<p>Long-term, concealed leakage from aging urban pipelines degrades the mechanical properties of unsaturated soils, increasing the likelihood of engineering failures when disturbed by shield tunneling. This study systematically investigates the coupled effects of long-term pipeline leakage and shield tunneling in unsaturated clay strata. Test materials were collected through field sampling, and the filter paper and temperature–humidity methods were used to measure the suction of unsaturated clay, revealing quantitative relationships between water content and matric suction, total suction, and osmotic suction. Incorporating direct shear tests, the influence of suction on soil shear strength was analyzed, and a correlation model relating water content, saturation, suction, and shear strength was developed. The results show that matric and total suction decrease markedly as water content increases, whereas osmotic suction initially increases and then stabilizes with further increases in water content. Both cohesion and internal friction angle decrease with increasing water content, and matric suction dominates the contribution to cohesion under low water content conditions. Based on the saturation–shear strength correlation model, the strength characteristics of the formation affected by pipeline leakage were reconstructed, and a three-dimensional numerical model was developed to simulate the coupled effects of pipeline leakage and shield tunneling. The evolution of vertical stratum stress, plastic zone development, and pipeline stress induced by single-line and double-line tunneling was analyzed under various conditions, including small, medium, and large leakage scopes, and leakage lengths of 9 m, 27 m, and 45 m. The results indicate that pronounced stress concentration zones develop in the stratum under large-scale leakage, and the extent of the plastic zone increases markedly as the leakage scope expands. When the tunnel face approaches the pipeline invert, large-scale plastic deformation inclined toward the excavation face develops in the stratum, and the disturbance is substantially greater than that observed under small-scale leakage or single-line excavation. The findings clarify the mechanical responses of strata and structures under coupled pipeline leakage and tunneling effects, providing a theoretical basis and technical support for risk assessment in similar engineering projects.</p>