Unequal-span small-clear-distance tunnel construction method suitable for weak surrounding rock

Abstract

The invention relates to the technical field of tunnels, in particular to an unequal-span small-clear-distance tunnel construction method suitable for weak surrounding rock. According to the unequal-span small-clear-distance tunnel construction method, the principle that a large section is advanced is followed, a tunnel with the large section is excavated through a middle partition wall method firstly, then a tunnel with the small section is excavated through the middle partition wall method, and it is guaranteed that the distance from the tunnel face of the large-section tunnel to the tunnel face of the small-section tunnel is 2-3 times of the clear width of the large-section tunnel. The unequal-span small-clear-distance tunnel construction method follows the principle that a large section is advanced, ground surface settlement and the area of a plastic zone of surrounding rock can be effectively reduced, and engineering safety is guaranteed. Compared with an existing tunnel construction method, the unequal-span small-clear-distance tunnel construction method has the remarkable advantages of maintaining the stability of the surrounding rock, the construction cost, the construction safety and the construction period, and the construction efficiency is improved on the premise that the stability of the surrounding rock is maintained.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnels, and in particular to a construction method for small clear - span tunnels with unequal spans applicable to soft surrounding rocks. Background Art

[0002] With the increasing number of Yue - ling tunnels, many tunnels cannot meet the excavation requirements of conventional separated tunnels with equal spans. This requires the application of the excavation mode of small clear - span tunnels with unequal spans. Generally, a small clear - span tunnel refers to a tunnel where the clear distance between two adjacent tunnels is less than 1.5 times the tunnel diameter. On the other hand, the geology in the western region of China is complex, and some tunnels need to pass through areas of soft and fractured surrounding rocks or fault zones. Due to the insufficient self - stability of the surrounding rocks, there are the following difficulties in tunnel construction: 1. Most of the projects are small clear - span tunnels with unequal spans. Due to the different cross - sectional sizes of the left and right tunnels, tunnel excavation will cause ground stress concentration, resulting in uneven pressure distribution on the tunnel. Moreover, the stress state and stability of the surrounding rocks of small clear - span tunnels with unequal spans are different from those of small clear - span tunnels with equal spans, and the excavation difficulty is relatively large.

[0003] 2. When planning the route of small clear - span tunnels with unequal spans in mountainous areas, it is often inevitable to pass through complex geological environments. Due to the insufficient self - stability of the surrounding rocks or insufficient support measures, tunnel construction often faces the risks of collapse or roof fall.

[0004] 3. When the face distance between the leading tunnel and the trailing tunnel is relatively close, the leading tunnel and the trailing tunnel will simultaneously affect the middle rock pillar during tunnel construction, which is not conducive to the stability of the middle rock pillar and will pose serious safety hazards to the tunnel; while when the distance is too far, it will invisibly extend the construction period and indirectly increase the construction cost.

[0005] 4. Tunnels are restricted by mountainous alpine regions. There is perennial snow accumulation in high - altitude areas. During the spring construction of cross - mountain tunnels, they are easily affected by snowmelt. The infiltration of snowmelt water causes great harm to the stability of slopes and is prone to engineering accidents. Summary of the Invention

[0006] In order to overcome the deficiencies in the background art, the present invention discloses a construction method for small clear - span tunnels with unequal spans applicable to soft surrounding rocks, and its purposes are as follows: 1. Control the settlement of soft surrounding rocks at the small clear - span tunnel part to ensure project safety; 2. During the construction process, ensure the stability of the middle rock pillar at the small clear - span tunnel part; 2. Improve construction efficiency.

[0007] To achieve the above - mentioned invention purposes, the present invention adopts the following technical solutions: A construction method for small - net - distance tunnels with unequal spans applicable to soft surrounding rocks follows the principle of excavating the larger - section tunnel first. First, the larger - section tunnel is excavated using the middle - diaphragm method, and then the smaller - section tunnel is excavated using the middle - diaphragm method, and it is ensured that the distance from the face of the larger - section tunnel to the face of the smaller - section tunnel is equal to 2 - 3 times the net width of the larger - section tunnel.

[0008] Furthermore, improving the technical solution, the tunnel excavation construction using the middle - diaphragm method follows the principles of short advance - length, strong support, early closure, and frequent measurement. The advance - length is 0.5 - 0.7m, and one steel frame is erected for each advance - length.

