Load reduction on high-filled cut-and-cover tunnel using discrete element method

Abstract

High-filled cut-and-cover tunnels (HFCCTs) can help satisfy the tremendous demand for usable land. However, this reclamation tunneling method involves massive backfill over a cut-and-cover tunnel (CCT), which induces high overburden pressure on the tunnel. A better understanding of the mechanisms that can reduce this load could also help to reduce safety risks and design costs. To this end, the use of relatively low-compacted (RLC) soil can reduce the load on top of CCTs; however, the RLC soil layer makes the load transfer mechanisms more complex. Previous studies have either ignored the distinct properties of soils or focused mainly on their macromechanical properties. Thus, if micromechanical soil properties can be considered properly, then the load transfer mechanisms can be better understood. Backfill with different relative compaction (‘R’) levels must be considered: e.g., R = 90% for major backfill and R = 80% for the RLC layer placed over a HFCCT.Via a discrete element method (DEM) software program, this study investigated changes in vertical earth pressure (VEP) on HFCCTs relative to the thickness and spread distance of the RLC soil layer, the ratio of the width of the valley to the width of the CCT (referred to as the B/D ratio), and the slope angle. Parametric DEM studies were conducted to characterize these influential factors. The DEM study results show that a proper thickness and spread distance of the RLC soil layer can optimize the soil arching effect and reduce VEP on top of CCTs. The results also show that the B/D ratio and slope angle are relevant to the reorientation of the VEP.