Abstract: Numerous deepburied tunnel projects in western China have failed to avoid some active faults due to key line control. Large deformation and damage phenomena of surrounding rocks, such as encroachment, offset pressure, collapse, and bottom failure, often occur across the fault zone, which seriously affect the safe and sustainable development of tunnel construction and operation. The risk of large deformation of the tunnel surrounding rock in the fault fracture zone can be controlled by adopting technical solutions such as advance grouting, multilayer steel arch passive support, and anchor cable active support. However, the low strength of the surrounding rock in the fracture zone contributed to the failure of advance grouting and active support anchoring. Therefore, the authors introduce a double-gradient grouting technology to enhance the strength of the surrounding rock of tunnels in the fault fracture zone, establish a double-gradient grouting conceptual model, and construct three double-gradient grouting modes to address the aforementioned problem. The adapting conditions of the grouting material particle size gradient and the grouting pressure gradient under specific working conditions are also determined. Finally, the double-gradient grouting mechanism and its control effect are explored through theoretical analysis, physical model tests, and field tests. Results show that the stability of surrounding rock is substantially affected by the tunnel arch shoulder and the structural stress with the increase in excavation sequence. Under the action of double-gradient grouting, the grout diffusion effect is good, the surrounding rock does not suffer large-scale detachment and damage, and the stress of the rock mass around the tunnel is evenly distributed. The double-gradient grouting forms a hard cross-grout vein skeleton, achieving a stress compensation effect and reducing the surrounding rock mass. The amount of rock deformation is controlled from the original 3 100 mm to less than 278 mm, achieving the goals of "zero arch replacement, zero intrusion limit, and zero inrush", which establishes a theoretical and practical foundation for the control of large deformation of surrounding rock in similar tunnel projects.

Key words: tunnel, fault fracture zone, large deformation of surrounding rock, double-gradient grouting, excavation compensation method