A rational construction method and deformation control system of tunnelling in extremely soft and fractured chlorite schist medium

The large deformation disaster is a major engineering problem that has not been sufficiently addressed in tunnelling in soft rock mass. Previous investigations essentially focused on large deformation control methods or measures for soft rock masses, with little attention to extremely soft and fractured rock masses. As a result, we here conduct a comparative field test of the cross diaphragm (CRD) method and the three-bench annular excavation method with reserved core soil. The case study site is the Lianchengshan Tunnel of the BaojiHanzhong Highway in Shaanxi Province and we could establish an appropriate construction methodology for the chlorite schist stratum. Following that, the mechanism of large deformation of extremely soft and fractured surrounding rocks is revealed, and the compatibility of the main methods of controlling large deformation in very soft and fractured layers is discussed. Thereafter, a suitable large deformation control approach and support system for extremely soft and fractured rock strata is proposed. Based on engineering practice, we also elaborate on the cooperative bearing principle of each component of the support system in detail. Our findings indicate that the CRD method has no outstanding effect on the deformation control of the Lianchengshan Tunnel. The three-bench annular excavation method with reserved core soil outperforms the CRD method in controlling the settlement of the tunnel arch, reducing the construction risk, and ensuring the construction progress. In general, it works better than the CRD method and should be preferred in tunnel construction. Considering the "soft" and "fractured" geological features of chlorite schist in the Lianchengshan Tunnel, a multi-layered and staged support system is developed with strengthening support as the core and stress release, as well as section optimization as the assistance. Under the support system, the tunnel deformation is effectively controlled. Compared to the single-layered primary support scheme via the same constructional approach, the maximum values of settlement and convergence of the measured section are reduced by 51 % and 56 %, respectively. The achieved results reveal that the overall stiffness of the single-layered primary support consisting of HW200 x 200 steel rib is about 3 times that of I22b steel rib, and the overall stiffness of the double-layered primary support consisting of HW200 x 200 steel rib is about 2.7 times that of a single layer. The results of this study not only contribute to the control concept and method of large deformation, but also provide a novel pathway and a substantial reference to solve the problem of control of large deformations of tunnels in extremely soft and fractured strata.