The enhancement of the durability of sulfoaluminate cement (CSA) in marine environments is of great importance. To this end, an investigation was carried out involving the placement of CSA concrete in the tidal zone of Zhairuoshan Island, Zhoushan, China, and subjected to a 20-month marine tidal exposure test. The comparison was made with ordinary Portland cement (OPC) concrete to evaluate the effectiveness of the former. The test findings indicate that the compressive strength of both types of concrete is reduced by seawater dry-wet cycling, and the porosity of the surface concrete is increased. However, the compressive strength of CSA concrete is observed to be more stable under long-term drying–wetting cycles. When the ettringite in the CSA surface concrete is decomposed due to carbonization and alkalinity reduction, its products will react with Ca²⁺ and SO₄²⁻ in seawater to regenerate ettringite to fill in the concrete pores, making the concrete strength more stable and hindering chlorine penetration. Furthermore, CSA concrete exhibits a higher capillary absorption capacity than OPC concrete, which results in chloride accumulation on its surface. However, the diffusion capacity of chloride in CSA concrete is significantly lower than that in OPC concrete.

The deformation pattern of the stratum caused by constructing a new metro tunnel crossing an existing tunnel is different from the deformation pattern caused by general tunnel construction. However, the prediction results by the ordinary surface settlement prediction model often bring significant errors because the complex influence of existing tunnels on the surface settlement caused by the excavation of new tunnels is always neglected. Based on the equivalent layered method and stochastic medium theory, a prediction model for the surface settlement due to excavating a new tunnel under an existing tunnel in the heterogeneous stratum was established. By equating the bending stiffness of an existing tunnel before and after applying the equivalent layered method, the layer index was determined. The critical parameters of the stochastic medium theory were derived based on the relationship between the critical parameters of both the Peck empirical formula and the stochastic medium theory. The surface settlement of some typical projects was predicted and compared by the prediction model in this paper and the ordinary prediction model. The comparison shows that the proposed prediction model and parameter determination method in this paper had high accuracy and applicability. The results of the prediction model in this paper fit the results of numerical calculation. The research of this paper can provide a new method for the theoretical prediction of surface settlement caused by the excavation of a new tunnel under an existing tunnel in the heterogeneous stratum and the determination of critical parameters of the stochastic medium theory.problem in the construction industry, and helps reducing the material waste and budget cost.

With the rapid development of high-speed railroads and subways, there has been an increasing number of bridge–tunnel overlapping structures. To study the dynamic response characteristics of bridge–tunnel structures under the synergistic effects of the vibration generated by high-speed railway and subway trains, the dynamic response characteristics of a bridge–tunnel structure under single-point vibration loading was analyzed by conducting numerical simulations and model tests, with the frequency response function and peak acceleration as the evaluation indices. The dynamic response characteristics of the overlapping structure under moving vibration loads of the high-speed railway and subway trains were further analyzed. The results showed that the dynamic response of the bridge–tunnel overlapping structure increased with the increase in the frequency under the full frequency domain single-point sweep vibration load. The dynamic response of the tunnel hance near the pile foundation side was significantly greater than the vault and invert. Compared with the effect of high-speed train loads alone, the dynamic response of the bridge–tunnel overlapping structure under the synergistic effects of high-speed railways and subways increased significantly and varied at different locations. This investigation provides theoretical support for the design and construction of bridge–tunnel overlapping structures under the synergistic effects of high-speed railways and subways, contributing to improving engineering quality and safety.