A Theoretical Model for the Circumferential Strain of Immersed Tunnel Elements Under Tidal Load

Abstract

In this paper, we assumed that the soil layer back-silted on the surface of the river bottom was impermeable. The tidal load was treated as a large-area overload periodically applied on the silt soil layer. Taking into account the coupling loading of the water and soil, we built a model to calculate the circumferential strain of the immersed tunnel elements. By using MIDAS GTS NX finite element software, we calculated the circumferential strain in the tunnel wall direction under a tidal load. The circumferential strains of the tunnel elements in the Zhoushan immersed tunnel were continuously monitored using fiber Bragg grating sensors for 24 h. The circumferential strain increments at each monitoring position were calculated based on the change of the tide level. The results were compared with the measured increments of the circumferential strains. Based on these data, we summarized the variation pattern of the circumferential strain increments under a periodic tidal load. In addition, in this paper, we calculated the total circumferential strain distribution in the inner and outer walls in different cross-sections of the tunnel elements and identified positions that should be monitored with extra care. The results show that the calculated circumferential strain increments at each measuring position of the tunnel elements are in good agreement with the measured values. This demonstrates that the theoretical calculation model is reliable. In cross-sections of the same tunnel element, the time-dependence curves of the measured circumferential strain increments at the symmetrical measurement points are approximately consistent with each other. Tidal load-induced strain accounts for approximately 12–15% of the total strain. The correlation between circumferential strain over a cross-section and tidal level depends on the constraints on both sides of the cross-section, and stronger constraints result in weaker correlations. It is important to note that tensile strain is large along the inner wall of the middle portion of the top and bottom plates as well as along the outer wall of the sides of the bottom plate.