Abstract: Grouting is commonly used in shield tunnels to stop water and sand inrushes by effectively sealing leakage points to prevent their further deterioration. To investigate the mechanism of different sealing materials for tunnel leakage in various strata, leakage accidents are divided into "seepage-type" and "cave-type" based on the deterioration characteristics of shield tunnels under various erosion stages of strata. Furthermore, the sealing grout is divided into "expansion type" and "bonding type" according to the chemical properties of the grouting materials. The working mechanisms of different types of slurries for various leakage issues are investigated through modeling tests. Results reveal the following. (1) The consolidated material exerts high positive stress on the contact surface through expansion or adhesion. When this positive stress on the contact surface exceeds the water pressure, the external flow of water cannot enter the tunnel, thereby stopping the leakage. (2) In the seepage-type strata, the spreading distance of slurry is primarily affected by the condensation time of slurry, permeability coefficient of strata, viscosity of slurry, grouting pressure, and hydraulic gradient. The empirical formula for the spreading distance of the slurry is given by tests. (3) In the seepage-type strata, the expansion of oil-based polyurethane generates a large positive stress that resists the external water pressure. The pressure on the contact surface of waterborne polyurethane is lower than that on oil-based polyurethane. This is because waterborne polyurethane consumes pore water and achieves leakage sealing under expansion pressure. (4) In the cave-type strata, oil-based polyurethane stops leakage by self-sealing, while waterborne polyurethane stops leakage by the combined action of adhesion and self-sealing. (5) In the cave-type strata, slurry diffusion is affected by the density of the slurry and its products. Further, appropriate slurries should be selected based on leakage locations.

Key words: shield tunnel, leakage issue, leakage sealing by grouting, modeling test, sealing mechanism