Taking Nanjing Metro 1 Line as the engineering background, through field tests, the mechanical behavior of segment structure of shield tunnel crossing viscous stratum during the whole construction process and stable period is systematically studied, and the finite element numerical simulation analysis is carried out by using beam-spring model considering the interaction between structure and stratum, and the results are compared and synthesized, and the design principles and methods of segment structure of subway shield tunnel under viscous stratum are put forward.
1 preface
The internal force and deformation calculation model of single-layer assembled segment lining structure of shield tunnel usually adopts homogeneous ring model. Although the joint effect of segment is considered in the beam-spring model, the actual load mode and the interaction mode between structure and surrounding rock are based on certain assumptions, and the results obtained by different parameters are quite different, and the specific interaction mode needs further study and determination [2]. In order to understand the mechanical behavior characteristics of shield tunnel segment structure in the construction process, this paper systematically studies the mechanical behavior of shield tunnel segment structure in the whole construction process and stable period through field tests, and compares it with the beam-spring model, and puts forward the design principles and methods of subway shield tunnel segment structure in viscous stratum.
2 test overview
2. 1 test section
The field test section is located at the pile number YK 1 3872 of TA 15 of Nanjing Metro1Line. The tunnel body is located in muddy silty clay, and the tunnel is covered with silty clay, with a thickness of about 4m. The surface layer is silty sand mixed with fine sand, and the buried depth of the tunnel is about 9m. See Figure 1 for geological survey. The shield tunnel in metro section adopts single-layer assembled reinforced concrete segments to construct the lining ring, with an inner diameter of 5.50m, a width of1.20m and a thickness of 0.35m. The lining ring consists of six sections, with the central angle of the covered section of 265,438+0.5, the central angles of the adjacent two sections of 68.0 and the central angles of the three standard sections of 67.5. Each section is provided with 65,438+06 longitudinal joints, which are arranged at an equal angle of 22.5. Segment ring is assembled with longitudinal staggered joints of 45, and lining segment ring is assembled and tested.
2.2 test content
The field test starts from the segment lining support ring until all test items are stable. The test contents include earth pressure, pore water pressure and internal force of segment structure of shield tunnel: XYJ-3 rigid string earth pressure box with a range of 0.3MPa is used to test earth pressure; The pore water pressure is measured by XJS-2 pore water pressure gauge with a range of 0.2MPa, and XJH-2 rigid string steel strain gauge with a range of 3000 microstrain is used to test the internal and external strain of the segment, and the internal force of the segment is obtained through the internal and external strain of the segment.
2.3 Layout of measuring points
There are 8 earth pressure, 8 pore water pressure and 16 pairs of internal force measuring points in the test target ring. The layout of measuring points is shown in Figure 3. The change of water pressure with the construction process is shown in Figure 4. As can be seen from Figure 4, within the range of 10 ring, the water pressure value acting on the target ring changes greatly from the start of the target ring support ring to the construction, and the water pressure value acting on the target ring tends to be basically stable after the construction range reaches beyond the target ring 20. In order of magnitude, the water pressure acting on the segment ring of the shield reaches 0.28MPa locally. As the maximum buried depth at the target ring is about 9m, it can be inferred that the water pressure of the segment ring from the starting point of the target ring to 10 measured by the pore water pressure gauge should also include other loads acting on the target ring. Figure 5 shows the water pressure distribution on the final target segment ring after the grouting pressure is stabilized.
3 Test results and analysis
3. 1 Water pressure change and distribution law
To sum up, due to the low permeability coefficient of cohesive soil, the grouting pressure can't dissipate quickly in the cohesive stratum. The water pressure acting on the shield segment ring is the superposition of grouting pressure and water pressure when the shield segment ring is constructed in a certain range (this paper thinks that the ring is about 15) and the construction range reaches a certain distance (this paper thinks that it is 40 ~).
3.2 Earth pressure change and distribution law
As shown in Figure 6, the target ring support ring changes with the earth pressure acting on the segment lining of the target ring after the test in the construction process. As can be seen from Figure 6, the changing law is basically the same as the water pressure. From the support ring of the target ring to the subsequent construction of the inner section of the 10 ring, the earth pressure acting on the target ring is relatively large, and the variation range of the earth pressure value is also relatively large. When the structural range reaches the target ring 30, it acts on the target segment ring. Figure 7 shows the earth pressure finally acting on the target segment ring.
