Water-reactive polyurethane (PU) grout is widely used for water sealing, yet its gel-forming capacity and property evolution under uncontrolled water addition remain insufficiently understood. In this study, PU:water volume ratios from 1:1 to 1:20 were investigated using gelation tests, Shore OO hardness and uniaxial compression tests, SEM, ATR-FTIR, and non-reactive all-atom molecular dynamics simulations. Increasing water addition prolonged the gelation time from approximately 15 s at PU:water = 1:1 to 22 s at 1:15. Stable gel formation was maintained up to a water-addition multiple of 18× (PU:water = 1:18, v/v), whereas stable gel formation failed at multiples of 19× and 20×. Meanwhile, the Shore OO hardness decreased from approximately 63 to 2, and the compressive stress at 50% strain decreased from 208 to 8 kPa between PU:water = 1:1 and 1:15. SEM revealed increasingly heterogeneous and locally loose pore structures at high water contents. ATR-FTIR confirmed substantial consumption of free –NCO groups and changes in the carbonyl/amide region after curing. MD simulations further showed composition-dependent changes in local water distribution and hydrogen-bond participation before reaction. These results establish the water-tolerance boundary and multiscale property evolution of water-reactive PU, providing a basis for controlling PU–water proportions in water-rich grouting applications.

The deformation prediction of large-span variable-section foundation pit excavations in water-rich soft soils and silty fine sand strata remains challenging. Therefore, the deformation characteristics of a large-span variable-section foundation pit were investigated by considering seepage-induced changes in the mechanical properties of silty fine sand and the creep behaviour of mucky silty clay. Based on a large-span variable-section foundation pit project located in the Yangtze River floodplain area, the characteristics of the upper muddy soil and underlying silty fine sand composite strata, together with the “L-shaped” construction method, were considered. A combination of laboratory tests and numerical simulations was adopted to investigate the soil saturation state, seepage characteristics and deformation behaviour of the retaining structure during the dewatering process. The results showed that the seepage conditions within the composite strata were significantly altered by dewatering, and the shear strength of the silty fine sand was exponentially reduced under seepage conditions. The results showed that accounting for seepage-induced changes in soil mechanical properties, together with soil creep, altered the location of the maximum deformation of the retaining structure and increased its deformation. Specifically, incorporating the modified mechanical properties of the silty fine sand increased the maximum horizontal displacement by up to 5.7% compared with the analysis considering soil creep alone. The influence of seepage within the silty fine sand layer on the deep deformation of the retaining structure was found to be significant. The seepage-induced displacement increment was mainly concentrated within the silty fine sand layer, with a maximum increase of 7.44 mm, and was jointly governed by creep duration, excavation depth and seepage conditions. Field monitoring results demonstrated that the numerical model incorporating the weakening effect of seepage on silty fine sand was capable of better reproducing the deformation evolution of the retaining structure during construction, thereby improving the accuracy of deformation prediction.