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.
Highlights
Understanding the true behavior of cohesionless soils immediately impacts the fields of internal erosion prediction, surficial sloughing in dams and levees, wave propagation, geo-sensor coupling, and geoenvironmental contamination and remediation designs
Research suggests that the use of effective stress principles is not indicative of nearsurface environments due to the application of confining pressures to maintain sample stability of granular materials
These specimens are weaker than the dry UDț specimens with nine specimens failed under a vertical applied stress ranging between 3.7 and 4.6 kPa an average stress of 4.1 kPa
Summary
Understanding the true behavior of cohesionless soils immediately impacts the fields of internal erosion prediction, surficial sloughing in dams and levees, wave propagation, geo-sensor coupling, and geoenvironmental contamination and remediation designs. Laboratory experimentation on cohesionless soil fabric behavior in zero-to-low vertical confining pressure environments relies on the use of effective stress principles to infer behavior [1,2,3]. Research suggests that the use of effective stress principles is not indicative of nearsurface environments due to the application of confining pressures to maintain sample stability of granular materials. Recent research has shown discrepancies between the prevailing mathematical representations of near-surface shear strength and the observed shear strengths, e.g., Equation 1. This classical approach has proved sufficient for soils that are not at, or in close proximity to, the elastic free boundary of the soil-atmosphere interface.
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