Face stability of EPB shield tunnels in multilayered ground with soft sand lying on hard rock considering dynamic excavation process: A DEM study

This paper aims at studying the face stability of earth pressure balanced (EPB) shield tunnels in multilayered ground with soft sand lying on hard rock considering dynamic excavation process. The analysis was performed using three dimensional discrete element method (DEM). The present work has two advantages: (1) it reveals the influence of soil disturbance induced by dynamic cutting process on tunnel stability in multilayered ground; (2) it reveals the influence of soil retaining provided by cutterhead panel on tunnel face stability in multilayered ground. Results show that tunnel face collapse undergoes two stages. The underlying hard rock restricts the movement of overlying soft soil, and face failure happens more abruptly with decreasing eta(eta= a/D. a = height of soft sand within tunnel cross section; D = tunnel diameter). The normalized limit support pressure pf/gamma D (p(f) = limit support pressure; gamma = soil unit weight) obtained from this paper is larger than that of existing research, i.e. existing research underestimate the limit support pressure. The weakening effect of cutterhead dynamic cutting process on face stability is larger than the strengthening effect of cutterhead panel retaining on face stability. The difference generally increases with increasing C/D (C = tunnel cover depth) and eta. Partial collapse occurs in multilayered ground with soft sand lying on hard rock. C/D mainly influences the shape of the failure zone, and eta mainly determines the size of the failure zone.