Model test study on the influence of support stiffness on the mechanical behavior of fractured phyllite tunnels in high seismic zones

Abstract Tunnel construction in fractured phyllite within high-seismic zones presents significant geotechnical challenges due to the rock’s pronounced rheology and poor self-stabilization. Using the Dujiashan Tunnel of the Guang−Gan Expressway as a case study, this paper investigates the mechanical response of the support system under varying stiffness conditions through field monitoring and scaled geomechanical model tests. The study reveals a critical ‘confinement-rheology coupling’ mechanism, demonstrating that while increasing primary support stiffness effectively restricts plastic zone dilation, it attracts disproportionately higher surrounding rock pressure, exemplified by a 64% pressure increase associated with thickened shotcrete, due to the passive earth pressure effect. Specifically, excessive stiffness induces shear failure risks at the arch foot, whereas insufficient stiffness leads to progressive loosening. Notably, the study confirms that systematic rock bolts exhibit limited efficacy in deformation control, as the deep excavation-induced relaxation zone in fractured phyllite exceeds the effective anchorage length. Consequently, an optimized support strategy is proposed based on ‘stiffness matching’, which prioritizes a primary support thickness of 30−35 cm, steel arch spacing of 40–50 cm, and a secondary lining thickness of 55 cm, while eliminating systematic bolting to streamline construction. This research provides a validated reference for balancing structural safety and cost-effectiveness in complex soft-rock tunneling.