Among the common subway station construction methods at present, the cover-and-dig top-down method has relatively little adverse impact on the engineering environment, and its comprehensive technical and economic indicators are ideal. Its pavement exposure operation time is short, which has little impact on the business and traffic environment around the project; As the supporting system of the envelope, the structure itself has high stiffness, which can significantly reduce the deformation of the envelope and the surrounding environment; Its cost is between open excavation and underground excavation, which is relatively low. Therefore, the top-down cover and excavation method has great application value in downtown areas or transportation hubs with prosperous commerce, dense buildings and heavy traffic. It has been applied in large-scale subway station projects in Beijing, Shanghai, Guangzhou and Nanjing.
Due to technical reasons, the top-down cover and excavation method also has limitations, which are mainly manifested in the following aspects: (1) the adverse impact of uneven settlement on the structural system during construction is more serious than that of the top-down method; The structure is constructed from top to bottom with many construction joints. Due to the influence of shrinkage and settlement in the hardening process of concrete structure, cracks will inevitably appear, which will adversely affect the stiffness, durability and waterproof of the structure. Most engineering components that meet at the same node are constructed asynchronously, and the connection accuracy is difficult to control. Laminates are generally constructed by soil formwork, and the appearance quality of concrete is difficult to control.
Xinjiekou Station is constructed by cover-and-dig method. In view of the above problems, corresponding technical measures have been taken and good results have been achieved. The relevant information is now introduced as follows:
2 Project overview
Xinjiekou Station is a transfer station between Line 1 and Line 2. Xinjiekou Station of North-South 1 Line is located in the south of Xinjiekou Circular Square and below Zhongshan South Road in the north of Huaihai Road and Shigu Road. Xinjiekou Station of East-West Line 2 is located underground of Hanzhong Road and Zhongshan East Road. The northern end of Xinjiekou Station on Line 1 is a large disc structure with an inner diameter of 50m, which is the intersection of the near-term and long-term stations.
Xinjiekou Station has a total construction area of 35,579.73 square meters. The station is 362.703m long, 24.2m wide (locally 3655m wide) and 17.24m high (locally 19.03m). Set a slope of 2‰, which is high in the south and low in the north.
The station is an island station with three floors underground and a platform width of 14m. The first to third floors underground are the commercial floor, the station hall floor and the platform floor respectively. The main enclosure structure of the station is an underground continuous wall with a thickness of 0. Part of the envelope structure in the south extension section is SMW pile. The central column is φ600 (a few φ700 and φ800) steel pipe column, and the foundation is φ 1 500 bored pile. The station * * * has 16 passengers and 3 air ducts.
According to the engineering geological survey report, the geological situation of the station is complicated. According to the age, genetic type and physical and mechanical properties of rock and soil, the engineering geology of the site is divided into four layers from top to bottom: ① artificial fill, ② alluvial soil in the middle and late Holocene, ③ alluvial soil in the late Neogene-early Holocene, and ④ sedimentary rocks of Gecun Formation in the Lower Cretaceous.
Groundwater in the construction area is divided into three layers: shallow pore aquifer, moderately weak confined aquifer and deep pore confined aquifer. The station structures with groundwater level between 0.8- 1.8m are all located in muddy clay layer, and the soil has the characteristics of high compressibility, high sensitivity, low permeability, saturation and soft flow plasticity.
The ground environment of the station construction area is very complicated. The buildings around the station include Jinling Hotel, China Bank and Xinbai Building, Central Shopping Mall, Commercial Building, Oriental Shopping Mall and Tianan Building, which are distributed around the central square. The surrounding pipe network is dense, with more than 50 water supply, sewage, electricity, telecommunications and other pipelines/kloc-0.
3 Xinjiekou Station Overall Construction Scheme
The top-down construction scheme of Xinjiekou Station is determined through many schemes optimization and demonstration according to the geological conditions, ground traffic, construction site conditions and construction period requirements of the station. The scheme is summarized as a top-down excavation method with one opening and three hidden covers. The first light means that the earthwork above the roof of the main structure is excavated in an open way, and the third dark means that the earthwork of the commercial floor, the station hall floor and the platform floor are all excavated in secret, which are applied to the laminate and the side wall structure from top to bottom in turn.
