Development and laboratory validation of a field-scale piezovane shear test device for use in intermediate soils

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 authors developed a new “piezovane” to measure PWP at the vane blade edges. To calibrate the device, piezovane shear tests were conducted on clayey silt tailings in a large-scale triaxial cell, measuring vane torque as well as PWP during insertion, rotation, and postshearing. PWP also was recorded using an external transducer at the top specimen boundary. Measured shear strengths were compared with published values, followed by an assessment of excess PWP trends. In this validation study, the authors did not investigate in detail the influence of rotation rate on the measured results.