Secondary lining
| 5.0 | 8.0 | 8.0 | -- | html l1-- | MechanicsDamping uses Rayleigh damping commonly used in engineering, because the rock mass is a weak surrounding rock, and the bottom of the boundary condition model uses a viscous boundary, which converts the velocity time range into a stress form input, and is surrounded by a free field boundary. Because there is no direct measured aftershock wave, the main seismic seismic wave in the tunnel area (Wenchuan Qingping seismic wave) is selected as the earthquake input in reference [11]. Generally speaking, the obtained seismic wave record is an acceleration time course curve. This time, the amplitude of the seismic wave is taken to be 0.33 g, as shown in Figure 9. The integration is converted into the speed time, and then the stress is converted into the bottom of the model by using formulas (1) and (2). σn=-2( ρ·CP)vn)vn) (1) σs=-2( ρ·CS)vs)vs (2) where: σn is normal stress; σs is shear stress ; ρ is media density; CP is media density; CP is media html of the html of the medium 2P wave velocity; CS is the S wave velocity of the medium; vn is the particle velocity in the vertical direction; vs is the particle velocity in the horizontal direction.  Figure 9 Input seismic wave acceleration time course curve 3.2 Consider the working conditions and monitoring scheme First consider three excavation methods: up and down step method, up and down step reserve core soil method, and three-step method to explore the impact of the excavation method on the dynamic response under the aftershock, and consider the extrusion effect of the soil in front of the palm, among which the upper step excavation reached 43.5 m, as shown in Figure 10. In view of the actual construction method of tunnels - the core soil method of reserved on the upper and lower steps, considering two processes during the construction process: the initial support construction completion and the secondary lining construction completion, to compare the seismic effects, surrounding rock-support dynamic response and structural safety encountered at different stages of construction. During the aftershock process, 8 positions at the initial support and secondary lining of the 10 m section were monitored, and the measurement points at different heights of the soil were monitored, as shown in Figure 11, where S1~S8 is the tunnel monitoring points and T1~T9 is the soil monitoring points. 4 Numerical calculation results and analysis 4.1 Surrounding rock-support dynamic response characteristics under different excavation methods 4.1.1 Displacement characteristics analysis The displacement difference of adjacent parts of the structure is an important basis for judging structural earthquake response. During the earthquake, the displacement of 8 measurement points at 10 m sections around the cave was monitored. The results showed that it generated the largest displacement difference between the tunnel arch and the center of the arch, and the displacement difference time course curve of different excavation methods was drawn, as shown in Figure 12. The change curves of the periphery of the tunnel tunnel under different excavation methods are similar, and the maximum displacement difference is close to 12.35 s (the peak point of positive acceleration of seismic waves). The maximum displacement difference between the three methods of the upper and lower step method, the core soil method of the upper and lower step reserve, and the three-step method is 1.00 cm, 0.86 cm, 1.04 cm, indicating that under the action of aftershock, the overall displacement difference of the tunnel is small, and the displacement difference of the three methods is not large, and the displacement difference shows a trend of the three-step method to reserve core soil methods for up and down steps. 4.1.2 Acceleration Characteristic Analysis Monitor the X direction acceleration in the tunnel during earthquake, and extract the acceleration time course curve of the tunnel arch and soil body at the same height, as shown in Figure 13. It can be seen from Figure 13 that the acceleration time course curve of the tunnel arch is almost the same as the surrounding rock at the same height, and is similar to the input acceleration time course curve, indicating that the implementation of the tunnel structure has little impact on the seismic dynamic response of the surrounding rock, which is the same as the conclusion obtained by Lai Jiongcheng [10], that is, the tunnel structure has obvious followings the response to the stratigraphic acceleration. Figure 10 Schematic of excavation method Figure 11 Monitoring point layout Figure 12 Tunnel displacement difference time course curve under different excavation methods Figure 13 Direction acceleration time curve of the initial branch and surrounding rock under different excavation methods is to compare the peak response of acceleration of different elevations in surrounding rocks. Under the action of aftershock, the peak acceleration of the X direction at the soil monitoring point at different locations is recorded, as shown in Figure 14. It can be seen from Figure 14 that as the distance from the bottom of the soil (the position of seismic wave application), the acceleration response gradually increases. Different excavation methods have little impact on the peak of the acceleration response, indicating that the soil acceleration response is mainly affected by the incident seismic wave and the rock body itself. Figure 14 The distribution of the plastic zone after the aftershock of the excavation method is shown in Figure 15. It can be seen from Figure 15 that the damage caused by rock mass is mainly shear failure, and the soil in front of the palm is damaged in large quantities. The plastic area of different excavation methods shows a tendency to reserve core soil method three steps for upper and lower steps, but the overall