Deformation mechanism and NPR anchor cable truss coupling support in tunnel through fault fracture zone

To address the issue of extensive deformation in the Tabaiyi Tunnel caused by the fault zone, nuclear magnetic resonance (NMR) technology was employed to analyze the physical and mechanical properties of water-absorbing mudstone. This analysis aimed to understand the mechanism behind the significant deformations. Drawing from the principle of excavation stress compensation, a support scheme featuring NPR anchor-cables and an asymmetric truss support system was devised. To validate the scheme, numerical analysis using a combination of the Discrete Element Method (DEM) - Finite Element Method (FEM) was conducted. Additionally, similar material model tests and engineering measurements were carried out. Field experiments were also performed to evaluate the NPR anchor-cable and truss support system, focusing on anchor cable forces, pressures between the truss and surrounding rock, pressures between the initial support and secondary lining, as well as the magnitude of settlement and convergence deformation in the surrounding rock. The results indicate that the water-induced expansion of clay minerals, resulting from damage caused by fissure water, accelerated the softening of the mudstone's internal structure, leading to significant deformations in the Tabaiyi Tunnel under high tectonic stress. The original support design fell short as the length of the anchor rods was smaller than the expansion depth of the plastic zone. As a result, the initial support structure bore the entire load from the surrounding rock, and a non-coupled deformation contact was observed between the double-arch truss and the surrounding rock. The adoption of NPR asymmetric anchor-cable support effectively restrained the expansion and asymmetric distribution characteristics of the plastic zone. Considering the mechanical degradation caused by water absorption in mudstone, the rigid constraint provided by the truss proved crucial for controlling the stability of the surrounding rock. These research findings hold significant implications for managing large deformations in soft rock tunnels situated within fractured zones under high tectonic stress conditions.