The overall process of top-down construction of Xinjiekou station cover is as follows: construction of concrete filled steel tubular column in the middle of underground continuous wall and station enclosure-earthwork excavation above the roof, formwork construction of the roof, construction of roof structure and waterproof layer and protective layer-backfilling earthwork above the roof-earthwork excavation of commercial floor, formwork construction, earthwork excavation of floor and side wall structure-earthwork excavation and structural construction of platform floor-platform floor and stairs. The construction procedure of the standard section is shown in figure 1.
4 Research and Practice on Main Key Technologies of Xinjiekou Station
4. 1 Main technical measures to prevent uneven settlement of underground continuous wall of retaining structure
The retaining structure of the foundation pit of Xinjiekou Station adopts C30S8 waterproof concrete diaphragm wall, which is not only used as the retaining structure for retaining soil and stopping water during construction, but also combined with the lining wall to form a composite wall as the side wall of the permanent structure. Diaphragm wall is 0.8m wide, 6m long and 35~39m deep.
In order to overcome the adverse effect of differential settlement of diaphragm wall on the overall quality of the main structure, the depth of diaphragm wall is adjusted at the design stage, so that its toe passes through the deep pore confined aquifer (③-3d2 fine sand layer and ③-4e coarse sand with gravel layer) and enters ⑤- 1 strongly weathered argillaceous siltstone-mudstone layer, and the depth is less than 0.5m, so that the toe of diaphragm wall is located in a stable and reliable bedrock bearing layer. Secondly, in order to improve the bearing capacity of the foundation under the wall and reduce the uneven settlement of the wall, two φ30 grouting pipe are reserved in the reinforcement cage of the continuous wall. After the concrete pouring of the wall is completed and the design strength reaches about 30%, 1: 2 cement mortar is injected into the toe of the wall. Practice has proved that this technical measure is effective. Pressure-injected cement mortar fills the gap in the toe of the wall and effectively strengthens the deposited layer on the toe of the wall, thus reducing the possibility of uneven settlement. Thirdly, in order to improve the overall stiffness of the diaphragm wall, a cast-in-place reinforced concrete ring beam of 0.8mx 1.0m is set at the top of the diaphragm wall to connect the diaphragm wall into a whole. Fourthly, in the construction stage of underground continuous wall, each working procedure should be carried out in strict accordance with the design and operation procedures, especially the construction quality in tank cleaning stage, and the next working procedure can be carried out only after ensuring that the mud proportion and sediment thickness meet the standards. First, clean the bottom by pulling and grasping, and then clean the bottom by sucking mud with a catheter. After bottom cleaning, the specific gravity of mud at the bottom of the tank is less than 1.25, and the thickness of sediment is not more than100 mm. After tank cleaning, measure the tank depth with a side hammer, and then use flat hammer to measure the tank depth. The difference between the two times is the thickness of the sediment. Then the supervisor will check the depth of the tank and the specific gravity of the mud, and the qualified unit will check the verticality of the tank wall with an ultrasonic detector. After all the indicators meet the requirements of the specification, the next procedure will be started.
Through the above technical measures, the overall construction quality of underground continuous wall is guaranteed, and the uneven settlement of the wall is effectively controlled. The maximum measured differential settlement of the wall is about 4 mm.
4.2 Main construction technology of station intermediate pile
Steel pipe column is an important engineering component for constructing underground station by reverse construction method. It is a temporary pillar in the construction stage and a permanent main vertical load-bearing and force-transmitting structure of the station in the use stage. The middle pile consists of two parts: middle pile and foundation middle pile. The outer diameter of the steel pipe column in this station is 600mm (700mm and 800mm in some parts), the wall thickness t of the steel pipe is 16mm, and the material is 16Mn. The core concrete is C50 micro-expansive concrete, and the columns are arranged in a grid shape on the plane. The middle pile is C30 reinforced concrete bored pile with a diameter of 1.5m, and the lower end of the steel pipe column is anchored on the column foundation for about 2m. Shear studs is set in the anchorage section of steel pipe column, and its structure is shown in Figure 2, and the position relation diagram of steel pipe column and steel casing.
