ABSTRACT
The excavation of a main tunnel is often preceded by that of a pilot tunnel for exploration, ground improvement or advance drainage. The question was raised in the past over the effectiveness of driving a coaxial pilot tunnel for stress relief in the surrounding ground, and thus improvement of the conditions prevailing during enlargement to the final cross-section. Previous investigations answered this question negatively for dry ground, concluding that it does not offer a tangible trade-off over the solution of tunnel profile over-excavation and the use of a yielding support. However, the question remains open for water-bearing ground, where besides stress relief the pilot tunnel also induces pore pressure relief over an extensive area. In the case of a pilot tunnel outside the tunnel cross-section, stress and pore pressure reliefs have been shown to lead to markedly reduced ground deformations during excavation in the main tunnel, which provides sufficient scope for a detailed investigation into this effect in the case of a coaxial pilot tunnel, as well. This is conducted in this paper based on coupled hydromechanical numerical simulations, considering light or heavy pilot tunnel supports. The results indicate a remarkable reduction of ground deformations during cross-section enlargement of nearly or more than 50% in some cases, highlighting the substantial benefits of applying this method in tunnelling practice.