To sum up, the segment lining is restricted by the grouting pressure at the tail of shield and the wrapping layer formed after the slurry hardens. When the shield segment is constructed in a certain range (about 15 ring in this paper), the earth pressure acting on the segment ring is the superposition value of grouting pressure and earth pressure. When the construction range reaches a certain distance (about 60 rings in this paper), with the grouting pressure gradually decreasing, that is to say, under the condition of cohesive stratum, the basic condition of stratum stability during construction is that the distance from the construction ring is greater than 60 rings, that is, about 12~ 14 days.
3.3 Variation and distribution law of segment internal force
As shown in figs. 8 and 9, the internal forces (axial force and bending moment) of the target ring segment change with the structure after the test. As can be seen from Figure 8 and Figure 9, due to the interaction of jack thrust, grouting pressure, formation pressure and assembly method, the internal force of the target ring segment is large in most positions, but with the continuous advancement of the shield machine, the jack thrust and grouting pressure gradually decrease, and the internal force value of the target ring segment gradually decreases and tends to. Figures 10 and 1 1 show the internal force distribution of the target section under typical working conditions. Figures 10 and 1 1 also prove the conclusions of Figures 8 and 9. At the same time, it can be seen from Figure 10 and Figure 1 1 that the internal force of segment ring of shield tunnel is characterized by staggered joints, whether at the moment of rigid support or at the final stage of stratum stability.
To sum up, under the specific conditions of cohesive stratum, the segment lining is restricted by the grouting pressure at the tail of shield and the coating formed after the slurry hardens. When the shield segment is constructed in a certain range (about 15 ring in this paper), the earth pressure acting on the segment ring is the superposition value of grouting pressure and earth pressure, and when the construction range reaches a certain distance (about 60 ring in this paper), it rises with the grouting pressure; In the tunnel, the longitudinal lining rings are connected by longitudinal joints with certain radial shear stiffness Kr and tangential shear stiffness Kt. The interaction mode between segment and surrounding soil is realized by radial and tangential springs, which can only be compressed around segment. These springs will separate automatically when they are in tension, and the stiffness of the springs is determined by the foundation resistance coefficient of the soil around the lining. The earth pressure acting on the tunnel is biased towards the total pressure of the safe soil column, and the lateral pressure is economical. The segmentation and loading mode is shown in figure 12. The parameters such as soil, assembly mode, joint stiffness, segment lining and buried depth are consistent with the field test.
4.2 Results and Analysis
Figures 13 and 14 list the results of finite element numerical simulation analysis of the water-soil beam spring model with high cost performance, and the measured values (black spots and values are field measured values) of the segment lining of the shield tunnel after stability are marked in the two figures. It can be seen from Figure 13 and Figure 14: ① Under the same assembling mode, the field test results are basically consistent with the theoretical analysis of the cost-effective water-soil beam spring model, and the position of the maximum internal force is basically the same, which shows that the load of the cost-effective water-soil beam spring model tunnel is basically the same as that of the field shield tunnel. (2) Under the same buried depth, the most unfavorable load of theoretical analysis of water-soil cost-effective beam spring model is smaller than the final stability result of field test, but it is close to the most unfavorable load of field rigid support. The main reason is that the beam-spring model ignores the dynamic load of jack thrust and grouting pressure at the tail of shield during construction, but adopts the assumption that the lining segment can bear all soil loads safely. It can be seen that it is reasonable to use the results of soil-water beam spring model with high cost performance as the design basis of shield tunnel segment lining under viscous stratum conditions in actual design.
5 conclusion
(1) Because the permeability coefficient of cohesive soil is small, the grouting pressure cannot disappear quickly under the condition of cohesive stratum, so the water and soil pressure acting on the segment ring is the superposition of the grouting pressure and the water and soil pressure within 20 circles behind the shield segment ring; When the construction scope reaches a certain distance (in this paper, it is 60 rings), the earth water pressure measured on site is the real earth water pressure acting on the segment structure.
(2) Due to the dynamic loads in the construction process, such as jack thrust and grouting pressure at the tail of shield, the internal force of segment lining reaches the maximum at the moment after the shield supports the segment ring, then decreases with the gradual reduction of various dynamic loads, and finally tends to be stable. Therefore, under the condition of cohesive stratum, the maximum internal force of segment lining in the whole construction process appears when segment ring support and grouting behind segment ring wall are completed.
(3) Under the condition of cohesive stratum, the load acting on the soil and water conservation beam-spring model tunnel has basically the same distribution law as that acting on the field shield tunnel. Although the dynamic loads such as jack thrust and grouting pressure at the tail of shield are neglected in the water-soil effect beam-spring model, the assumption that the lining segment can bear all soil loads safely is adopted, and the results of the water-soil effect beam-spring model are used as the design basis of shield tunnel segment lining under cohesive stratum conditions in actual design.
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