Ground Subsidence Hazards due to Crushing and Removing Large Isolated Boulder by Tunneling

Abstract

To date, cases of severe ground subsidence or abrupt cave-in failure of roadway without warning after crushing and removing boulders in the sand matrix by tunneling have been frequently reported in practice, which jeopardize the safety of pedestrians and vehicles and even cause casualties. Despite the severe consequence, it is still difficult for engineers to give reliable estimations on the ground subsidence hazards and then take suitable countermeasures or remedial measures in advance to prevent or mitigate relevant risks. In this study, failure mechanism of ground subsidence due to removing large isolated boulder in the sand matrix by tunneling and its adverse effects on tunnel performance were explored using numerical modeling. It is discovered that after removing the boulder, the soil stratum above the cavity maintained stability momentarily, and no clear sign of subsidence could be immediately observed at ground surface. With more and more sand particles falling off into the cavity over time, the soil particles nearby were mobilized which finally led to subsidence or cave-in of ground surface. Removing the boulder also incurred redistribution of bending moments and increase of axial stresses in the tunnel lining. In the end, the effects of tunnel cover depth and position of boulder along the tunnel circumference on ground subsidence and tunnel performance were also investigated. The findings in this study can serve as references for evaluating the risk of ground cave-in failure due to future tunneling work in similar geological conditions.