1 Introduction
The tunnel between Guomao Station and Laojie Station of the Shenzhen Metro Phase I project is located in the prosperous Guomao and Laojie areas of Shenzhen, with ground roads crisscrossing it. Due to the influence of the subway station location and the foundation of the buildings along the line, the right and left lines of the section are arranged in an upper and lower overlap. The tunnel structure is designed as a unique single-hole double-layer overlapping structure, and is constructed using the shallow burial and concealed excavation method.
Due to the complex stratigraphic effects and structural mechanical behavior caused by the construction of single-hole double-layer structures, this will bring great difficulties to the structural safety and surface settlement control during construction. Therefore, it is very necessary to understand its stress mechanism and control of surface settlement. This paper studies it through three aspects: numerical analysis, model testing and field measurement.
2 Numerical simulation analysis of overlapping tunnel construction
2.1 Numerical analysis method
The simulation calculation of single-hole double-layer overlapping tunnel construction uses the large-scale finite element software ANSYS. The excavation and lining construction are simulated through the method of unit activation and change of material parameters. The calculation uses a range of 80mx 80m. The boundary conditions are all displacement boundary conditions. The upper boundary is the ground, which is a free surface. Both sides and bottom surfaces are subject to normal constraint.
For the construction of overlapping tunnels, the upper layer is constructed first and then the lower layer is constructed. The specific steps are as follows: ① Calculation of the self-gravity stress field of the surrounding rock; ② Advance support and excavation of the upper step; ③ Bolt and shotcrete support of the upper step Protect and construct temporary inverts; ④ Perform advanced support and excavate the lower steps; ⑤ Perform anchoring and shotcrete support and construct the lower steps; 6 to 9. Excavate the lower layer, the same as 2 to 5 above.
2.2 Analysis of calculation results
2.2.1 Lining internal force
See Table 1 for the superstructure.
2.2.2 Surface settlement
Through calculation, the surface displacement is small, with a maximum value of only 6.7mm, which will not cause major damage to the surface. The calculation results are shown in Figure 1.
This article also calculated and analyzed the working conditions from bottom to top. When the lower hole is excavated first, the surface settlement is about 50% of the total surface settlement, but the total surface settlement is excavated from bottom to top. It is more scientific and reasonable to use the top-down excavation method in the construction of a single-hole double-layer tunnel than to excavate from top to bottom. It is more scientific and reasonable to pre-support the arch to form a support arch, and then excavate the lower part under its protection. of.
3.1 Introduction to Plane Model Test
The geometric similarity ratio of the planar model is taken as 10. The plane test model is based on the tunnel cross-section size and burial depth, taking into account boundary effects, and the size of the model slot is 3.40m (width) x 3.70m (height) x 0.48m (thickness).
The test strictly simulated the construction process, and the positive step construction method was used for the excavation of the single-hole double-layer structure.
3.2 Plane model test result analysis
3.2.1 Lining internal force
The test results of lining bending moment and lining axial force are shown in Figure 2 and Figure 3. In the figure, the model bending moment unit is N·m, the severe bending moment unit is kN·m,
The model axial force unit is N, and the prototype axial force unit is kN.
Through the analysis of internal forces, it can be seen that the construction of the top first and then the bottom can better play the role of the arch reinforcement ring and the lining of the upper hole. It should be large, but it can make the upper hole lining fully play its role in the weak surrounding rock and protect the excavation of the lower hole. When constructing a single-hole double-layer structure first up and then down, the wall base should be widened to prevent excessive pulling force.
3.2.2 Surface settlement
The surface settlement curve is shown in Figure 4 (the prototype values ??are in brackets in the figure). The test results show that the settlement in the first step of single-hole double-layer excavation accounts for the vast majority of the total value, and there is almost no overlap in subsequent excavations, and the effect is very obvious.
4 On-site measurement analysis
4.1 Surrounding rock pressure
The changing pattern of surrounding rock pressure is relatively obvious. As the surrounding rock is further disturbed and relaxed by the excavation of the adjacent steps, the surrounding rock pressure at the vault and arch foot has been increasing, but the increase rate is small, which is basically synchronized with the rate of vault sinking.
As the excavation surface advances forward, the surface subsidence of the section, the subsidence of the vault, and the internal space displacement tend to converge, and the surrounding rock pressure also tends to be stable. The surrounding rock pressure at the vault is 0.13MPa. If the shallow tunnel is The calculated value of the overlying soil pressure is 0.29MPa, which shows that the "arch effect" of the reinforcement circle is obvious. The same rock pressure at the arch foot is 0.23MPa. After the deformation stabilizes, the surrounding rock pressure at each measuring point of the entire section and its distribution are shown in Figure 5. The value range is 0.15~0.23MPa, which shows that the structure has a certain degree of safety.
4.2 Surface subsidence
The proportion of settlement in each stage of construction to the total subsidence is: advanced subsidence 17, upper step excavation 34, second step excavation 29, three-step excavation 13, four-step excavation 7. According to the 13 measuring points arranged on the ground surface, the influence range of tunnel excavation on the cross-section surface settlement is about 1.5D from the center line of the tunnel. The settlement distribution in the cross-sectional direction caused by the excavation of each step is shown in Figure 6. Although the actual measured surface settlement value is greater than the calculation, test and design benchmark value (30mm), the rule that the settlement caused by the excavation of the upper hole accounts for the majority and the superposition of the lower hole is very small is consistent with the test and calculation, reflecting the upward first and then the lower. benefit.
5 Conclusions and Suggestions
(1) For overlapping tunnels, the process has been optimized through numerical simulation analysis, and it is recommended to adopt the first-up-then-down construction method to reduce surface settlement;
(2) The numerical analysis was verified through model tests and on-site measurements. The rule that the excavation of the upper tunnel of the overlapping tunnel caused the majority of the settlement and the superposition of the lower tunnel was very small is consistent with the numerical calculation. The "arch protection effect" of the upper tunnel lining "Obviously, it reflects the benefits of going up first and then going down;
(3) The surface settlement value measured on site is greater than the calculated and experimental value. This is because of precipitation, steps, surrounding rocks and construction closure time. caused by differences.
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