ON THE POROMECHANICAL MODELLING AND INTERPRETATION OF FIELD VANE TEST

Resumo

The field vane test is probably the most used apparatus for evaluation of undrained shear
strength of clay deposits. Although being designed for low permeability soils, its application in
intermediate permeability materials can also be found in geotechnical investigation practice. Since the
standard rate of shearing may not ensure undrained conditions in those cases, attention must be paid to
partial drainage effects in the soil surrounding the vane, leading to an increase in the soil resistance
and an erroneous estimation of the undrained strength. This paper aims to investigate the drainage
effects by means of a nonlinear poroelastic model, conceived to capture the transient flow effects in
the medium surrounding a rotating cylinder, which can be viewed as a simplified conceptual model for
the vane geometry. The model relies on a nonlinear poroelastic stress-strain analysis addressed by the
Biot’s poroelasticity framework, where closed-form expressions for pore pressure distribution were
derived while stress and displacements are computed numerically through a finite difference scheme.
The nonlinear poroelastic model is briefly presented and validated through experimental results in low
permeability soils. A parametrical analysis is then conducted to evaluate the response of theoretical
materials varying strength, stiffness and influence radius size. Finally, the numerical model is applied
to the interpretation of experimental data in zinc mining tailings, viewed as intermediate permeability
materials. It has been shown that the proposed poroelastic model is a good tool in evaluating the
drainage effects in vane tests, allowing for the identification of test patterns that ensures the desired
drainage behavior.