Geohazards of tunnel excavation in interbedded layers under high in situ stress

In situ stress is one of the most important parameters that affect rock mass stability during tunnel excavation. High in situ stress may induce a series of geohazards, such as rockburst in strong rock and squeezing in weak rock. This paper presents a case study of a tunnel excavated using a tunnel-boring machine across interbedded strong and weak rock layers under high in situ stress in Pakistan. To estimate the induced geohazards during tunnel excavation, in situ stress was measured and the stability of the tunnel was analyzed using the finite element method. In situ stress measured in the strong rock layers shows that the maximum compressive stress is very high, and its direction is almost perpendicular to the tunnel axis. Using a simple analytical method, stress distribution in weak layers was determined and the results show that the magnitude of the maximum compressive stress in the weak layers is much less than that in strong layers, and the direction is almost parallel with the tunnel axis. The FEM simulation reveals that the redistributed stress due to the tunnel excavation is more concentrated in strong layers and less concentrated in weak layers. Both the initial in situ stress distribution and the stress redistribution after tunnel excavation predict geohazard in strong layers rather than in weak layers. This point is consistent with the field records, which indicated that rockbursts were frequent in the strong rock layers and no squeezing occurred in the weak rock layers.