Stability of shallow sub-surface cavities under surface loading using granular media through model testing

Abstract

Buried cavities are one of the commonly occurring Ground related issues. Every year, thousands of cave-in episodes are documented as a result of the collapse of underground cavities, damaging infrastructure and frequently resulting in fatalities. Cavities at shallow depths are critical and can seriously harm the surface infrastructure, which in most cases are the highways and the roads with traffic loadings. The fault or cracks in subsurface structures, such as box girders, pipelines, etc., primarily contribute to the cavity initiation at shallow depths. The cavities remain stable until the soil arching and the capillary forces in the unsaturated media can collectively resist the pressure of overburden soil. The stability of the shallow cavity is of concern when it lies beneath the heavy traffic loads of the highways. In this study, a particular model test equipment was prepared, having a soil chamber, water level indicator, and a base with a slit opening to induce a cavity in the soil. Model tests were performed using granular media of spherical and non-spherical (clumped and deformed) glass particles under unsaturated conditions to form a stable shallow cavity. The resultant cavity was tested with surface loading using surface plates and having variable loading rates. It was found that particle shape has a significant effect when surface loading is applied to the stable cavity. Clumped particles showed approx. Ten times higher peak resistance to surface loading compared to the spherical particles, both having equivalent mean particle size. The loading rate was found to be effective for the cases with unsaturated conditions; however, for dry samples, the loading rate was found to have insignificant effects on surface loading.