Simplified study on the evolution of the stored and released energy in the middle rock pillar of a twin-tunnel and its influence on the neighbour tunnel

In this study, the evolution of energy in the middle rock pillar (MRP) of a soft rock twin tunnel is analysed using dynamic FEA simulation. The most affected region by energy storage is the MRP; however, despite accumulating substantial energy, the energy storage is independent of the in-situ stress according to vertical components output. The energy release is significant in the adjacent tunnel when horizontal components are considered. The plastic radius of both tunnels and their geometry play a significant role; for instance, the overlapping part of both plastic radii (Re) registers the most convergences (horizontal displacements) at the bench level of the tunnel. The energy evolution is also linked to the geometry of the tunnel, where the more arched the tunnel shape becomes, the more energy dissipates in that region of the tunnels, and vice versa at the tunnel arch waist, where the geometry is rather straight. These results can efficiently contribute to the body of knowledge in underground engineering, particularly in twin- tunnel excavation cases.