Experimental Study on Additional Stress Induced by Grouting with Polyurethane-Modified Cementitious Materials under Confined Conditions

The rapid development of urban rail transit has posed increasing construction and operational challenges for metro tunnels, often leading to structural damage. Grouting technology using cement-based materials is widely applied to address issues such as seepage, leakage, and alignment correction in shield tunnels. This study investigates the additional stress induced by grouting in silty soil layers, using cement-based grouts with different water-to-cement ratios and polyurethane-modified cement-based materials. Results show that additional stress decreases with depth and is more influenced by horizontal distance from the grouting point. In staged grouting, the first injection phase contributes about 50% of the peak additional stress. A lower water-to-cement ratio (e.g., 0.6) increases additional stress but reduces grout flowability, while a higher ratio (≥1.0) improves diffusion but increases the risk of grout loss. The polyurethane-modified cement-based material enhances stress transfer performance, increasing peak additional stress by approximately 10%. These findings provide theoretical guidance for optimizing material selection and grouting design in metro tunnel repair within silty soil layers.