[0009] Furthermore, improving the technical solution, during the tunnel excavation construction using the middle - diaphragm method, the bench length is controlled within 8 - 10m, the longitudinal spacing between the excavation working faces on the left and right sides of the pilot tunnel of the larger - section tunnel is greater than 30m, and the longitudinal spacing between the excavation working faces on the left and right sides of the pilot tunnel of the smaller - section tunnel is greater than 15m.

[0010] Furthermore, improving the technical solution, during the tunnel excavation construction using the middle - diaphragm method, the temporary support of the middle - diaphragm uses I - beams as stiffening arch frames and is connected to the steel frames.

[0011] Furthermore, improving the technical solution, after the initial support of the larger - section tunnel is completed and its strength reaches 70%, the excavation of the smaller - section tunnel can be carried out.

[0012] Furthermore, improving the technical solution, after the initial support construction is completed, the temporary middle - diaphragm is removed section by section, and the removal length of each section of the temporary middle - diaphragm is less than 5m.

[0013] Furthermore, improving the technical solution, during the tunnel excavation construction process, the ground settlement and crown settlement are monitored, and the monitoring frequency is at least once a day.

[0014] Furthermore, improving the technical solution, during the tunnel excavation construction process, pipe - shed is used as the advanced support, and the middle - rock pillar is reinforced by grouting the hollow anchor rods.

[0015] Furthermore, improving the technical solution, before the tunnel excavation, the pipe - shed grouting advanced support is arranged in the portal section.

[0016] Furthermore, improving the technical solution, anti - skid piles and catch - water ditches are set on the side of the tunnel with a slope.

[0017] After implementing the above - mentioned technical solution, compared with the background technology, the beneficial effects produced by the present invention are as follows: 1. Following the principle of excavating the larger - section tunnel first, excavating the larger - section tunnel first and then the smaller - section tunnel can effectively reduce the ground settlement and the area of the plastic zone of the surrounding rocks, ensuring the project safety; 2. Using the middle - diaphragm method to excavate the tunnel following the principle of excavating the larger - section tunnel first has significant advantages in terms of the stability of the surrounding rocks, construction cost, safety, and construction period; 3. Practice has proved that when the distance between the heading face of the large-section tunnel and the heading face of the small-section tunnel is equal to 2-3 times the net width of the large-section tunnel, the construction efficiency can be improved on the premise of maintaining the stability of the surrounding rock. Brief Description of the Drawings

[0018] Attached Figure 1 shows the surface settlement curves of the first excavation of different sections.

[0019] Attached Figure 2 shows the distribution state diagram of the plastic zone of the surrounding rock when different sections are excavated first.

[0020] Attached Figure 3 shows the change curves of the crown settlement with the increase of the excavation depth under different excavation methods.

[0021] Attached Figure 4 shows the schematic diagram of the transverse construction process of the excavation by the middle diaphragm method.

[0022] Attached Figure 5 shows the schematic diagram of the longitudinal construction process of the excavation by the middle diaphragm method.

[0023] Attached Figure 6 shows the horizontal displacement difference curve of the middle rock pillar.

[0024] Attached Figure 7 shows the distribution diagram of the plastic zone of the middle rock pillar under different tunnel heading face spacings. Detailed Embodiment

[0025] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principle of the present invention and are not intended to limit the protection scope of the present invention. It should be noted that in the description of the present invention, the terms indicating the direction or positional relationship such as "front", "rear", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. are based on the direction or positional relationship shown in the accompanying drawings, which is only for the convenience of description and should not be construed as a limitation to the present invention.

[0026] Taking the Wuyi Expressway Tunnel located in the mountainous area east of Baluntai Town, Hejing County, Bayingolin Mongol Autonomous Prefecture, Xinjiang as an example, the present invention discloses a construction method for unequal-span small clear-distance tunnels applicable to soft surrounding rocks.

