Voids behind linings may affect the safety and performance of structures. In this paper, the applicability of the extended finite element method for simulating crack propagation was verified firstly through comparisons between numerical simulations and model tests. Moreover, the mechanical behavior of double-arch tunnels under effects of voids on the top of the middle wall was investigated numerically. Two factors, including void size and tunnel shape, were mainly investigated. The main results obtained were explored including internal forces, deformation and fracturing of the liner. The results showed that voids produced adverse effects on the liner. Internal forces on the liner experienced significant changes and the deformation of the liner increased. Besides, larger crack depth was observed at the crown and the connection between the spandrel and middle wall, indicating a significant decrease in bearing capacity of the structure compared with tunnels without voids.

The presence of voids behind lining seriously affects the safety of the symmetrical double-arch tunnels during service life. It is essential to find out the impact of voids on tunnel safety due to the increasing demand for the construction and maintenance of symmetrical double-arch tunnels. Model tests and numerical analyses were conducted in this research. The results attained were explored including earth pressure, internal force, and lining failure. Results reveal that the presence of voids has a large influence on the internal force in the lining of symmetrical double-arch tunnels, generally in the form of asymmetrical failure patterns of the lining. The failure patterns of the lining are greatly influenced by the size and location of voids with respect to the symmetrical double-arch tunnel circumference. Significant changes in the lining internal forces were found at the areas in the close vicinity of the void whereas a few changes were found at the bottom of the sidewall, the invert, and the central wall far away from the void. The propagation laws of lining cracks of asymmetrical double-arch tunnels are more complicated than symmetrical tunnels with a void behind the central wall. The location of the initial cracking of symmetrical and asymmetrical double-arch tunnels is the same, while the lining failure of the large-section tunnel is the most complicated.

This article proposes a new 3D failure mechanism to analyze the stability of the tunnel face driven by a closed shield in cohesive-frictional soils. The failure mechanism consists of a single block in the lower part defined by logarithmic-spiral and a distributed force caused by the weight of a half-ellipsoid in the upper part. The logarithmic-spiral surface represents the shear band and the distributed force is applied to simulate the arch effect. The arch effect and rotational mechanism are considered simultaneously. Comparisons among the proposed mechanism, numerical simulation and existing mechanism are conducted. The results show that the proposed mechanism provides improved limit support pressure values, and the failure patterns of the proposed mechanism agree well with those of the numerical simulation and experimental tests. This verifies that the proposed mechanism is effective and reasonable.

Closely spaced twin tunnels were excavated vertically beneath an adjacent subway station in service in Beijing, China. Because of restrictions on the settlements of the existing subway station in the area, a special protection methodology was adopted, and an analysis was performed using real-time monitoring data to investigate the response of the overlying subway station to twin tunnelling and surface settlement characteristics. The results indicated that advanced grouting realizes regional integral uplifts within the range of a reinforced area. The lifting work with jacks is critical to restraining the trend of settlements. The final settlement trough of the columns displays a double-trough “W” shape that is induced by nonuniform lifting loads, and the total ground surface settlement profile displays an asymmetric single-trough “U” shape. The moment when temporary inverts and middle diaphragms are demolished is important, as it causes potential excessive settlements. With monitoring, this tunnel engineering was completed without interrupting the subway traffic, and the safety of the operating subway station and ground surfaces was successfully guaranteed. This is a valuable project case for establishing criteria to guide the construction of closely spaced twin tunnels in similar conditions.