Failure mode transitions of unconfined granular media from dry to unsaturated to “quasi-fully” saturated states

¹ U.S. Army Corps of Engineers, Engineer Research and Development Center, Geotechnical and Structures Laboratory. 3909 Halls Ferry Rd, Vicksburg MS, USA
² GeoEngineering Centre at Queen’s—RMC, 13 General Crerar, Sawyer Building, Room S3418, Department of Civil Engineering, Royal Military College of Canada, Kingston, ON, Canada

^* e-mail: katherine.e.winters@usace.army.mil

Abstract

Recent research has shown discrepancies between the prevailing mathematical representations of near-surface shear strength and the observed shear strengths. This investigation focuses on three granular materials, i.e., 1) poorly-graded, medium-fine silica-quartz sand, 2) an engineered silica-quartz mix of 3.38-mm and 0.638-mm sub-angular particles, and 3) an angular fused quartz sand. Specimens were tested under load-controlled conditions at variable saturations in order to identify and quantify the influence of suction on the granular structures and failure modes. All three materials exhibited localized radial particle force chain buckling failures in unconfined drained dry (UDκ) conditions and classical shear failures in the unconfined drained unsaturated shear (UDP) conditions. In unconfined drained suction failures (UDS) conditions, the poorly-graded, medium-fine silica-quartz sand exhibited a bulging and sloughing failure without weeping, while the other two materials wept and then held loads before failure. Thus, it is suggested that the pore fluid had a predominate lubrication (strength weakening) effect, and the assumption of structure stiffening (strength increase) from matric suction may not be valid at near-surface conditions for sub-angular silica-quartz materials but is valid for the angular fused quartz.