Disengagement of invert filling in shield tunnel induced by overlying excavation: Mechanisms and data-driven assessment

To assess how staged vertical excavation of an overlying foundation pit affects invert-filling disengagement in a shield tunnel, a three-dimensional model was developed to explicitly incorporate the invert-filling, staggered-joint segmental lining, reinforcement cages, and bolts, with diaphragm walls and steel struts serving as temporary supports. A framework based on a coordinate-mesh index was established to locate and quantify interface disengagement. Nonlinear regression and k-means clustering were employed to derive deformation-control thresholds from disengagement indicators, enabling quantitative risk assessment and supporting early-warning decisions. The main findings are as follows: (1) As excavation deepens, the tunnel-invert-filling interface undergoes three progressive stages: edge-induced tension-shear degradation, waist-centered disengagement, and central detachment with residual compressive contact at both ends. (2) Vertical settlement rebound ratio (SRR) and lateral convergence rebound ratio (CRR) exhibit nonlinear correlations with disengagement volume. Disengagement remains Stable when SRR < 6.364 % and CRR < 8.403 %; Alert when SRR is between 6.364 % and 24.572 % or CRR between 8.403 % and 23.483 %; and Critical beyond these limits. (3) Volumes of 0.0014‒0.0959 m 3 denote the Alert state, whereas volumes >0.0959 m 3 indicate severe disengagement. (4) CRR governs disengagement-volume prediction (>96 % importance), with a 95 % probability of reaching Critical state at threshold—exceeding the 91 % probability associated with SRR.