Distress mechanism and treatment measures in construction of large cross-section tunnel passing through Q₂ soft-plastic loess layer

This paper presents a case study of a large cross-section high-speed railway tunnel constructed in a binary stratum of Q₂ loess—soft-plastic loess sandwiched between hard loess. The distresses during tunnel construction include lining cracking, water seepage, falling blocks at the excavation face, cavities behind the lining, and collapse of surrounding loess. The day-lighting collapse of surrounding rocks induced subsidence of the ground surface. The relationship between the soft-plastic loess permeability coefficient and the void ratio is obtained by triaxial seepage unloading tests and embedded into a 3D fluid-mechanical coupled numerical analysis. The results show that when the tunnel passes beneath the soft-plastic loess layer, the instability of the surrounding loess first initiates at the excavation face and further develops with the water migration. The soft-plastic loess extrudes into the free face, and the benches are immersed in water. The shear failure of overlain surrounding loess finally occurs along the weak surface in the arch section. The collapse also causes damage to the tunnel’s supporting structure. The arch crown fails due to excessive tensile and shear stresses, and the sidewalls experience failure due to excessive compression. A scheme of grouting plus pipe roofing was proposed to treat the collapse zone, and field monitoring proved the effectiveness of the proposed method.