# Research on Lateral Deformation Control Criteria of Metro Shield Tunnels with Excessive Ellipticity
## 4 Development of Lateral Deformation Limits for Shield Tunnels  
Analyzing structural bearing capacity reveals that when the foundation spring coefficient is 30 MPa/m, the tunnel nears yielding at an ellipticity of 20%.. However, when the coefficient is 5 MPa/m, there is no clear yielding even when the tunnel's ellipticity exceeds 35%.. This suggests that a higher foundation spring coefficient results in reduced tunnel deformation corresponding to structural deformation when the tunnel yields entirely. From a safety perspective, excessive lateral deformation can diminish the tunnel's sectional headroom, potentially endangering ceiling equipment within the tunnel and posing threats to moving trains. In addition, such deformation can lead to roadbed unevenness, necessitating slowed train movement.  
Factors such as the tunnel design parameters, surrounding ground conditions, existing issues, and deformation safety margin reservations should be integrated to limit the precast concrete tunnel segment's lateral deformation in the metro shield tunnels. Excessive elliptical deformation in shield tunnels irreparably damages concrete structures. Large lateral deformations from external loading, even if promptly unloaded, make it challenging to rectify the tunnel segment's lateral deformation [25]. This directly affects the safety margin for enduring structures like shield tunnel structures. Hence, the shield tunnel damage treatment principle should be: thoroughly inspect and address potential issues surrounding the tunnel early during deformation. The structure should be reinforced if deformation continues to grow post-inspection and shows no convergence signs.  
Considering the actual conditions, the standard warning value for lateral deformation based on the ellipticity evaluation of the Guangzhou Metro shield tunnels must be 20%, and the control value at 25%. Depending on various foundation spring coefficients, the finite-element simulation results for the shield tunnel, considering increasing lateral deformation of structural damage, are listed in Tables 2 and 3.  
When the tunnel's single-ring elliptic variation reaches 15-20%, the relevant departments overseeing subway protection must monitor the interval along the tunnel more closely and investigate whether engineering activities surrounding the tunnel can seriously impact its structural safety. If the tunnel's single-ring elliptic variation reaches 25% and the adjacent multi-ring tunnel segment ellipticity is 20% or more, and continuous monitoring results indicate that the deformation lacks a convergence trend and the tunnel interior exhibits signs of cracking, seepage, or crumbling blocks, then timely measures are necessary to reinforce the tunnel within the damage interval. In addition, for shield tunnels in hard  
Figure 15: Variation curve of bolt stress with increasing ellipticity.  
surrounding rock, the deformation control value should be adjusted in accordance with the standards mentioned above. The above safety limits and the corresponding safety control levels under the corresponding deformation states can be applied to judge the safety state of a 6-m diameter jointed metro shield tunnel. This study mainly focuses on 6-m diameter shield tunnels. Future research could be conducted on deformation control criteria for shield tunnels of different diameters.