difference is not large. Figure 15 Plastic zones of different excavation methods 4.1.4 Initial support stress distribution Aftershock, the initial support stress mainly bears compressive stress. During the process, the main stress at the 8 measurement points of the initial support was monitored to extract its minimum main stress, as shown in Figure 16. It can be seen from Figure 16 that under different excavation methods, the initial support stress distribution is similar, and the initial support compression stress is relatively large at the arch shoulder and foot. Among them, the maximum initial support stress of the upper and lower steps method, the core soil method of the upper and lower steps reserved at the arch foot method and the three-step method are 34.29 MPa, 33.53 MPa, and 33.35 MPa respectively. This shows that the excavation method has little impact on the initial support under the action of aftershocks, but it exceeds the compressive strength of concrete , which means that under the action of earthquake, the initial support is in a relatively unfavorable stress state, and concrete may fall. Figure 16 Initial support stress distribution under different excavation methods 4.2 Characteristics of surrounding rock-support dynamics in different construction stages 4.2.1 Displacement and acceleration characteristics analysis Draw a time chart of the displacement difference of tunnels in different construction stages, as shown in Figure 17. The change curves of the displacement difference of the tunnel periphery under different construction stages are similar. The maximum displacement difference between the initial support and the initial secondary lining are 0.86 cm and 0.61 cm respectively. The application of the secondary lining reduces the tunnel displacement difference by 17.44%, indicating that the application of the secondary lining is conducive to reducing the displacement difference caused by earthquake effects around the tunnel. Figure 17 Time course curve of tunnel displacement difference in different construction stages is the same as excavation method. The peak acceleration in the direction of different positions of the surrounding rock X (see Figure 11 for the surrounding rock measurement points), as shown in Table 3. It can be seen from Table 3 that the application of the second lining has little effect on the acceleration response of the surrounding rock, indicating that the application of the tunnel structure has no effect on the acceleration response of the surrounding rock, which is the same as the conclusion drawn in the previous article. 4.2.2 Analysis of initial support stress From the previous article, it can be seen that under the action of an earthquake, the initial support is mainly under pressure, and the initial support compressive stress takes the maximum value at the arch foot. Therefore, the minimum main stress time curve of the initial support left arch foot is drawn in different construction stages, as shown in Figure 18.It can be seen from Figure 18 that the initial support compressive stress changes in different stages of construction are similar. The maximum compressive stress of the first support left arch foot under the combined action of the initial support and the initial support second lining are 33.53 MPa and 28.12 MPa respectively. The application of secondary lining reduces the initial support stress by 16.13%, indicating that the second lining is conducive to sharing the stress of the initial support. Table 3 Weiyan X test point | T1 | T2 | T2 | html ml1T3 | T4 | T5 | T6 | T6 | T7 | T8 | T9 |
initial branch effect
| | .10 | | .06 | | .06 | | .83 | .80 | .72 | .64 | .61 | .39 | .04
initial branch two lining joint
| | .11 | | .11 | | .06 | | .83 | .80 | .73 | .65 6 | .62 | .38 | .04 Figure 18 The minimum main stress time curve of the first support left arch foot at different construction stages 4.3 Internal force response analysis of lining structure Under the joint working condition of the initial support second liner of the tunnel, the peak of the internal force of the tunnel was extracted and the results of the internal force of the second liner excavated to this section were compared, as shown in Figures 19 and 20. From Figures 19 and 20, it can be seen that under the action of static power, the tunnel bending moment is symmetrically distributed along the tunnel axis, and the extreme value is obtained at the left and right arch feet; the axial force is symmetrically distributed along the 45° direction of the tunnel, and the extreme value is obtained at the left and right arch shoulders. Under the dynamic case, the maximum bending moment is 278.09 kN·m, and the maximum axial force is 4 129.73 kN. Under the action of power, the bending moment of the tunnel increases sharply, which is most obvious at the arch shoulder. The dynamic bending moment is 8.19-8.49 times the static bending moment. The remaining parts increase by 0-2 times, and the dynamic axial force growth value is less than 1 times, which is prone to large eccentricity and instability failure. Figure 19 Two-lined bending moment peak Unit: kN·m Figure 20 Two-lined shaft force peak Unit: kN Unit: kN Highlight the safety coefficient of each part of the second lining during the tunnel earthquake based on the internal force value of the second lining, and extract its minimum value, as shown in Figure 21. The tunnel safety factor is smaller at the right arch shoulder and the left arch foot, which is 2.83 and 2.80 respectively, both greater than the safety factor specified in the "Highway Tunnel Seismic Design Code" [16]. Compared with the static effect, the minimum safety factor is reduced by 16.91%, indicating that under the aftershock, the two-lined structure that has been constructed in the tunnel is safer, but seismic protection should be paid at the arch shoulder and arch foot. Figure 21 Tunnel two-lined safety factor 