Effect of Suction and Confining Pressure on Shear Strength and Dilatancy of Highly Compacted Silty Sand: Single-Stage versus Multistage Triaxial Testing

Abstract

Abstract Evaluating short- and long-term performance of engineered earthen structures and slopes requires proper understanding of the behavior of compacted and highly compacted soils. However, limited studies exist in the literature of unsaturated soil mechanics experimentally testing highly compacted soils, which often possess complex shear strength and dilatancy characteristics. The main objective of this study is to investigate the effect of suction and confining pressure on shear strength and dilatancy of a highly compacted silty sand through multistage and single-stage triaxial testing. Multistage triaxial testing offers a time and cost efficient approach that can reasonably represent field conditions where soils experience fluctuations in external loading conditions (e.g., flood loading/unloading). This article presents the results of consolidated drained multistage and single-stage triaxial tests on highly compacted silty sand under saturated and unsaturated conditions. Soil specimens were isotropically consolidated at a constant matric suction of 0 (saturated), 20, 50, and 95 kPa under net confining pressures of 50, 100, and 200 kPa. Unsaturated specimens consistently exhibited higher shear strength in multistage and single-stage tests compared to saturated specimens. Peak state lines for all unsaturated tests were plotted and found to be near parallel to the saturated peak state line. All tested specimens exhibited dilatancy following a small initial contraction. Although the dilatancy during shearing in multistage tests was comparable to that in the single-stage tests, the cumulative volumetric strain resulted in a net increase in the specimen volume in the case of unsaturated tests, and a net decrease in the case of saturated tests. This difference in volumetric strain behavior resulted in a lower peak shear strength of the saturated specimens tested using multistage testing compared to single stage but a higher peak shear strength in the case of the unsaturated specimens.