Shaping force-transfer arch to retain subsurface cavity in coarse sandy ground

Abstract

Ground cave-ins, which are the collapse and discontinuous subsidence of the ground surface, are thought to be caused by the expansion and upward movement of subsurface cavities due to fluctuations in the groundwater table or earthquakes. Compared to cohesive clays or plastic silts, cohesionless sands are more vulnerable to cavity formation and subsequent ground cave-ins. With recent technology, such as ground-penetrating radar, geometrical information on cavities, e.g., location and shape, can be detected. In practice, the soil cover thickness-to-cavity width ratio ($H/B$) is often used for risk assessments of cave-ins. However, it is questionable whether $H/B$ alone is sufficient for these risk assessments since the mechanical responses, such as the resistance of the remaining soil above the cavity, are not considered. For this reason, the aim of the present contribution is to understand the mechanism underlying the subsurface cavity stability by considering the force transfer around the cavity. Suction measurement, cavity retention, and needle penetration model tests were conducted using various coarse granular materials. The results revealed that suction is essential to preventing cavities from collapsing, and that suction is higher for smaller particles, particles with lower degrees of saturation, and particles with angular shapes and smoother surfaces. In addition to $H/B$, the mechanical interlock from angularity or roughness contributes to cavity stability. Laboratory needle penetration tests revealed the existence of a force-transfer arch between the sound and weakened zones around a cavity, which is related to the cavity stability. Furthermore, the position of the arch is affected not only by $H/B$, but also by the particle characteristics (e.g., friction angle) and cavity roof shape. Therefore, considering the material type and the shape of the cavity roof, along with $H/B$, will lead to enhanced assessments of the cave-in potential of subsurface cavities.