Tunnel excavation in granular media: the role of force chains

We investigate how force chains in a granular packing influence and change during a tunnel excavation process. A two-dimensional (2D) frictional cohensionless packing is considered under gravity and a set of contiguous particles are removed in the interior. Using discrete element simulations on realistic non-spherical soil grains, we investigate the role of force chains in the stability of the resulting tunnel. We illustrate that force disturbance due to excavation is transmitted over considerable distance by force chains. Such force chains behave as one-dimensional (1D) load carrying members, leading to nonlocal influences on tunnel stability as these chains rearrange around tunnels. Based on these observations, we posit the non-existence of a local stability prediction criterion that examines only the set of grains adjacent to the tunnel boundary. Finally, we study the mechanics of transition to the new equilibrium configuration by examining how the various components of force disturbance vary with distance from the tunnel. This work lays the framework for a systematic analysis of granular excavation process by examining how forces applied in the domain interior are transmitted into the granular media.