# Full-Scale Model Test of Subway Contact Channel under Mechanical Construction +
## 1 Introduction  
Urban traffic problems have become more serious with the continuous expansion of cities and rapid population growth. In this regard, the construction of subways is an effective measure to alleviate traffic congestion. A previous specification [1] stipulated that a contact channel should be set up when the continuous length between two single-track running tunnels of urban rail transit is longer than 600 m. At present, the methods of contact channel construction include the open excavation method, the freezing method [2], and the emerging mechanical method [3, 4]. The open excavation method is relatively simple, but it is generally suitable for areas with good geological conditions, stable soil, and no heavy load around. The freezing method is used to excavate soil with a high water content after freezing, which has the advantages of causing less pollution and not being affected by the surrounding environment. However, it needs high technical requirements for construction, and will cause large surface settlement. The mechanical method, compared with the above two methods, has the advantages of a higher degree of mechanization, shorter construction period, safer construction, and smaller ground surface settlement. Mechanical connection channels have been applied in underground projects at home and abroad, which have unveiled its broad application prospects. These projects include the safety corridors of the Fourth Elbe River Tunnel in Hamburg, Germany, the brunch lines of the Emisor OrienteTunnel in Mexico, and the rail transits in Ningbo [5; 6], Wuxi [7], Hangzhou [8; 9], and Beijing [10]. Some scholars carried out research on the construction, design, and calculation method for mechanical contact channel technology. [PERSON] et al. [3] led the development of the engineering practice for mechanical connection channels in China and put forward a new construction concept characterized by \"micro-reinforcement, machinable, tight sealing and strong supporting\", and expounded the cutting method and matters needing attention. [PERSON] et al. [4] studied the initial process of the mechanized drilling of contact tunnels through two full-scale tests and found that tunnel segments, together with the internal supports, carry the additional loads resulting from the cutting process. Meanwhile, other experts studied the security issue of mechanical connection channels under train loads and seismic loads using the finite element method and concluded that joints are prone to fatigue damage under dynamic loads [11; 12]. [PERSON] et al. [13; 14] took the practical application of a mechanical contact channel in a coastal area as the background, evaluated the construction safety, and verified the applicability of mechanical contact channel construction in a high-water-pressure mudstone stratum by numerical simulation. In recent years, it has been found in engineering practice that steel-ring secondary lining composite segments should be used at connecting positions to ensure a good seal between the contact channel and the main tunnel, but its deformation law in the cutting process has not been extensively studied yet [15]. This is because steel-ring secondary lining composite segments are of great significance for both the safety of cutting construction and the long-term stability of the main tunnel and the contact channel. Accordingly, studies on the stress and deformation of steel-ring secondary lining composite segments in main tunnels have become extremely significant in the design and construction of mechanical connection channels. At present, the research on the application and mechanical properties of steel-ring secondary lining structures in tunnels is relatively simple [16; 17; 18; 19], and studies on the special positions of tunnels are especially lacking. In a previous study, [PERSON] et al. [20] conducted bending and thrust tests on two kinds of concrete-filled composite segments.