Influence of unloading disturbance on adjacent tunnels

• Unloading-induced stress concentration around adjacent tunnels was examined.
• Dynamic responses of tunnels subjected to unloading disturbance were modelled.
• The influencing factors of unloading disturbance were investigated.
• Unloading excavation under high stress can serve as a part of vibration excitation.

A theoretical model was first established in order to evaluate the physical possibility of a dynamic response occurring around an existing tunnel under conditions of unloading wave incidence. Based on the steady state solution of the wave expansion approach, transient solutions subjected to unloading waves with different unloading times were obtained. A three-dimensional numerical model was then constructed in order to simulate dynamic responses around an existing tunnel under unloading disturbance forces. In the simulation, dynamic unloading was carried out through a new excavation after stress redistribution had been finalized in the examined tunnel, and parametric studies were then conducted for various unloading times, stress levels and tunnel spacing configurations. The results show that dynamic effects are induced around an existing tunnel under high levels of initial stress and at high unloading rates. The results of the dynamic analysis show that the PPV of unloading-induced microseism can be as high as that of an explosion-induced wave, and damage can be inflicted on existing tunnel walls (especially on the incident side of a tunnel). This dynamic effect diminishes dramatically with an increase in tunnel spacing and unloading times.