This paper presents a numerical investigation on the improvement mechanism of dynamic compaction (DC) in saturated soil of a weak layer with high levels of groundwater, using an improved fluid–solid coupling method with a Drucker–Prager–Cap soil model. The numerical model is verified by comparing with published test data at first, which show that the dual-phase coupling method can approximately reflect the development law of excess pore water pressure and the improvement effect of layered saturated soil foundation under DC. Then, based on the numerical model, the influences of the thickness and depth of the weak layer as well as tamping energy on the development and dissipation of excess pore water pressure, effective stress, and the relative degree of reinforcement during DC were investigated. The results showed that the embedment depth and thickness of the weak interlayer may greatly affect the effective reinforcement depth of DC. Meanwhile, the tamping energy and the groundwater table also play a great role in the improvement of the layered saturated soil under DC. The groundwater table should be lowered by dewatering or adding a drainage layer to achieve a better compaction effect during DC.

Addressing the current lining cracking problem in coastal tunnels, this paper independently introduces a novel type of repair material for tunnel lining cracks—the composite repair material consisting of waterborne epoxy resin and ultrafine cement (referred to as EC composite repair material). Through indoor testing, we have analyzed the change rule of the mass change rate, compressive strength, flexural strength, and chloride ion concentration of the repair material samples in erosive environments, with the dosage of each component in the EC composite repair material being varied. We have also investigated the working performance, mechanical properties, and microstructure of the repair material. The results of this study show that when the proportion of each component of ultrafine cement, waterborne epoxy resin, waterborne epoxy curing agent, waterborne polyurethane, defoamer, and water is 100:50:50:2.5:0.5:30, the performance of the EC composite repair material in a chloride ion-rich environment is optimal in all aspects. When the mixing ratio of each component of the EC composite repair material is as stated above, the repair material exhibits the best performance in a chloride ion erosion environment. With this ratio of components in the EC composite repair material, the fluidity, setting time, compressive strength, flexural strength, and bond strength of the repair material in a chloride ion erosion environment can meet the requirements of relevant specifications, and it is highly effective in repairing tunnel lining cracks. The polymeric film formed by the reaction between the waterborne epoxy resin emulsion and the curing agent fills the pores between the hydration products, resulting in a densely packed internal structure of EC composite repair material with enhanced erosion resistance, making it very suitable for repairing cracks in tunnel linings in erosive environments.