Numerical investigation on the thermo-mechanical behaviour of twin energy tunnels

Energy tunnel provides an innovative and efficient approach to harvesting geothermal energy. Some investigations on a single energy tunnel have been reported in the literature. This study investigated the behaviour of twin energy tunnels in two configurations (i.e., parallel and perpendicularly crossing) by using a thermo-hydro-mechanical coupled numerical model developed and validated in previous work. The responses of ground and a tunnel (denoted by ET1) to three thermo-activation modes were determined: cooling ET1 itself alone (ET1active), cooling a nearby tunnel alone (ET2active), cooling both tunnels simultaneously ((ET1 + ET2)active). The computed results reveal that the interaction between two tunnels becomes negligible when the clear distance reaches four times the tunnel diameter. When the clear distance is smaller, the interaction could impact the tunnel and soil displacements. In ET1_active and (ET1 + ET2)_active, the vertical responses (i.e., ground settlement, ET1 settlement and vertical convergence) in both tunnel configurations can be close to or even exceed the allowable values in tunnel management guidelines, depending on ground and tunnel conditions. The horizontal responses of ET1 (i.e., horizontal displacement and convergence) are always insignificant. The thermo-activation of ET2 alone has minor effects on ET1, except a strong impact on ET1 settlement in the case of crossing tunnels.