dependent soil model for the undrained shear strength of sensitive clays is incorporated in the FE simulation. The first set of simulations is performed for only one element of thickness (i.e., two-dimensional conditions) by applying rotation to a rigid vane. Three-dimensional simulations are then performed by penetrating the vane to the desired depth, followed by a rotation. The rotation initiates the formation of a rounded square-shaped failure block which turns into a circular one after the peak torque. The penetration of the vane can significantly disturb the surrounding sensitive clay and thereby reduce the maximum torque during rotation. The extent of disturbance depends on several factors, such as blade thickness, soil sensitivity, and post-peak strength degradation parameters.

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