# Waterproof Performance of Sealing Gasket in Shield Tunnel: A Review
## 2 Mechanical Behavior of Lining Structure and Joints
### Double-Layer Gasket Configuration and Corresponding Test Technology  
Tests and simulations have been conducted for large-diameter shield tunnels with the double-layer gasket configuration. Aforementioned methods are used to determine the functionality under high water pressure. Waterproofing of tunnel joint gaskets depends on the contact condition of the gasket-gasket contact surface and the gasket-segment contact surface, namely, the rubber-rubber contact and the rubber-concrete contact. It is difficult to ensure the smoothness of the in-use segment with reinforced concrete casting and, therefore, it is also hard to determine the contact behavior between the gasket and rubber. Hence, in the tests, the grooves cannot be accurately simulated by a precisely fabricated steel clamp, which makes the results unreliable. Given this, the author participated in the research and development of a novel joint waterproof testing apparatus based on the Nanjing Weisan Road Tunnel (2018), and a full-scale concrete segment experiment was performed.  
The contributions above on the waterproof testing of tunnel joints are mainly focused on urban railway tunnels and highway tunnels, with a single-layer gasket and the steel clamp specimen. In practice, gaskets are glued in concrete grooves, and the surfaces are rougher than that of steel clamps, so the test accuracy based on steel clamps needs to be further investigated ([PERSON] et al., 2019). The most significant difference between the large-diameter deep  
Figure 2: Detection results of different models (Reprinting with permission from [PERSON] et al., 2022).  
drainage tunnel and the traditional traffic tunnel is the bi-directional leakage characteristics caused by different drainage and function modes (empty pipe and full pipe).  
With the construction of deep drainage tunnels, attention has been drawn to the joint waterproofing of the deep drainage tunnels. [PERSON] et al. [11] conducted the test of gasket selection according to the Suzhou River test section of the Shanghai Deep Drainage Tunnel. Based on the data collected from the Donghaochong test section of the Guangzhou Deep Drainage Tunnel and the Suzhou River test section of the Shanghai Deep Drainage Tunnel, [PERSON] et al. [32] demonstrated the necessity of adopting a double-layer gasket configuration for joints. However, the above indoor tests of deepwater drainage tunnels adopt the experimental idea of separate testing of inner and outer sealing gaskets or by simply using steel clamps to simulate the contact behavior of sealing gaskets in the segment grooves. Therefore, the test results can not reflect the bi-directional leakage characteristics of the double-layer gaskets in deep drainage shield tunnels under different operating scenarios with high external pressure and variable internal pressure.  
Throughout the current studies, the purpose was concentrated on the design and selection of the joint single/double-layer gaskets for urban subway tunnels, urban deep municipal tunnels, and cross-river or cross-sea traffic tunnels, considering the influence of different load conditions (static, earthquake, fire, soil erosion) on the waterproof capacity. To have a scientific and in-depth understanding of the actual leakage failure behavior of the double-layer gaskets in deep drainage shield tunnels under complex internal and external high water pressure, it is urgent to carry out corresponding targeted research and waterproofing performance testing of the double-layer gaskets.