The construction area of this project is mainly muddy clay, so the installation of steel pipe column adopts wet operation first insertion method. The main technological process of 522 12 is as follows: drilling holes for bored piles-integrally lifting the reinforcing cage and steel sleeve of bored piles-pouring concrete for pile foundation to the preset position for the first time-pumping out the mud in the steel sleeve-grouting under pressure at the bottom of the pile-installing an automatic locator in the steel sleeve-lifting and installing the steel pipe column to complete the positioning of its upper and lower ends-pouring concrete for pile foundation cup to the designed position-pouring concrete for steel pipe column to the designed position. See the pile construction flow chart in Figure 3.
The influencing factors of the comprehensive quality of steel pipe columns mainly include the construction quality of pile foundation, the processing quality of steel pipe columns, the installation and positioning quality of steel pipe columns and the construction quality of concrete in columns. This paper focuses on the latter two points.
One of the key problems in quality control of steel tubular columns is to ensure the installation accuracy of the columns. According to the specification, the allowable deviation between the center line of the column and the center line of the foundation is 5mm, and the verticality error of the column should not be greater than 1‰ of the column length.
The installation and positioning of steel pipe column mainly adopts the upper and lower two-point positioning method. The positioning of its lower end depends on the automatic positioner, and the positioning of its upper end adopts four adjustable screws, which are placed between the steel casing and the steel pipe column and located in the same plane. The automatic positioner is a pre-processed conical device. After accurately correcting its plane position, elevation and verticality, connect it with the installation leg pre-welded on the steel casing wall with four bolts. After pouring pile foundation concrete, the locator is firmly anchored in the concrete. Its structural characteristics determine that it can realize the functions of extradition, limit and accurate positioning of steel columns.
Measure the design plane position of the intermediate pile with total station, and protect the pile in the transverse direction outside the pile casing construction area to ensure the position of the pile center. Using the combination of level and 30m steel rule, the elevation of the bottom of steel pipe column is measured, and four points are made in the transverse direction of the steel casing wall to control the installation elevation of the positioner. The measurement method of column center is as follows: firstly, a vertical ball is used to guide the pile center from the ground to the pile foundation surface in the steel casing, so as to calibrate the initial installation position more accurately; Then put the1/200,000 point drop meter directly at the pile center on the ground with the total station, measure the pile center directly at the center of the locator, and then command the locator to install accurately and pour concrete.
Another key problem in the construction of intermediate column is how to ensure the concrete pouring quality of intermediate column. C30 ordinary underwater concrete of bored pile of central column foundation is poured by conduit method. The core concrete of steel tube column adopts C50 expansive concrete. Compared with the three pouring methods specified in the specification, namely, pumping jacking pouring method, vertical artificial pouring tamping method and high-position vibration dropping method, combined with the structural characteristics of the steel pipe column in this project, the high-position vibration dropping method is selected to pour the concrete in the column. In order to overcome the shortcoming that the core concrete is not compact because the concrete is wrapped with bubbles during the implementation of this method, a high-energy vibrator is inserted into the column to vibrate the whole concrete. This high-position throwing combined with vibrating construction technology effectively ensures the compactness of concrete in the column. See table 1 for the mixture ratio of C50 micro-expansive concrete.
Table 1C50 Mix Proportion of Micro-expansive Concrete
4.3 Soil formwork construction technology
Xinjiekou Station has a roof thickness of 0.8m, a laminate thickness of 0.4m and a concrete strength grade of C30. The station structure is divided into 16 construction sections.
The average section is about K22m.
The roof and deck of the station are located in the muddy clay layer, which has the characteristics of high sensitivity, high compressibility, low strength, saturation, plasticity or soft plasticity, and is not suitable for natural foundation. Large-scale application of formwork and structural concrete in this stratum is easy to cause uneven settlement. After research and test, it is decided to adopt composite structure formwork, namely 10cm thick macadam layer and 8cm thick C20 plain concrete layer. According to the specific geological conditions, local replacement treatment is carried out to ensure the strength, rigidity and construction accuracy of the formwork and minimize the uneven settlement of the laminated structure during construction.
