Abstract: To mitigate the early-age cracking risk in mass concrete of precast immersed tunnel elements, a high-fidelity multi-field coupling numerical model integrating hydration, temperature, humidity, and stress fields is established, systematically revealing the cracking mechanism and evaluating the effectiveness of prevention measures. Taking the Mingzhu Bay Tunnel project as a case study, the evolution of early-age temperature, stress, and cracking risk in concrete is systematically simulated. The concrete temperature, stress, and cracking risk coefficient are quantitatively compared under four working conditions (i.e., baseline, continuous cooling, concentrated cooling, and postcast section). The results indicate that: (1) The combined effect of significant temperature gradients induced by hydration heat and external ultra-high constraints is the primary cause of cracking, with the surfaces of the base slab and roof slab identified as the most vulnerable areas. (2) Cooling measures function to “reduce the peak heat release”, controlling the maximum temperature rise in the core region to about 54 ℃ and reducing the cracking risk coefficient below 0.7. Notably, a 2-day concentrated cooling period shows equivalent effectiveness to 30-day continuous cooling, offering greater economic feasibility. (3) The post-cast strip exhibits a special mechanical state characterized by “low temperature rise and high constraint”, where constraint effects, rather than temperature rise, dominate the cracking risk when ordinary concrete is used. Engineering practice verifies that the coordinated control strategy of “early-stage concentrated cooling + segmented casting” proposed based on the model can successfully maintain the core concrete temperature below 55 ℃, achieving crack-free conditions in both the main tunnel element and the postcast section joint.

Key words: immersed tunnel, mass concrete, cracking risk coefficient, multi-field coupling, early-age cracking