[0027] The tunnel site area of the Wuyi Expressway Tunnel belongs to the high-middle mountain and canyon geomorphic area. The Quaternary system is widely distributed in the surrounding valleys. The surface of the tunnel section is mainly bedrock. In the valleys, it is mainly composed of fresh alluvial-colluvial gravel and boulders. Locally distributed on the slope is fresh residual-colluvial gravel, and the underlying bedrock is the Variscan intrusive granite. The tunnel portals are mainly medium-strong weathered granite, with the surrounding rock grade of IV-V. The strongly weathered granite has a massive structure, well-developed joints and fissures, and the rock mass is relatively broken. The stability of the surrounding rock is very poor. The entrance slope and the side slopes on both sides are prone to collapse, resulting in difficulties in the construction of entering the tunnel. If the excavation plan or support measures for entering the tunnel are not selected reasonably, it is extremely easy to cause engineering accidents such as the collapse of the portal section or the landslide of the side slopes. Therefore, both the stability of the tunnel surrounding rock and the problem of snow cover on the slopes are construction difficulties.

[0028] The planned and designed Wuyi Expressway Tunnel is a small clear distance tunnel with unequal spans. The left tunnel is three lanes (with an additional climbing lane), with a clearance of 15×5m and a total length of 780m. The left tunnel is a large-section tunnel. The right tunnel is two lanes, with a clearance of 11×5m and a total length of 738m. The right tunnel is a small-section tunnel. The distance between the entrance lines is 15.4m, the distance between the exit lines is 15m, and the maximum buried depth is about 141m.

[0029] To solve the construction difficulties pointed out in the background technology, the construction method of this unequal-span small clear distance tunnel follows the principle of advancing the large section first. First, the middle diaphragm method is used to excavate the tunnel with a larger section, and then the middle diaphragm method is used to excavate the tunnel with a smaller section, and it is ensured that the distance from the face of the large-section tunnel to the face of the small-section tunnel is equal to 2-3 times the clear width of the large-section tunnel.

[0030] After the tunnel is excavated, in the evaluation index of the stability of the surrounding rock, the ground settlement plays a crucial role. To reduce the ground settlement, first, according to the actual engineering situation, the finite element software MIDAS GTS NX is used to establish a calculation model.

[0031] Refer to Appendix Figure 1 , Appendix Figure 1 shows the ground settlement curves of different sections excavated first. It can be seen from Appendix Figure 1 that when the large-section tunnel on the left is excavated first, the ground settlement value is smaller, and when the small-section tunnel on the right is excavated first, the ground settlement value is larger. It is not conducive to controlling the ground settlement when the small-section tunnel on the right is excavated first.

[0032] Refer to Appendix Figure 2 , Appendix Figure 2The figure shows the distribution state diagram of the plastic zone of the surrounding rock when different cross-sections are excavated first. According to the analysis of the calculation model, the plastic zone of the large cross-section tunnel on the left is distributed at the arch waists on both sides, while the plastic zone of the small cross-section tunnel on the right is distributed near the tunnel perimeter. By comparison, when the small cross-section tunnel on the right is constructed as the pilot tunnel, the area of the plastic zone of the surrounding rock is larger than that of the large cross-section tunnel on the left excavated first, and at this time, the plastic zones of the left and right tunnels are connected, damaging the stability of the middle rock pillar.

[0033] Refer to Appendix Figure 3 , Appendix Figure 3 The figure shows the change curve of the crown settlement with the increase of the excavation depth under different excavation methods. It can be seen from Appendix Figure 3 that as the excavation depth increases, the change trend of the crown settlement value of the right tunnel under different construction schemes is generally the same. Generally, it first rises slowly, then increases sharply, and finally tends to be flat. Among them, when the middle diaphragm method is used for construction, the cumulative crown settlement of the right tunnel is the smallest, which is 17.49 mm; the double-side drift method is the second, which is 19.96 mm, an increase of 2.47 mm compared with the middle diaphragm method, an increase of 14.12%; when the three-step method is used for excavation, the crown settlement is the largest, which is 23.12 mm, an increase of 32.19% compared with the middle diaphragm method. The numerical simulation results show that: when the middle diaphragm method is used for tunnel excavation, the control effect on the crown settlement is the best, the double-side drift method is the second, and the three-step method is the worst.

