Shield tunnels may experience performance degradation caused by uncertainties of the construction conditions and long-term service. Various strengthening methods have been applied to tunnel structures, among which steel plate – concrete composite (SPCC) strengthening is a newly developed approach. It can be applied to strengthen deformed tunnel structures. In this way, a multilayer structure of higher strength and stiffness is obtained. Such strengthened tunnels, however, exhibit a complex mechanical behavior. It is the consequence of the multilayer structure and the existence of joints. A numerical investigation is conducted to study the mechanical behavior of continuous-joint shield tunnels, strengthened by the SPCC strengthening technique. A model with three layers of materials, i.e., a reinforced concrete layer, a layer of self-compact concrete, and a steel plate, is the basis for the analysis of the failure mechanism and the bond forces between different material layers of a strengthened tunnel lining. A parametric study is carried out to investigate the key factors of influence of the structural performance. It is shown that inside the bonds the radial tensile forces reach a maximum at the joints. The tangential shear forces attain a maximum at the joints, with a change of their sign. The results also reveal that the bond strength is the key factor to guarantee the structural strength and ductility. Furthermore, the strength of the steel plates and the degree of damage have a significant influence on the strengthened structure, whereas the strength of the new concrete and the thickness of the concrete layer concerned have a minor influence on the load-bearing performance of the structure. To sum up, this paper contains the presentation of a nonlinear model of a SPCC-strengthened tunnel structure with both reasonable effectiveness and reasonable accuracy. It is validated by experimental tests and refined models. The presented model also provides an effective approach for the analysis of a multilayer composite structure with two interfaces. Furthermore, results from the parametric analysis provide useful suggestions for engineering practice, helping to optimize the strengthening design of existing tunnel structures.
