# Experimental Study on Vertical Shear Behaviors of an Immersion Joint with Steel Shear Keys
## 1 Introduction  
Since the first immersed tunnel was constructed in the USA in 1894, the technology of the immersed tunnel has developed quickly and spread around the world for over a century [1]. As a type of tunnel constructed by prefabricated components, the immersion joints connecting contiguous elements are the weakest [2; 3; 4]. Of the mechanical properties of an immersion joint, the shear behavior is a significant factor when subjected to differential settlements [5; 6; 7], earthquakes [8; 9], wave impacts [10], etc. Flexible immersion joints, consisting of rubber gaskets, vertical shear keys and transverse shear keys, have been widely applied in immersed tunnel projects for years [11; 12; 13]. Since the shear keys are the vital components of an immersion joint, studies of their shear behaviors have been paid much attention. However, as the structures and arrangements of vertical shear keys and transverse shear keys in immersion joints are different, there is a difference between the shear resistance modes of immersion joints in the two directions. Hence, the shear behaviors of an immersion joint should be investigated in both directions. The shear behaviors of an immersion joint subjected to seismic shaking were studied in a previous study [14]. Although a suitable design criterion for immersion joints was suggested,the deformation mechanisms of the joint were discussed only under longitudinal and transverse seismic excitations, respectively. Several studies have been performed on the immersed tunnel of the Hong Kong-Zhuhai-Macau Bridge (HZMB) project. A numerical model of the immersion joint, composed of GINA rubber gaskets and shear keys, was presented. Analysis of the shear behavior of the joint in the transverse direction was implemented, and the results showed that different compressive loads in the longitudinal direction had little influence on the shear behaviors [15]. Hybrid fiber-reinforced concrete (HFRC) shear keys of a segmental joint were developed in a study, and an experiment was performed to investigate the seismic behaviors of the HFRC shear keys in the transverse direction. Based on the experimental results, analytical models were achieved to predict the ultimate transverse shear capacity of the shear keys [16]. A 1/10 scale experiment was designed to explore the mechanical behavior of an immersion joint under transverse shear action. The transverse shear stiffness and the capacity of the joint were obtained, and the transverse shear failure mode with steel shear keys was discussed [17]. The foregoing studies either focused only on the transverse shear behaviors of the immersion joint or did not take the vertical shear keys into consideration. According to the static equilibrium, an analytical mechanical model of the immersion joint was established. The longitudinal, transverse and vertical displacement of the immersion joint were calculated by the corresponding mechanical relationship [18]. However, this theoretical analysis was based on many idealized assumptions, which cannot be guaranteed in engineering practice. A simplified numerical model of the immersed tunnel was established, where immersion joints were simulated by non-linear hyperelastic springs. The vertical shear behaviors of the joints subjected to seismic shaking were investigated, and a surprising conclusion emerged from the results, wherein the overstressed joints improved the safety of the immersed tunnel after the seismic shaking [19]. Another numerical simulation study on the vertical shear behavior of the segmental joint in an immersed tunnel was also performed. Considering the stiffness of rubber gaskets and the friction between contiguous tunnel segments, the vertical shear capacities of the concrete shear keys in different kinds of segmental joints were investigated [20].