Shear Strength of Sand at Extremely Low Pressures

Data contributing to the understanding of the nature of shear strength at extremely low pressures are presented. The expanding use of models for solving soil stability problems has aroused considerable interest in the behavior of sand at confining pressures below 5 psi. Triaxial compression tests were carried out varying the cell pressure between 0.20 psi and 35 psi. Experimental evidence shows that the increased dilatancy of sands sheared at extremely low pressures produces an increase in the principal stress ratio at failure. The overall increase in strength is mostly represented by the introduction of an apparent cohesion intercept. Values of this intercept for the quartz sand tested range from 0.14 psi at loose states to 0.22 psi at dense states. Values of the angle of shearing resistance at extremely low pressures are smaller than those obtained at moderate pressures by less than 0.5°. For loose states, shear can be associated with expansive volume change if the normal pressures are low enough. The shearing behavior of loose sands at very low pressure is quite similar to that of dense sands at moderate pressure.