Influence of Discontinuities on the Squeezing Intensity in High In Situ Stresses (a Tunnelling Case Study; Actual Evidences and TBM Release Techniques)

Abstract

Squeezing rock condition has always been a major concern as one of the principle geological hazards when tunnelling in weak overstressed rock masses. This phenomenon often occurs in shear faulted zones, extremely jointed regions or metamorphic rocks with thin schist layers. Due to the inherent vulnerability of shielded TBMs to jamming, lack of sufficient accurate understanding upon existing squeezing conditions could lead to TBM jamming and long delays in the project duration as a result of TBM downtime and excessive associated costs. In this research, attempts were made to describe the squeezing conditions encountered in July 2014 in the long tunnel of Zagros as a special case study in Iran. The results of the conducted analyses as well as the experiences obtained from this project are presented accordingly. The research procedure in this paper is explained in two separate stages. In the first stage, joint characteristics and mechanical properties of rock mass are estimated, and the behaviour of TBM jamming section of the tunnel is simulated using FLAC and UDEC as the continuous and discontinuous modelling codes. By taking advantage of these simulations and the comparisons, the influence of existing discontinuities and the intensifying effect of overburden stress on the encountered harsh squeezing conditions are analysed accordingly. In the next stage, main reasons and actual indications of TBM jamming that occurred in the squeezing regions of the Zagros Tunnel are investigated. These results are presented in three separate sections including geological indications such as the amount of injected pea gravel and cement grouting as segmental lining backfill materials, field observations regarding the segmental lining and effects on the operational parameters of machine including thrust force, cutterhead torque, Field Penetration Index and the pressure applied by the auxiliary thrust cylinder jacks. Lastly, the most efficient implemented techniques for releasing the machine are explained step by step.