# Waterproof Performance of Sealing Gasket in Shield Tunnel: A Review
## 4 Waterproof State Simulation Methods Development
### Fine Simulation Method  
To further investigate the impacts of progressive water pressure intrusion on the waterproof performance of joint gaskets, [PERSON] et al. [62] introduced the fluid-structure coupling algorithm based on the Euler-Lagrange grid method to perform a fine simulation of the joint seepage process. [PERSON] et al. [63] further extended the algorithm to the seepage process simulation of t-shaped joints. However, the above calculation method assumes that the fluid has no pressure, and there is a certain deviation from the actual situation. [PERSON] et al. solved the problem while still unable to account for the progressive open leakage process of two-layer gasket waterproofing structures under different water pressure directions [7].  
To investigate the tunnel structure and joint deformation under partial joint leakage, a lot of work has been accomplished in recent years. [PERSON] et al. [64] and [PERSON] et al. [65] established two-dimensional/three-dimensional finite element models based on the formation-structure method and analyzed the evolution of seepage formation and structural deformation under the condition of localized joint seepage. However, the above models adopt the idea of weakening joint stiffness from the Homogeneous Ring method, and the simulation results are not verified by collected data. To tackle this dilemma, [PERSON] et al. [66] proposed a self-defined joint leakage unit, however, the joint opening process in the seepage process is not considered.  
The main difficulty of the fine simulation lies in the reappearance of the continuous - discontinuous process of water pressure expansion on the contact surface, and the key to it is overcoming the convergence problem caused by large deformation. [PERSON] et al. [7] adopted the idea of numerical simulation of a large deformation process of static penetration test for a reference [67; 68]. Based on the built-in \"mesh-to-mesh Solution Mapping\" technology of FE commercial package ABAQUS/Explicit, the Mesh Regeneration Method, illustrated in Figure 3, was proposed, and the accurate simulation of the dynamic process of water pressure expansion on the contact surface was realized. A Continuous-Discontinuous Fine Finite Element method for calculating the joint waterproofing capacity of the shield tunnel under normal temperature was established.  
The existing fine numerical simulation method of waterproof performance of joint gaskets for shield tunnels and the large numerical simulation model considering the interaction between stratum and tunnel structure did not consider the double-layer gasket configuration, and the adopted model only considers the application of external water pressure, which does not apply to deep drainage shield tunnels.  
Therefore, it is necessary to focus on the upcoming requirements of joint bi-directional waterproof sealing of deep drainage shield tunnels under complex internal and external water pressure conditions, and carry out research on the seepage formation mechanism of segment joints and the waterproof failure mechanism of double-layer gaskets.