5 Conclusion This paper relies on the Lanjiayan Highland stress soft rock tunnel, firstly analyzes the internal force and safety of the tunnel's second-lined lining and its safety through on-site measured data, and then for different excavation methods: up and down step method, up and down step reserve core soil method, and three-step method, through numerical simulation, the impact of the tunnel excavation method on the surrounding rock-support dynamic response under the aftershock effect is studied, and the following conclusions are drawn. () The internal force of the tunnel second lining is similar to its security coefficient. In the first 0 d tunnel second lining internal force increases rapidly, causing the safety coefficient of the tunnel second lining to decrease. The change rate of 0~60 d gradually slows down, and the internal force and safety coefficient of the second lining after 0 d after is stable. Under the aftershock, the overall displacement difference of the tunnel is small, and the displacement difference shows a trend of the three-step method to reserve core soil methods for the up and down steps. The acceleration response of the tunnel structure has obvious follow-up to the acceleration response of the formation. The acceleration response of the surrounding rock is mainly affected by the incident seismic wave and the properties of the soil, and has nothing to do with the excavation method and the implementation of the tunnel structure. () The earthquake caused rock mass damage mainly by shear damage, and the soil in front of the palm is damaged in large quantities. The plastic area of different excavation methods shows a tendency to reserve core soil method three-step method up and down step methods, but the overall difference is not large. Under the action of an earthquake, the initial support is subjected to greater compressive stress, and concrete may fall. () The combined effect of the first branch two linings is compared with the first branch alone. The tunnel displacement difference is reduced by 7.44%, and the maximum compressive stress of the initial support is reduced by 6.13%, indicating that the application of the second lining is conducive to reducing the displacement difference caused by earthquakes around the tunnel and sharing the initial support force. Under the aftershock of (| ) the tunnel bending moment increases sharply, which is most obvious at the shoulder. The dynamic bending moment is | .19~8.49 times of the static bending moment. The remaining parts increase by ~2 times, and the dynamic axial force growth value is within 1 times of html, which is prone to large eccentricity and instability damage. Compared with static effects, aftershock effects reduce the minimum safety factor by 6.91%. A smaller value is obtained at the right shoulder and the left foot of the arch, and attention should be paid to seismic protection. References [1] Zhu Anlong. Large deformation mechanism of railway tunnels with high ground stress steep tilts of Qianzi Rock formations[J]. Modern Tunnel Technology, 2019, 56(S2):231-238. [2] Lai Hongpeng, Yang Wanjing, Xie Yongli. Deformation and loading characteristics of soft rock large deformation bias highway tunnel [J]. Journal of Central South University: Natural Science Edition, 2014, 45(6):1924-1931. [3] Xu Guowen, He Chuan, Dai Cong, et al. Research on the mechanism and excavation method of soft rock tunnel under complex geological conditions [J]. Modern Tunnel Technology, 2017, 54(5):146-154. [4] Zhong Yujian, Miaomiao, Wang Yadong, et al. Monitoring and finite element simulation of deformation law of Qianmei Rock Tunnel in large span shallow buried bias [J]. Highway, 2019, (6):277-283. [5] Guo Xiaolong, Tan Zhongsheng, Li Lei et al. Research on the timing of secondary lining construction of Qianmei Rock Tunnel in highland stress [J]. Journal of China Highways, 2020, 33(12):249-261. [6] Wang Bo, He Chuan, Zhou Yi, et al. Seismic fracture characteristics of weak rock mass in strong earthquake zones and their problems faced in tunnel construction [C]//Tunnel Engineering Branch of China Highway Society, Chongqing Municipal Transportation Committee. Proceedings of the National Highway Tunnel Academic Conference in 2013, 2013. [7] Zhang Jing, He Chuan, Wang Bo, et al. Research on the aftershock dynamic response laws of tunnels under construction on the Guanggan Expressway [J]. Journal of Underground Space and Engineering, 2016, 12(1):268-274. [8] Zhao Wei. Analysis of dynamic effects of thousands of rocks surrounding rocks in Dujiashan Tunnel [D]. Chengdu: Chengdu University of Science and Technology, 2012. [9] Ling Yao. Research on the collapse mechanism and aftershock impact of soft rock tunnel in Guanggan Expressway [D]. Chengdu University of Science and Technology, 2012. [10] Lai Jiongcheng. Research on the dynamic response characteristics and stability of earthquakes during the construction period of the cracked rock tunnel [D]. Chengdu; Southwest Jiaotong University, 2013. [11] Xu Jinhua. Research on the instability mechanism and safety control measures of soft rock tunnels in the cracked rock tunnel area [D]. Chengdu: Southwest Jiaotong University, 2014. [12] JTG 3370.1-2018 Highway Tunnel Design Specification Volume 1 Civil Engineering[S]. [13] Niu Hongtao. Research on the technology of tunnel forming a complex surrounding rock tunnel in the loess-covered soil-rock contact zone [D]. Xi'an: Xi'an University of Technology, 2010. [14] Huo Runke, Wang Yanbo, Song Zhanping, et al. Analysis of initial support performance of loess tunnels [J]. Geoscience Mechanics, 2009, 30(S2):287-290. [15] Dai Cong. Research on excavation and support of soft rock tunnels in highland stress fields [D]. Xi'an: Southwest Jiaotong University, 2018. [16] JTG/T 2232-01—2019 Seismic design specifications for highway tunnels [S].
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