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| Number | 1 | 2 | 3 | 4 |
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| Name of tunnel | Sikewan Tunnel of Lanhai Expressway, Gansu Province [42] | Beijing metro line 9 [37] | Gangudun Tunnel, Longnan, Gansu Province [43] | Chengdu metro line 1 [44] |
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| Geological survey | Medium-dense gravel grounds are a foot-slope accumulation layer with high obliquity and large porosity. Clastic media such as sandy soil, mudstone, and mudstone are filled between cobble and gravels, which have poor cementation, strong permeability, well-developed groundwater, and large water content. | The excavation of Q4 cobble bed and Q3 strongly weathered conglomerate bed has large cobble size, high content, and good cementation. In the whole conglomerate layer, the thickness of the dome conglomerate layer is 1.4–1.6 m, the groundwater is abundant, and the conglomerate layer is hard. | The tunnel passes through strata of Q4 diluvial fine breccia soil, alluvial coarse sand, fine cobble soil, and coarse cobble soil, and underlying bedrock is phyllite and limestone. | In dense sand and cobble grounds, the average diameter of cobble is 130 mm and the maximum diameter is 400 mm. Tunnel is located below the groundwater level without precipitation. It is easy to collapse without precipitation. The stability is greatly improved after precipitation. The permeability coefficient is 2.0∗10−2 cm/s. When the excavation span is 3 m, the face has 10-hour self-stabilization ability, which is conducive to construction. |
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| Engineering problems | (1) Ring and oblique cracking of the initial branch, with the maximum crack of 6 mm.
(2) The cobble is easy to collapse from the leading small catheters, and the cobble above the arch waist is easy to fall off and collapse on a small scale.
(3) During the construction of arch frames on both sides, groundwater in the side wall flows out in a linear or femoral shape, and the water accumulation is serious. | Because the hardness of conglomerate layer is bigger and smaller, the grouting effect is unsatisfactory, and there are many continuous seepage phenomena in the tunnel, and the original design of advance small pipe grouting is not applicable. | A large amount of water is produced between the sandy cobble soil layer and phyllite layer, which concentrates on the arch foot position on both sides of the upper and middle guide. The water output is more than 700 m3/d, and the settlement deformation is large. | (1) Tunnels are located below the groundwater level and collapse easily without precipitation.
(2) Φ42 small conduit is easy to deflect and deviate when it is penetrated. It can be solved by using PH150 bolt drill and erecting steel cone head. |
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| Construction/excavation method | (1) Shallow burial and subsurface excavation, upper and lower step excavation, and buried depth of about 15–25 m.
(2) The maximum deformation is over 300 mm.
(3) After strengthening the support, the deformation is reduced by more than 50%. | Shallow buried excavation, upper and lower step excavation, and upper step reserved core soil. | Construction of three steps and seven parts excavation method. | (1) Shallow excavation method with a depth of 13 m.
(2) Reserved deformation of 50 mm, 0.5 m per foot.
(3) CD excavation, the ground settlement is less than 10 mm, the settlement in the tunnel is less than 5 mm, and the convergence is less than 10 mm.
(4) The upper and lower steps are excavated with reserved core maps, and the lower steps are divided into left and right sections. The deformation of the lower steps is very close to that of the CD method, but the construction disturbance is smaller.
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| Support system | The original design adopted Φ4.2 m long 42 small conduit, system bolt was 3.5 m long D25 grouting bolt, I20a steel spacing was 0.75 m, reserved deformation was 12 cm, and C25 reinforced concrete was used as secondary lining. (2) Later stage support was strengthened, leading small conduit was changed to double layer, length was adjusted to 1.2 m and 2 m, steel frame spacing was 0.5 m, and feet reinforcement bolt group was changed to double group. | The penetration and grouting tests ofΦ25, Φ30, and Φ32 self-propelled bolts were carried out in the tunnel, and YT28 gas leg drill was used to penetrate the bolt. It was found thatΦ30 was the most suitable, and the air outlet of the bit was optimized. | Composite lining design of general mountain tunnel | (1) The original design of 3.5 m Φ42 small conduit for advance anchor rod, lock foot anchor rod is Φ32 self-drilling anchor rod, the second lining is 40 cm reinforced concrete, and I22 I-shaped steel.
(2) After precipitation, the grounds are stable, and the unsystematic bolt reduces the disturbance, which is beneficial to the ground stability.
(3) The test of steel frame internal force and surrounding rock pressure shows that the maximum of steel frame is only 12 kN, the earth pressure is only 80 kPa, the surrounding rock pressure is small, and the supporting structure is safe. |
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| Technology summary or breakthrough | Water-rich sandy cobble strata are summarized as follows: small footage excavation, strong support means, appropriate grouting technology, strictly prohibit blasting, and manual or mechanical excavation is needed to reduce disturbance and destruction of strata.
High water content leads to the whole ground softening and loss of bearing capacity, which is the cause of large deformation and lining cracking. | The original design of vault 1.8 m long Φ25 small conduit was changed to 2.5 m long Φ30 self-propelled bolt, the water outlet of vault was improved, and the effect of grouting reinforcement was obviously improved. | The whole section radial grouting technology is adopted to stop water. The grout is cement water glass slurry, and the number of rock bolts is strengthened. The deformation of surrounding rock is improved. | (1) It is necessary to dewater before excavation below the groundwater level.
(2) The deformation of surrounding rock by the CD method is similar to that by upper and lower step method, and it does not exceed the reserved deformation.
(3) The construction speed of the step method is faster and the disturbance to construction is smaller, so priority should be given to it.
(4) Sand and cobble strata have good stability, and less disturbance should be given priority, followed by strong support. |
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