Mechanism and control method of large deformation for large-span chlorite schist tunnel

Based on the field test of deformations and stresses of supporting structures in the Lianchengshan Tunnel of Baoji-Hanzhong Expressway (double-hole six-lane), the characteristics and mechanism of large deformation disasters of large-span chlorite schist tunnels were analyzed, the comprehensive control method of large deformation disasters of the tunnel was summarized, the classification standard of large deformation of large-span chlorite schist tunnel was established, and the corresponding support parameters of each deformation grade were proposed. Analysis result shows that the large-span chlorite schist tunnel mainly has the settlement deformation during the tunnel excavation, which is mainly manifested as the overall settlement of the primary support. After the primary support is closed, the tunnel deformation is mainly manifested by the extrusion deformation of the side wall and the bottom heave of tunnel invert caused by the settlement of tunnel foot. The large deformation disasters are mainly as follows: the instability and collapse of tunnel face, primary support invasion and failure, the unsoldering and failure of feet-lock pipe, the cracking of secondary lining, the sinking of side wall, and the uplift and cracking of invert backfill. The chlorite schist is extremely weak and broken, and the invert base is softened by water, which are the fundamental causes of large deformation disaster of the tunnel. The large span of tunnel excavation (the maximum excavation span is 19.6 m), the flat tunnel section, and the lack of bearing capacity and effective restraint for arch foot foundation aggravate the deformation intrusion and instability failure of tunnel structure. The limited bearing capacity of primary support, causing loads to be continuously transmitted to the secondary lining, is the direct cause of the cracking of secondary lining. The deformation mechanism of surrounding rock can be summarized as the continuous downward slipping and loosening mechanism caused by the difficulty of the cohesion of arch rock mass in overcoming its own weight, and the plastic flow mechanism of soft rock caused by the low-strength stress ratio at the tunnel foot and invert. The large deformation disaster of the tunnel can be effectively avoided by adopting the comprehensive control method of "three-step core soil method+large reservation deformation+double-layer HK200b steel frame+large-diameter feet-lock pipe+radial grouting of surrounding rock+deepening tunnel invert", and managing the large deformation hierarchically at the same time. 5 tabs, 18 figs, 32 refs. © 2021, Editorial Department of Journal of Traffic and Transportation Engineering. All right reserved.