Mechanism of load transfer and tunnel-soil interaction in shield tunnel bottom grouting for deformation recovery

This study aims to elucidate the load transfer mechanism of shield tunnel bottom grouting for deformation recovery, a process summarized as "grouting pressure → additional earth pressure → additional tunnel load → tunnel deformation recovery". While previous studies have separately clarified how grout properties influence diffusion modes and how stratum types affect final uplift, the mechanical link transforming grout diffusion into a specific tunnel deformation response remains undefined. To bridge this gap, this paper integrates grouting unit model experiments with a shield tunnel bottom grouting lifting model experiment across three strata (sandy cobble, sand, and clay). The unit experiments are used to define a parameter termed "pressure-forming efficiency" (η), which quantifies the effectiveness of converting source grouting pressure into sustained additional earth pressure for different diffusion modes. This concept is then applied to interpret the tunnel uplift experiments. Results demonstrate that the grout diffusion mode dictates the magnitude and spatial distribution of the additional earth pressure field, which in turn acts as the direct additional load on the tunnel. The tunnel's longitudinal flexure and transverse convergence are a coordinated structural response to this load, with their magnitude and pattern highly dependent on the "pressure-forming efficiency" and location of the grout source. A theoretical model combining cylindrical cavity expansion and a Timoshenko beam on a Vlasov foundation is formulated to quantitatively describe this mechanism. By linking the characteristics of the grout source to the structural response, this study provides a physical basis for the mechanistic design and refined control of grouting uplift.