Stability analysis and failure mechanism of multiple parallel soft-rock tunnels considering the random distribution of rockmass joints

Abstract

In order to investigate the failure mechanism of multiple parallel tunnels considering the random distribution of rockmass joints, this paper conducts in-depth study on the instability characteristics, deformation mechanism and active deformation control methods of deep jointed soft-rock tunnels. A rockmass joint generation technique considering the random distribution characteristics of geometric parameters has been established and validated. Compared to the ordinary single-hole tunnel, the failure mode of the double-hole tunnels with parallel adit is much more complex, with the deformation mainly reflected on the outer side of the left and right holes. The surrounding rock pressure is concentrated at both ends of the normal direction of the bedding plane. Narrowing the distribution range of joint dip angle will exacerbate the asymmetry of surrounding rock deformation, and a larger range of joint dip angle can make the stress characteristics of the supporting structures more uniform. Increasing the joint trace length will gradually increase the overall displacement of the surrounding rock and reduce the number of yielding blocks. As the density of joints increases, the stress in the surrounding rock becomes more uniform, but it can lead to more fractured rockmass, reduced the deformation stability, and is not beneficial for the formation of key structural layers. The active deformation control method suitable for deep jointed soft-rock tunnels is proposed and applied. Under conventional passive support schemes, the surrounding rock faces a high risk of instability collapse, with a failure probability exceeding 20%. The active deformation control concept represented by prestressed support and grouting reinforcement can effectively improve the safety factor of surrounding rock and reduce the instability probability of rockmass. For the deep stratum with developed joints, it is difficult to effectively apply high prestressing force to the internal stable structural layer. Therefore, a long-bolt with full-face grouting scheme shall be considered to actively reinforce the deep jointed rockmass.