Research Seminars @ Illinois

Development and Laboratory Validation of a Field-Scale Piezovane Shear Test Device for Use in Intermediate Soils

Advisor: Professor Scott M. Olson

Abstract

Historically, the field vane shear test (FVST) has been used worldwide to measure the undrained shear strength of soft to medium-stiff natural clays and silts. Recently, it has been used to test intermediate soils with relatively high permeability, such as mine tailings. A key question is whether the measured torque reflects undrained, drained, or partially drained conditions. Additionally, it is unclear whether excess porewater pressure (PWP) is more significant during vane insertion or rotation, and divergent criteria are used to estimate the degree of drainage during testing. Current FVST practice does not directly measure PWP to resolve these issues. Therefore, the author developed a new "piezovane" to measure PWP at the vane blade edges.

This dissertation presents the development and laboratory validation of a novel field-scale piezovane shear test device designed to directly measure excess PWP at the vane blade edges during both insertion and rotation. The device integrates a miniature pressure transducer within the vane blades to measure PWP. By enabling real-time PWP measurements, the piezovane provides a direct means to assess drainage conditions and improve the reliability of FVST interpretation in soils where partial drainage is likely to occur.

A comprehensive laboratory testing program was conducted using reconstituted clayey silt iron ore tailings representative of fine mine tailings deposits. Specimens were prepared to controlled initial void ratios and isotropically consolidated to specified effective stresses. The experimental program investigated the influence of consolidation stress and void ratio on peak and remolded undrained shear strength; the generation and dissipation of excess PWP during vane insertion and rotation; the effect of waiting time between insertion and rotation; and the influence of vane rotation rate on measured shear strength. Results showed that a large fraction of excess PWP (typically 40-80% of consolidation stress) was generated during vane insertion, while rotation contributed relatively little under fully undrained conditions. Allowing dissipation prior to rotation resulted in increased peak strength due to local consolidation, consistent with trends reported in the literature, though the magnitude and timescale of strength gain were material dependent.

The effects of partial drainage and rotation rate were shown to be particularly significant for remolded shear strength. Tests conducted with open drainage during rotation exhibited progressive post-peak strength increases that did not represent true undrained behavior. Increasing rotation rate reduced drainage effects, but excessively high rates introduced viscous (rheological) strengthening, consistent with classical observations in clays and recent field studies in tailings. An optimal range of rotation rates was identified in which both drainage and viscous effects were minimized. These findings highlight that immediate rotation after insertion is required to obtain representative peak undrained strength in intermediate soils, but

is often insufficient to ensure representative remolded strength unless drainage during rotation is prevented or overcome.

The development of the shear failure surface and zone of influence was investigated through post-test excavation using colored sand marker layers and through finite element modeling employing a coupled Eulerian-Lagrangian (CEL) approach in ABAQUS. Experimental and numerical observations revealed a narrow annular shear zone localized immediately adjacent to the vane blades, with limited radial and vertical extent.