# Study on the Seismic Response of Shield Tunnel Structures with the Preload Loss of Bolts
## 5 Conclusions  
A three-dimensional finite element model of the soil-structure system with viscoelastic artificial boundaries was established to study the effect of high-strength bolt preload force loss on the seismic response of the tunnel structure at the segment joints. The research investigated the impact of different levels of preload force loss on various aspects, including  
Figure 17: Cloud map of structural damage under seismic load. (**a**) SDEG of inner side of the crown. (**b**) SDEG of inner side of the arch bottom. (**c**) Tensile damage. (**d**) Compression damage. Figure 18: Cloud map of structural damage under seismic load. (**a**) Damage of segment BP when preload is 280 kN. (**b**) Damage of segment BP when preload is 160 kN. (**c**) Damage of segment BP with no preload. (**d**) Damage of segment A1 when preload is 160 kN. (**f**) Damage of segment A1 with no preload. overall structural damage, segment joint opening, ellipticity, Arias intensity, radial strain, and more. The following conclusions were drawn:  
(1) During the early stages of seismic loading, the joint areas are subjected to compression. As seismic loads act, the joints' opening value continuously fluctuates, with more pronounced peaks in gap size as the peak acceleration increases. Under different seismic waves, with increasing levels of preload loss, the peak gap size of the tunnel segments also increases. Preload can effectively reduce the gap size of the tunnel segments and prevent excessive amplitudes of cyclic loading, thereby preventing issues such as water leakage at the segment joints, aging of rubber gaskets, fatigue damage to local components, and local tensile damage to concrete segments. (2) Preload exerts a certain restraining effect on the radial deformation of the structure. As the degree of preload loss intensifies, the circumferential restraint capacity on the overall deformation of the structure gradually weakens, leading to an increase in the radial deformation of the tunnel segments. The Arias intensity, as a measure of structural damage, shows an increasing trend with the worsening of preload loss. This increasing trend is more significant as the PGA increases. (3) The influence of seismic waves with PGA of 0.1 g and 0.3 g on the radial displacement of the structure remains relatively consistent. However, under the effect of seismic waves with a PGA of 0.5 g, it becomes evident that the irreversible radial deformation gradually increases with the intensification of preload loss. Compared to the static phase, the dynamic loads have a more pronounced effect on the irreversible deformation. (4) Under the influence of seismic loads with different PGA, the relative longitudinal settlement of the structure increases with the preload loss rate increases. This suggests that the application of preload has an optimizing effect on the reduction in longitudinal stiffness and the control of vertical deformation in the multi-ring structure under seismic loads. As the preload loss rate increases, the stiffness reduction gradually becomes more significant. The influence of preload loss on stiffness reduction is more pronounced in the initial stages of preload loss changes compared to the later stages. Additionally, as the preload loss rate increases, the rate of localized tensile damage to concrete segments significantly increases. This indicates that preload plays a crucial role in enhancing the overall structural performance. (5) After the application of seismic loads, local structural damage primarily occurs at the crown, shoulder, inner part of the arch bottom, outer part of the arch waist, and the top block. The main type of damage observed is tensile damage to the concrete segments. [PERSON] wrote the first draft of the paper and performed all the simulations. [PERSON] put forward technical route of numerical simulation. [PERSON], [PERSON] and [PERSON] edited the paper and investigated the simulation work. [PERSON] and [PERSON] provide fund support and the method of data processing.