[0034] From the perspective of the stability of the surrounding rock, the displacement of the surrounding rock caused by using the double-side drift method and the middle diaphragm method during construction is relatively small, and the distribution state of the plastic zone is relatively good. The two are not much different as a whole, and the double-side drift method is better than the middle diaphragm method. However, the displacement of the surrounding rock caused by the three-step method during construction is relatively large and the plastic zone is connected. From the perspective of construction organization analysis, when using the double-side drift method for construction, there are more working faces, and the mutual influence between processes is greater, and the construction period is longer; while the excavation face of the middle diaphragm method is slightly larger than that of the double-side drift method, which helps to speed up the construction progress; although the excavation face of the three-step method is very large, this tunnel is a shallow-buried small clear-span tunnel with a surrounding rock grade of grade Ⅴ. Using the three-step method for excavation is likely to cause the surrounding rock to become unstable and lead to disasters such as tunnel collapse. By comprehensively considering factors such as the stability of the surrounding rock, construction cost, safety, and construction period, the middle diaphragm method is finally selected for the excavation of the portal section.

[0035] Refer to Appendix Figure 4 and Appendix Figure 5 , Appendix Figure 4 The figure shows the schematic diagram of the transverse construction process of the middle diaphragm method excavation, Appendix Figure 5The figure shows the schematic diagram of the longitudinal construction process of the excavation by the middle diaphragm method. The middle diaphragm method is divided into the excavation of the left pilot tunnel and the right pilot tunnel, and each side of the pilot tunnel is further divided into two bench steps. The middle diaphragm method follows the principles of short advance, strong support, early closure, and frequent measurement. The mechanical-assisted manual method is adopted for the excavation of the surrounding rock. The advance per cycle is controlled according to the spacing of each steel frame, and the advance is 0.5 - 0.7m. One steel frame is erected for each advance.

[0036] During the construction of the tunnel excavation by the middle diaphragm method, the bench length is controlled within 8 - 10m. The longitudinal spacing between the excavation working faces on the left and right sides of the pilot tunnel of the large-section tunnel is greater than 30m, and the longitudinal spacing between the excavation working faces on the left and right sides of the pilot tunnel of the small-section tunnel is greater than 15m. The temporary support of the middle diaphragm uses 120a I-beams as stiffening arch frames and is connected to the steel frames. After the initial support of the large-section tunnel is completed and the strength reaches 70%, the excavation of the small-section tunnel can be carried out. After the construction of the initial support is completed, the temporary middle diaphragm is removed section by section, and the removal length of each section of the temporary middle diaphragm is less than 5m. In addition, the ground settlement and crown settlement are monitored during the tunnel excavation construction process, and the monitoring frequency is at least once a day.

[0037] The key to ensuring the overall safety of the construction of the small clear distance tunnel is to ensure the stability of the middle rock pillar. Especially in the geological environment where the surrounding rock is relatively weak, even if the distance between the heading faces of the left and right tunnels of the small clear distance tunnel meets the specification requirements, it is necessary to reinforce the middle rock pillar to ensure the tunnel safety. Therefore, pipe roofs can be used as the advanced support, and the middle rock pillar can be reinforced by the method of grouting with hollow anchor rods. Grouting reinforcement is to inject the slurry with a certain strength and cementing performance into the fissures or pores of the rock pillar body through a certain pressure and path, discharge the filled air and water, cement the loose or broken rock into a whole, improve the mechanical index of the rock, and thus enhance the bearing capacity of the middle rock pillar. Specifically, the anchor rods use D25 hollow grouting anchor rods with a length of 3.5m, the grouting uses cement mortar, the water-cement ratio is 1:1, and 5% of the expansion agent and 0.5 - 1% of the early strength water reducer are added.

[0038] Before the tunnel excavation, the advanced support of pipe roof grouting is arranged at the portal section, which improves the self-stabilizing ability of the surrounding rock and is beneficial to controlling the vertical deformation of the surrounding rock and the crown settlement.

[0039] Refer to Appendix Figure 6 , Appendix Figure 6 shows the horizontal displacement difference curve of the middle rock pillar. From Appendix Figure 6 it can be seen that the horizontal displacement difference of the middle rock pillar shows a trend of first increasing and then decreasing, which indicates that the excavation of the small-section tunnel can restore the deformation of the middle rock pillar to a certain extent. When the distance between the heading faces of the left and right tunnels is 15m, the displacement of the middle rock pillar caused by the construction is very small, while increasing or decreasing the distance will cause the horizontal displacement difference to increase, and the maximum horizontal displacement difference is 4.37mm. Therefore, in order to maintain the stability of the surrounding rock, the distance between the heading faces should be not less than 15m.

