Stress path dependence of gassy soil in triaxial testing and DEM simulations exemplified by shield tunnelling

Gassy soils, often encountered in shield tunneling, can hinder construction progress. To assess their impact, four stress paths induced by tunneling were designed. A combination of triaxial tests and DEM simulations was used to study the effects of varying saturation levels and stress paths on gassy soil deformation. A new method was proposed, using the stress path forward angle ( ψ ) and the stress increment ratio ( Δ q / Δ p ) to jointly describe the dependence of soil deformation characteristics on the stress path. The results show that the influence of the shield tunneling stress path on the deformation characteristics of gassy soil is greater than that of the initial saturation. The stress path determines the fundamental characteristics of the stress–strain and volumetric strain curves of gassy soil. Under the premise of these fundamental characteristics, the initial saturation induces the gassy soil sample to develop strain softening and dilatancy. The dependence of gassy soil deformation characteristics on the stress path is primarily controlled by the sample’s consolidation stress ratio (Kc), stress increment ratio ( Δ q / Δ p ), and stress path angle ( ψ ). The consolidation stress ratio (Kc) determines the initial stress state of the gassy soil sample, while the stress increment ratio ( Δ q / Δ p ) and the stress path angle ( ψ ) determine the loading direction of the stress path in gassy soil. When the stress path angle is in the range of 22° to 90°, the stress–strain curve tends toward strain hardening, the initial shear modulus increases, and the volumetric strain curve tends toward contraction. When the angle is between 90° and 180°, the stress–strain curve softens, the initial shear modulus decreases, and the volumetric strain curve dilates. The comparison between the DEM simulation and triaxial test results validates the reliability of the experiments and supports the relationship between the stress path angle and the strain hardening and softening behaviors.