The construction points of soil model foundation soil layer are as follows: according to the structural geometry, design elevation and 2‰ longitudinal design slope, the foundation soil plane is leveled; Before earthwork excavation, 20 days in advance, the groundwater in the foundation pit shall be treated with large-diameter deep well dewatering, and some earthwork covered with excavation in the hole shall be treated with light well point dewatering and open ditch drainage to ensure that the surface of foundation soil is basically dry; In the process of earthwork excavation, it is forbidden to overbreak, and 20cm of manual bottom check is reserved; Strengthen the replacement treatment of cement lime soil in local weak areas.
Key points of soil model structure construction of plate beam structure in each layer: the elevation of the top surface of plate structure soil model is increased by 2cm as the reserved settlement; After the foundation soil is compacted and leveled and passed the inspection, 10cm macadam layer and 8cm thick C20 plain concrete shall be laid, and the elevation shall be accurately controlled before finishing and grouting. The side formwork structure of the center beam and longitudinal edge of the station roof adopts 12cm thick brick formwork, and the surface is coated with 5cm thick mortar leveling layer. After the application of soil formwork, apply long-acting release agent to ensure the demoulding effect.
According to the measured data, the maximum cumulative settlement of the roof formwork is 6mm, while the middle plate has no settlement. Except for the poor demoulding effect of the roof caused by rain, the lines in other parts are smooth and the apparent quality of concrete is good.
4.4 Comprehensive waterproof construction technology
Compared with cut and cover, the subway station constructed by cover-excavation method has more horizontal construction joints. Therefore, the guarantee of waterproof quality is extremely important. The waterproof project of Xinjiekou Station has adopted comprehensive treatment technical measures and achieved good waterproof effect.
The main structure and pedestrian passage of the station are designed according to the waterproof grade-grade standard, and the waterproof grade of the air duct and air shaft structure is grade II. The key points of structural waterproof construction include: first, taking effective measures to block the leakage of underground continuous wall of envelope structure; C30 high-performance shrinkage-compensating waterproof concrete is adopted in the station concrete structure, and its impermeability grade is not lower than S1O.. In the construction process of each working procedure, strictly abide by the operating procedures to ensure the compactness of concrete. In particular, special technical treatment measures are taken for the frightened horizontal construction joints. According to the technological characteristics of top-down cover excavation method, the plate beam structure is poured first, and then the lining wall structure below it is poured. Due to the shrinkage and subsidence of concrete, the horizontal construction joint between them is prone to micro-cracks and difficult to be compacted. The fake corbel pouring and secondary vibrating technology are adopted here. Set a special inclined formwork to ensure that the top surface of the false support is 20cm higher than the construction joint; The station roof adopts polyurethane coating which can be closely attached to the structure, and the underground continuous wall is continuously laid to a waterproof layer with a width of 30cm, and a waterproof layer made of the same material is added at the corner; In order to improve the impermeability of concrete, the interface between underground continuous wall and each layer of structure is chiseled, and the cement-based permeable crystalline rigid waterproof layer is painted.
The key points of waterproof technology for special parts are as follows: when conditions permit, 250mm steel putty waterstop shall be used for waterproof sealing of construction joints, and 30mmx20mm water-swellable rubber strip shall be used for waterproof sealing of parts where steel putty waterstop cannot be used. At the horizontal construction joint between the laminate and the side wall, strong waterproof treatment is carried out by pre-embedded φ30 pipe grouting. The deformation joints are internally waterproof with 300mm-wide embedded PVC waterstop, and externally waterproof with PVC waterstop (without roof). Through the above comprehensive waterproof technical measures, the overall waterproof quality of Xinjiekou Station meets the design requirements.
5 concluding remarks
In the top-down construction of Xinjiekou station, practical and reliable technical measures were taken to prevent the uneven settlement of underground wall and intermediate pile and effectively ensure the safety and quality of the structure. The whole set of construction technology of concrete-filled steel tubular column in the middle is safe and reliable, and the installation accuracy is very high, which effectively ensures the construction quality of steel tubular column; The formwork and structure construction technology under weak geological conditions effectively ensure the internal quality and appearance quality of the structure; Comprehensive waterproof technology ensures that the overall waterproof quality of the station meets the design and specification requirements. The top-down construction experience of Xinjiekou station provides reference for similar projects in China.
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