[0040] Refer to the attached Figure 7 , the attached Figure 7 shows the plastic zone distribution map of the middle rock pillar under different tunnel face spacings. From the attached Figure 7 , it can be seen that as the tunnel face spacing increases continuously, the difference in the plastic zone of the middle rock pillar becomes larger and larger. Let the net width of the left tunnel (large-section tunnel) be D. In this embodiment, D = 15m. When the tunnel face spacing is 0 - 1.6D, the plastic zone penetrates the entire middle rock pillar, resulting in severe deformation and extremely low stability. This is because the overlapping effect of the stress fields generated during the excavation process makes it difficult for the middle rock pillar to bear the huge pressure from the upper surrounding rock. When the spacing is 2D, the plastic zone no longer penetrates; when the tunnel face spacing reaches or exceeds 3D, the plastic zone is completely separated, and the core area is less damaged. At this time, the middle rock pillar shows better stability. Therefore, the reasonable tunnel face spacing should be not less than 2D.

[0041] By comparing and analyzing the ground settlement, crown settlement, horizontal displacement difference of the middle rock pillar, and plastic zone of the middle rock pillar under different tunnel face spacings, the reasonable tunnel face spacing under this surrounding rock condition is 30 - 5m, that is, 2 - 3 times the net width of the large-section tunnel. Therefore, when the tunnel face spacing of 30m is selected for construction at the tunnel site, the mutual disturbance caused by the excavation of the left and right lines of the tunnel is small, meeting the requirements of safe construction.

[0042] During the construction of the small clear-span tunnel with unequal spans, the stability of the slope is related to the safety of the entire tunnel. The entrance section of the tunnel is in a shallow-buried and bias-pressure terrain. In Xinjiang, winter comes earlier, there is more snowfall, and the snow depth is large. Even if the clear distance of the tunnel meets the corresponding design specifications, corresponding reinforcement measures should be taken. The tunnel slope is a large-deformation concentrated area. Therefore, anti-skid piles are set on the side of the tunnel with a slope, and a catchment ditch is used to intercept and treat the snowmelt.

[0043] The parts not described in detail are the prior art. Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The protection scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks, characterized by: Following the principle of excavating large sections first, the middle partition wall method is used to excavate tunnels with larger cross-sections first, and then the middle partition wall method is used to excavate tunnels with smaller cross-sections, and it is ensured that the distance from the heading face of the large-section tunnel to the heading face of the small-section tunnel is equal to 2-3 times the net width of the large-section tunnel.

2. A method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: The tunnel excavation construction of the next-door method follows the principles of short advance, strong support, early closure and frequent measurement. The advance is 0.5-0.7m, and a steel frame is built for each advance.

3. A method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 2, characterized in that: During the tunnel excavation construction using the next-door method, the step length is controlled at 8-10m, the longitudinal spacing between the excavation working faces on the left and right sides of the large-section tunnel pilot pit is greater than 30m, and the longitudinal spacing between the excavation working faces on the left and right sides of the small-section tunnel pilot pit is greater than 15m.

4. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 2, characterized in that: During the middle partition wall method tunnel excavation construction, the temporary support of the middle partition wall uses I-beams as stiffening arch frames and is connected to the steel frame.

5. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: The excavation of small-section tunnels can only be carried out after the initial support of large-section tunnels is completed and the strength reaches 70%.

6. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: After the initial support construction is completed, the temporary middle partition wall is removed section by section, and the removal length of each section of the temporary middle partition wall is less than 5m.

7. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: During the tunnel excavation construction, the surface settlement and vault settlement shall be monitored at least once a day.

8. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: During the tunnel excavation and construction process, pipe sheds were used as advance support, and hollow anchor grouting was used to reinforce the middle rock columns.

9. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: Before tunnel excavation, pipe-roof grouting advance support is laid out at the entrance section.

10. The method for constructing tunnels with unequal spans and small clearances suitable for soft surrounding rocks as claimed in claim 1, characterized in that: Sliding piles and intercepting ditches are set on the side of the tunnel with a slope.

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