Centrifuge modeling of ground thaw settlement during metro tunnel construction using AGF method

Abstract

As a reinforcement technique developed specifically for water-rich soft ground, the Artificial Ground Freezing (AGF) method has been extensively adopted in metro tunnel construction. However, the thawing of the frozen wall upon tunnel completion induces significant ground settlement, posing adverse environmental impacts. To address this, a centrifugal modeling system for AGF construction was designed based on a metro tunnel horizontal freezing project, and a series of tests were conducted under 30g centrifugal acceleration. This study aims to elucidate the mechanical response mechanisms of both ground and tunnel structures during frozen curtain thawing under in-situ stress conditions. The test results show that the natural thawing time of the frozen wall was approximately 2.11 times longer than the active freezing. And the formation and thawing rates the frozen wall varied substantially across cross-sections, with peak efficiency observed at freezing pipe ends. During thawing, tunnel structures exhibited: increased radial compression, reduced hoop compression, enhanced axial compression. The soil pressure and pore water pressure surged rapidly during initial thawing, accompanied by substantial outward water migration and pronounced ground settlement, and stabilized during later thawing stages. Post-thawing drainage surfaces formed by moisture diffusion is at an approximate 45° angle to the tunnel horizontal axis. The ground surface settlement accelerated initially before stabilizing, and test results demonstrating close agreement with field measurements. The reliability and applicability of centrifugal modeling for investigating the laws and mechanisms of ground thaw settlement are verified.