Rate effects and vane shear strength of sandy clay

Geotechnical applications such as new rapid pile testing methods and push-in pile installation techniques have highlighted the need for a better understanding of rate effects in sandy clay material. In the case of press-in piles a correlation between driving force and in-situ soil strength could help to confirm or modify design assumptions. In order to gain an insight to the problem, a new Rowe Cell-Vane Shear apparatus has been developed to rotate shear vanes inserted in reconstituted beds of sandy clay consolidated at 200 kN/m2 over a wide range of shearing velocities. The apparatus consists of a modified triaxial test frame, double Rowe cell consolidometer, associated pressure unit, laboratory vanes with servo motor and gear head connected to a motion controller, torque transducer and data logging device. Three vanes 20mm in diameter and length 20mm, 40mm and 80mm were used. The vanes were rotated at a range of peripheral velocities from 0.5mm/s to 400mm/s to generate correlations of undrained shear strength and applied rate of shear. The results of the investigation highlight that the peak shear resistance recorded increases with increasing peripheral velocity and conform to power and semi logarithmic correlations. It was found that the magnitude of vane displacement required to achieve full mobilisation of soil shear strength was dependant on the applied rate of shear. The failure of the material at low velocities was ductile and became more brittle with increasing velocity of shearing. Furthermore, secondary shear resistance is proposed as a possible technique to provide a better estimate of shear resistance as the impact of rate effects are reduced.