WHY DO WE RUN THE VANE SHEAR TEST SO SLOWLY?
The peak torque is usually observed within 20° of rotation. The Piezocone Penetration Test (CPTu), which can test one vertical linear meter of soil in approximately 50 seconds, is much faster than the VST. In silty-soils the CPTu penetration resistance can be partially drained and may vary with penetration rate. This technical bulletin covers the rotation-rate effects with the VST.
The undrained strength of clay depends on the strain-rate. Bjerrum (1974) presented laboratory results from a series of undrained triaxial compression tests performed at different rates (Figure 1). The strength decreased at slower rates. In these undrained triaxial compression tests the drainage lines were closed and the specimen was sealed by an impermeable membrane. This prevented all partial-drainage effects, even when tested at very slow shear-rates. Bjerrum (1974) attributed the increase in strength at faster rates to a viscosity-related property of the water in the specimen.