A Simple Method for Assessing the Peak Friction Angle of Sand at Very Low Confining Pressures

This article presents a simple experimental method that may be used to assess the peak friction angle of sand at very low effective confining pressures (i.e., < 10 kPa). Soil strength at very low confining pressures is important to a variety of geotechnical problems, including small-scale 1-g physical modeling and micro-gravity environments. It is often very difficult to assess friction angles at very low confining pressures in conventional element tests because some factors, such as specimen self-weight and machine friction, become significant. Currently, there are no published methods available to measure the peak friction angle of sands below a mean effective stress at failure of about 6 kPa. To address this need, a simple tilt test method is proposed that involves preparing a soil to a specified relative density within a steel mold and then tilting the mold to induce a shallow slope failure. Based on infinite slope analysis, the tilt angle at which the slope fails is equal to the peak friction angle. A modified stress-dilatancy relationship is also proposed that can be calibrated for a specific soil using a combination of tilt test and triaxial test data. This relationship can be used to predict peak friction and dilation angles over the low stress range.