EFFECT OF DISTURBANCE DUE TO INSERTION ON VANE SHEAR STRENGTH OF NORMALLY CONSOLIDATED COHESIVE SOILS

Abstract

Laboratory vane tests were conducted on two cohesive soils with plasticity index of 50 and 20. Pore pressure measurements were attempted during the insertion and rotation of the vane by two small transducers embedded in samples and one transducer connected to porous tips at the vane blades. The embedded probes produced satisfactory results, whereas the measurement by the porous tips was not very successful.It was found that high pore pressures were developed by the insertion of the vane and that the subsequent dissipation was completed in about four hours. The pore pressure change during the vane rotation was unexpectedly small. It was confirmed that the time elapsed between the vane insertion and the start of rotation was a very important controlling factor of vane strength. The ratio of the shear strength to the vertical consolidation pressure for long elapsed time increased by about 20% and 45% for soil with plasticity index of 50 and 20 respectively in comparison with the ratio for short elapsed time. The ratio for long elapsed time was slightly smaller than that observed in the vane tests free from the disturbance due to the insertion of the vane. The ratio free from the disturbance agreed fairly well with that obtained with K₀ consolidated and undrained triaxial tests. The rate of increase of the ratio with the plasticity index for short elapsed time compared well with that obtained by Skempton for the vane test.