In the construction of Beijing Metro Line 5, the shield method was used for the first time to build a subway tunnel, which runs under the city center. Many of these areas are old cities and central business districts, and the control requirements for land deformation and land subsidence are extremely strict. Therefore, it is necessary to analyze the law of ground deformation and ground settlement in the process of shield tunneling, and take corresponding construction methods and technical measures to control it, so as to meet the environmental requirements in the process of shield construction.
1 Cause analysis of formation deformation
The basic causes of ground deformation caused by shield tunnel construction can be summarized as follows.
(1) Movement of soil on the excavation face: During tunnel excavation, the horizontal supporting stress of soil on the excavation face may be greater than or less than the original lateral pressure, and the soil in front of the excavation face will sink or uplift.
(2) Settlement caused by the gap between buildings: the soil is squeezed into the gap at the tail of the shield, and the soil around the tunnel behind the shield loses its original three-dimensional equilibrium state due to untimely grouting, insufficient grouting quantity and inappropriate grouting pressure, resulting in stratum loss; When curved shield tunneling or rectification tunneling, the actual excavation section is not circular but elliptical, resulting in stratum loss; When the shield moves in the soil, a layer of clay is attached to the surface of the shield shell, and the gap formed around the tunnel behind the shield tail is greatly increased. If the grouting quantity does not increase correspondingly, the formation loss will increase.
(3) Lining deformation and settlement: under the action of earth pressure, tunnel lining deformation will also cause a small amount of stratum loss. When the settlement of tunnel lining is large, it will cause stratum loss that cannot be ignored, and lining leakage will also cause settlement.
(4) Consolidation and re-settlement of disturbed soil: due to factors such as squeezing during shield tunneling and grouting at the tail of shield, a super-porous water pressure zone is formed in the surrounding strata, which can only dissipate and recover after a period of time. In this process, the ground subsidence is caused by the drainage consolidation deformation of the stratum.
2 Strata subsidence control method
2. 1 Investigation of stratum and buildings along the line
In order to effectively control the ground subsidence during the construction, the first premise is to investigate the ground conditions and buildings along the tunnel construction before the shield tunnel excavation. After obtaining the relevant original data, the foundation conditions and the state of buildings (structures) along the line are evaluated and graded, and the control standards to ensure the stability of foundation and buildings (structures) in the construction process are determined in combination with the requirements of relevant codes.
2.2 Optimization and matching of shield tunneling parameters
After determining the control standards of strata and related buildings along the line, it is necessary to adjust the shield tunneling parameters according to the control objectives, so that the shield can achieve the optimal control tunneling state in the construction process.
The optimal shield tunneling means that it has the least influence on the surrounding strata and the ground, the strength of the strata is slightly reduced, the disturbance is small, the overpressure is small, the ground heave is small, and the sudden subsidence amplitude of the shield tail when it comes out of the hole is small. This is the primary condition and fundamental method for shield construction to control ground subsidence and protect the environment.
In order to achieve the above-mentioned optimal state, reasonable parameters must be selected to guide the construction according to the tunnel depth, geological conditions, ground load, design slope, turning radius, axis deviation, shield posture and other factors. Therefore, it is necessary to monitor the surface deformation along the line, constantly optimize the combination, guide the next excavation construction, and truly achieve the purpose of optimizing construction parameters.
2.3 Try to determine the parameters to guide the construction.
The main working medium of shield construction is rock and soil, and the interaction between buildings (structures) and rock and soil within the influence range of construction needs to take a certain excavation section as the excavation test section according to the stratum conditions and building (structure) conditions along the line.
Generally speaking, the initial excavation of 100m is taken as the trial excavation section. In the actual excavation process, the trial excavation section of 100m can be divided into three sections: the first section is 15m, which is the initial excavation, and * * * sets three sets of excavation parameters to explore the stratum change law and axis control through surface monitoring; The second section is 35 meters long. According to the ground conditions, buildings and underground pipelines, the three groups of parameters set in the first stage are adjusted to get the optimal parameters. The third section, 50m long, is the preparation stage for formal excavation. Through this section of excavation, the control measures for ground subsidence, tunnel axis control and lining installation quality are formulated, and the construction parameters are basically mastered, which can be used to guide construction by information feedback. Through the exploration of excavation parameters and stratum deformation law of 100m trial excavation section, a good foundation is laid for the determination of construction parameters in the whole excavation process.
2.4 the setting of the earth warehouse pressure
In the whole process of tunnel excavation, the setting of earth pressure is a very key parameter. If the set value of earth pressure is too small, it will lead to the increase of stratum subsidence, while if the set value of earth pressure is too large, it will lead to stratum uplift.
