Experimental investigation of the influence of the timing of strengthening on the structural behavior of segmental tunnel linings

If the serviceability of an existing shield tunnel structure is endangered, strengthening is required. A newly proposed technique of strengthening of segmental tunnel linings, called steel plate - concrete composite structure (SPCCS) strengthening, was experimentally investigated in previous research, in which the tunnel lining was strengthened at the ultimate state. Such a strengthening scheme is denoted as delayed strengthening. Experimental results have shown that the new strengthening technique is advantageous in enhancing the design life of shield tunnels, as compared to traditional strengthening techniques. However, in engineering practice, the tunnel lining must be strengthened before the ultimate state is reached. Hence, the timing of strengthening is a critical parameter in the design of the strengthening process. In the present research, a full-scale test was conducted such that the segmental tunnel lining was strengthened ab initio, i.e. before being subjected to external loading. This mode of strengthening is termed as initial strengthening scheme. The ultimate capacity, the failure process, and the failure mode were investigated. The failure mechanism, the ultimate capacity, the ductility, and the toughness of segmental tunnel linings, reinforced by the SPCCS strengthening technique, were compared with the ones of initial and delayed strengthening schemes. It is shown that the timing of strengthening does not have much influence on the ultimate capacity. However, it has a significant influence on the stiffness, the ductility, and the toughness. The ultimate load bearing capacity of the strengthened structure in case of initial strengthening was found to be only 5.7 % larger than the one in case of delayed strengthening, whereas the increase of the structural stiffness, the ductility, and the toughness is 520.6 %, 46.1 %, and 64.6 %, respectively. Regarding the utilization efficiency of the steel plate, the results have shown that the earlier the structure is strengthened, the greater the utilization efficiency of the strengthening material.