Rehabilitation of Concrete Pipelines with Invert Voids by Non-Water-Reactive Polymer Grouting: The Interface Expansion Pressure Evolution and Lifting Response

Invert voids beneath underground concrete pipelines can cause support loss and settlement, while the spatial evolution of interfacial forces generated by confined polymer foaming remains poorly understood. Small-scale curved-interface grouting tests were conducted to investigate the effects of grout density, interface arch height, and circumferential position on local expansion forces, and a quadratic response surface model was developed for force prediction. Full-scale pipeline lifting tests were subsequently performed to investigate the relationship between interfacial expansion and structural displacement. The expansion response exhibited three stages—filling, rapid foaming, and stabilization—with pronounced spatial nonuniformity. Increasing grout density from 0.26 to 0.36 g/cm3 increased the peak expansion force by 58.9–69.7%, whereas increasing arch height from 8.14 to 14.60 mm resulted in an increase of only 5.3–9.2%. The response surface model showed good agreement with the experimental data used for model development (R2 = 0.9896). The full-scale test achieved a pipeline uplift of 50 mm, with the bell section lifting approximately 2.1 times faster than the spigot section during rapid uplift. A simplified equilibrium analysis yielded a theoretical critical grout density of 0.29 g/cm3 for initiating pipe uplift providing a preliminary reference for grouting parameter design and pipeline rehabilitation.