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Large deformation has always been a focus and difficult issue in the construction of deep-buried tunnels in squeezing rock. Previous studies mainly focused on the large deformation of medium and small span railway/highway tunnels in soft ground. However, there are limited researches on the large deformation control methods for large-span (three-lane) highway tunnels constructed in unfavorable geological environment. Based on the Lianchengshan Tunnel of the Baoji-Hanzhong expressway in Shaanxi Province, China, this paper studied the deformation behaviors and mechanical mechanisms of a large-span tunnel excavated in chlorite schist formation with single primary lining method and double primary lining method by in-situ test and numerical simulation. The achieved results indicate that the double primary lining method is much more effective than that of the single primary lining method in restraining the deformation of surrounding rock, and the maximum vertical displacement and horizontal convergence are reduced by 67% and 66%, respectively. The support method of double HK200b-type steel sets combined with large-diameter foot reinforcement bolt (FRB) and deep invert could effectively control the large deformation of the case tunnel, which effectively avoided the supporting structure failure, repeated clearance invasion and multiple reshaping work caused by the single primary lining method and conformed to the energy-saving construction concept of "no clearance interfering, no support reshaping" of tunnels in squeezing ground. Simulation analysis of surrounding rock deformation, supporting structure stress and plastic zone distribution was performed to evaluate the support effect of the two deformation-controlled methods. Finally, the deformation and stress characteristic curves of rock-support of the two deformation-controlled methods were established, which revealed the supporting mechanism of double primary linings for large-span tunnels in chlorite schist. The research results can provide a theoretical basis and practical reference for the large-deformation control of similar large-span tunnels in squeezing rock. [ABSTRACT FROM AUTHOR]

• A field comparison test of rational construction methods for tunnels was conducted. • The applicability of main large-deformation control methods to strata is discussed. • A deformation control system is proposed and its synergistic principles are elaborated. • The load-bearing process of a multilayered and staged support system is analyzed. 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 Baoji-Hanzhong 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 × 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 × 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. [ABSTRACT FROM AUTHOR]

• Deformation of 18 sections in Grade V rock mass and the initial support structural force of 2 sections are analyzed. • FLAC 3D numerical simulation is used to study the deformation of the rock mass. • The supporting parameters of fifth-grade surrounding rock for large-span loess highway tunnel are designed. When a large span loess tunnel is excavated, the rock mass is disturbed, and it is difficult to form a load-bearing arch under shallow depth region. As a result, the structural deformation is complicated. This paper analyzed the deformation of 18 sections in Grade V rock mass and the primary support structural force of 2 sections in a case study of a highway loess tunnel in the large span loess highway tunnel group of Huangyan Traffic Capacity Expansion Engineering in Northern Shaanxi Province-Jianzicha No.2 Tunnel. The analyzed results were combined with the FLAC 3D numerical simulation to study the deformation of rock mass in Jianzicha No.2 tunnel and the stress characteristics of primary support. The results showed that: (1) The time-space relationship curve of arch deformation indicated three stages of "rapid deformation stage -continuous deformation stage-slow deformation stage" in the construction by annular excavation with reserved core soil method. (2) The rock mass pressure distribution of tunnel basically presented the bilateral symmetry characteristics. The rock mass pressure distribution of arch was relatively uniform, and the rock mass pressure at the invert was relatively large. (3) The outer and inner sides of primary support arch steel rib was subjected to compression, and the stress distribution was relatively uniform and large. (4) The primary support shotcrete was mainly subjected to compression, and the stress distribution was basically characterized by bilateral symmetry. The stress distribution of shotcrete at the arch was relatively uniform and large, and the stress of shotcrete at the inverted part was relatively small. This paper studied the time-space law of supporting structure deformation in the shallow section of Ⅴ grade surrounding rock, and designs the supporting parameters of Ⅴ grade surrounding rock for large-span loess highway tunnel, which has guiding significance for the construction of similar projects in the future. [ABSTRACT FROM AUTHOR]