## 2 Analysis of the Working Mechanism of the Buried Waterstop
### Waterproof Mechanism of a Buried Waterstop  
The waterproofing principle of a buried waterstop mainly includes the following two aspects:  
1. Extrusion sealing: in a similar way to the sealing principle of the shield tunnel rubber gasket [28; 29], the rubber sealing belt transfers pressure to its contact surface. When extrusion occurs between the waterstop and the concrete lining, contact pressure is generated. Without considering the bonding force between the waterstop belt and concrete, leakage occurs when the water pressure is greater than the contact pressure (Figure 2a).
2. Bonding water plugging: There is a bond between the contact surface of the buried waterstop and the concrete lining. Without considering the sealing of the waterstop belt, when the water pressure is greater than the bond force, the bond area experiences \"intact-damage-failure\" and then leakage occurs (Figure 2b).  
If the extrusion seal and bonding of the waterstop are simultaneously considered, when the water pressure is greater than the contact pressure, the contact pressure will fail. The bonded area then begins to be stressed, and the magnitude of the force is the water pressure minus the contact pressure. As the water pressure increases, the bonded area enters the damage stage, and then completely fails, at which point the waterstop belt fails to block the water, and the tunnel joints leak.  
The force of the waterstop preventing groundwater from passing through the seepage channel is called water infiltration resistance (Figure 3). The waterproofing ability of the waterstop depends on its water infiltration resistance, involving contact pressure and adhesion force. When water infiltration resistance is greater than water pressure, the waterstop can play a waterproofing role, that is:  
\[\mathrm{P_{wl}}\geq\gamma\mathrm{P_{w}} \tag{1}\]  
\[\mathrm{P_{wl}}=\mathrm{F+C} \tag{2}\]  
where \(\mathrm{P_{wl}}\) is the water infiltration resistance, \(\mathrm{P_{w}}\) is the theoretical water pressure, \(\gamma\) is the safety factor, \(\mathrm{F}\) is the contact pressure, and \(\mathrm{C}\) is the adhesion force. At the same time, the long seepage path formed by the complex cross-sectional form of the waterstop can  
Figure 2: Waterproofing mechanism of a waterstop. (**a**) Extrusion sealing, (**b**) Bonding water plugging.  
improve the waterproof reliability of the joint, remove the head pressure, and increase the difficulty for groundwater to seep through the tunnel joint.  
As can be seen from the preceding discussion, tunnel joint deformation constantly changes with temperature, load, etc., and with the development of deformation, the contact pressure between the waterstop and the concrete may change (Figure 4). The bonding of the waterstop to the concrete may also break with deformation, and the resistance of the waterstop to water seepage will also change. The water infiltration resistance after waterstop deformation can be regarded as short-term, \(\mathrm{P}_{\mathrm{wl}}^{\mathrm{st}}\), and the water infiltration resistance when the waterstop is not deformed can be regarded as long-term, \(\mathrm{P}_{\mathrm{wl}}^{\mathrm{lt}}\).  
Figure 4: The deformation contact pressure of the waterstop. (**a**) Tensile contact pressure. (**b**) Compressive contact pressure. (**c**) Settling contact pressure.  
Figure 3: Water infiltration resistance of tunnel joints.  
At the same time, according to relevant research reports, the service life of artificial rubber and other polymers can reach more than 100 years when the stress level does not exceed 20% of their tensile strength at fracture [30]. According to the specification, the tensile strength of the rubber waterstop is not less than 10 MPa, so the dangerous stress value is 2 MPa. In the following section, using test and finite element analysis, the failure characteristics of a waterstop will be studied from two aspects: water infiltration resistance and forced deformation of the waterstop.