During the shield construction of Beijing Metro Line 5 17, the ground deformation is strictly controlled because it is located in the old city, with more than 1000 houses on the ground and dense underground pipelines. According to the theoretical calculation results of earth pressure and the empirical data of the trial excavation section, the earth silo pressure is set to 105% ~ 65438+ of the theoretical value.
2.5 Determine the optimal value of synchronous grouting parameters for shield tail.
In the process of shield tunneling, the key measure to control or reduce the surface deformation is to adopt appropriate grouting pressure, grouting quantity and reasonable grouting material, and synchronously inject grouting into the gap behind the lining outside the shield tail. The key parameter control in the process of shield tail synchronous grouting mainly includes the following points.
(1) Slurry with reasonable mixture ratio: the consistency value is controlled at 10.5 ~ 1 1.0, and the bulk density is close to that of undisturbed soil.
(2) Grouting pressure: The suitable grouting pressure is about 5 ~ 6 bar. Because the actual grouting amount is greater than the calculated grouting amount, only the super-volume slurry can be injected into the gap of the soil at the tail of the shield, and the grouting pressure is appropriately higher than the calculated grouting pressure.
(3) Grouting time: The pressing time of shield tail grouting has obvious influence on grouting construction effect. The injection time of slurry should be synchronized with the pulling out of the shield tail of the segment, and the time for evenly injecting slurry should be the same as the time for the segment to advance one circle.
(4) Grouting quantity: Generally speaking, the control of synchronous grouting quantity at the tail of shield can be obtained according to the calculation of the gap at the tail of shield. However, in the actual grouting process, the actual synchronous grouting amount of the shield tail is much larger than the theoretical calculation amount because of the non-compact or void soil at the shield tail and the disturbance effect of shield construction on the surrounding soil. According to the experience of our unit, the suitable grouting amount for sandy pebble stratum should be160% ~ 220% of the theoretical grouting amount; The suitable grouting amount of silt and cohesive soil should be 140% ~ 180% of the theoretical grouting amount.
(5) Distribution of grouting positions: grouting pipe distributed in the shield tail shell at equal angles is selected for grouting, and the grouting pressure and grouting quantity of each grouting pipe are determined according to different geological conditions and control standards, which can make the tunnel tail suspended in the slurry generate controllable displacement, which can not only improve the original deviation of the tunnel axis, but also effectively improve the jamming between the segment and the shield tail.
2.6 Information construction guidance
In the process of shield tunneling, according to the comparative analysis of monitoring results and various construction parameters, the construction parameters can be further revised, so as to optimize and match the tunneling parameters and effectively control the surface deformation.
3 engineering practice effect
Beijing Metro Line 5 is the first subway tunnel built by shield method in Beijing. Shield tunneling in this section has the following characteristics.
3. 1 Geological conditions are representative.
The buried depth of the tunnel in this section is14.8 ~15.6m, and the upper stratum of the tunnel is 23 layers of silt, ② layer of silt, ③ 1 layer of medium coarse sand, ③ 1 layer of round gravel, ④ layer of cohesive silt and ⑤ 65438 layer of silty clay. The tunnel enters the phreatic layer and part of it enters the confined water layer.
The buried depth of the other section of the tunnel is 8.2 ~ 9.5m The upper stratum of the tunnel is clayey silt, sandy silt ② and silty clay ② 1, and the stratum that the tunnel passes through is clayey silt, sandy silt ② 1, silty sand ③ and round gravel ④, while the lower stratum of the tunnel is round gravel ④ 1 and round gravel ④. In some sections, the bottom of the tunnel enters the aquifer, but does not enter the confined water.
3.2 The surrounding environment is complex and there are many construction constraints.
The two sections of ground buildings in this bid section are complex, and the ground roads along the line are the main traffic arteries in the north and south of the city, with a large traffic flow. The tunnel passes through the bustling commercial street, and the ground traffic on this section is busy, so the ground traffic cannot be interrupted during the subway construction.
The ground buildings are complex, and there are tunnels under the dense commercial buildings on the east side of the road. Most of the buildings were built earlier, and most of them were dangerous buildings. There are many underground pipelines and civil air defense buildings, including sewage, water supply, electricity, heat and telecommunications pipelines.
Combined with the actual engineering conditions, in the actual construction process, the control method described in this paper has achieved the construction goal of effectively controlling ground deformation. For example, in many hutongs crossed by tunnel construction, the ground deformation control effect is ideal, and the ground settlement value is basically within 10 mm, far less than the 30 mm required by the code.
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