Research on the reinforcement effect of frozen wall for tunneling in locally-frozen sandy cobble strata

Purpose The artificial freezing method is one of the most effective techniques for ground improvement and underground construction in water-rich strata. The reinforcement effect of frozen wall during tunneling is perceived as a main concern. Design/methodology/approach In this paper, considering the temperature difference of frozen zone and unfrozen zone, a numerical method for simulating tunnel excavation in locally-frozen sandy cobble strata was established, and three-dimensional simulations were conducted in unfrozen ground and frozen ground, respectively. Then, from the view of deformation performance and stress redistribution, an index for evaluating the reinforcement effect of frozen wall was proposed with particularly focus on the rock content. Moreover, the freezing parameter optimization of frozen wall was carried out; the optimal frozen wall thickness and the optimal frozen wall temperature were given. Findings The analysis result shows that the computed stress–strain curves and failure modes of specimens agree well with that of physical tests. The frozen wall can significantly reduce the ground performance induced by tunnel excavation, as well as the lining. However, the reinforcement effect will be affected by the rock content; a higher rock content contributes to a weaker reinforcing effect of frozen wall in sandy cobble strata. At the rock content of 50% and tunnel depth of 12.0 m, 2.0 m is the optimal frozen wall thickness and −10°C is the optimal frozen wall temperature. Originality/value At present, the research on artificial freezing method in sandy cobble strata mainly focuses on the freezing construction technology. The corresponding theory on ground deformation and stability analysis are still at an initial stage. Relevant research rarely considers the tunnel excavation process, and there is little attention paid to the ground disturbance characteristics for tunneling in frozen sandy gravel strata. It is of great scientific and engineering value to study reinforcement effect of frozen wall and ground deformation for tunneling in locally-frozen sandy cobble strata.