At present, the construction methods of subway stations at home and abroad include cut and cover, cover excavation method, underground excavation method and shield method. After nearly 40 years of development, China's subway construction methods have developed from the initial single open excavation and cover excavation to the coexistence of various methods such as open excavation, underground excavation, shallow underground excavation and shield method. With the continuous development and improvement of construction technology, a special discipline system has been initially formed.
With the rapid development of China's socialist economic construction and the enhancement of comprehensive national strength, the scale of cities is increasing, the urban population flow is still increasing, and the traffic situation is deteriorating year by year. In order to improve the traffic environment, people have taken various measures, among which the construction of underground railway has been widely recognized. For example, in recent years, Beijing, Guangzhou, Shenzhen, Zhengzhou and other cities have built a large number of underground railways.
The construction method of urban underground railway is greatly influenced by ground buildings, roads, urban traffic, hydrogeology, environmental protection, construction equipment and capital conditions, so the construction methods adopted are not the same. The following will introduce the construction methods of urban subway respectively.
The choice of construction method should be determined after comprehensive comparison according to the nature, scale, geological and hydrological conditions of the project, as well as the above-ground and underground obstacles, construction equipment, environmental protection, construction period requirements and other factors.
1. Fill as you dig.
Open excavation refers to the construction method of digging the ground, excavating the earthwork from top to bottom to the design elevation, starting from the basement and constructing from bottom to top in turn, completing the main structure of the tunnel, and finally backfilling the foundation pit or restoring the ground.
Cut and cover is the preferred method for subway construction in various countries, and cut and cover is usually adopted when the ground traffic and environment permit. Cut and cover and cut and cover are often used in shallow-buried subway stations and interval tunnels, which belong to deep foundation pit engineering technology. Because subway projects are generally located in urban areas with dense buildings, the main technical difficulties of deep foundation pit projects are to protect the undisturbed soil around the foundation pit, prevent surface subsidence and reduce the impact on existing buildings.
The advantage of cut and cover is that its construction technology is simple, fast and economical, and it is often regarded as the first choice. However, its shortcomings are also obvious, such as traffic jams for a long time, noise and vibration and other environmental effects.
Applicable conditions of subway: Generally, when the ground conditions permit, the subway tunnel should be covered by open excavation, but it has a great impact on the social environment and is only suitable for use in places with few people, many cars and few pipelines.
General cut and cover construction procedures can be divided into four steps: maintenance structure construction → internal earthwork excavation → engineering structure construction → pipeline repair and soil covering, as shown in figure 1.
Huangxing Road Subway Station of Shanghai Metro M8 Line is located below the center line of Kongjiang Road on the east side of the intersection of Kongjiang Road and Jingyu Road in Shanghai. The station is an island station with two floors underground, with a length of 166.6 m, a standard section width of 17.2 m, and a wellhead width of 2 1.4 m at the north and south ends. The standard section is a reinforced concrete structure with single column and double span, and the end well part is a double column and double span structure. * * * There are two air wells and three people. The main body of the station adopts the underground continuous wall as the foundation pit enclosure structure. The underground continuous wall in the standard section is 26.8m deep and 0.6m thick. SMW piles are used for the retaining structure of foundation pit for station population and wind shaft.
2. Cover excavation method
Cover excavation method is to close the top after digging down from the ground to a certain depth, and the rest of the lower works will be constructed under the closed top cover. The main structure can be built forward or backward.
Building subway stations in busy urban areas often takes up roads and affects traffic. When the subway station is located on the main road, the traffic can not be interrupted, and a certain traffic flow requirement must be guaranteed, the cover-and-dig method can be selected.
2. 1 cover excavation method
Cover-and-dig construction method is to put prefabricated standard cover-and-dig structures (including longitudinal beams and pavement slabs) on the soil retaining structure to maintain traffic after the soil retaining structure is completed in surface operation, and repeatedly excavate downwards and cross-support until the design elevation is reached. From bottom to top, construct the main structure and waterproof measures, backfill soil and restore pipelines or bury new pipelines. Finally, remove the exposed part of the retaining structure and restore the road as needed. The construction sequence is shown in Figure 2.
When the main building is built under the condition that the road traffic cannot be interrupted for a long time, the cover-and-dig method can be considered.
Engineering example: Huaqiang Road Station of Shenzhen Metro Phase I Project is located at the west side of the intersection of Shennan Middle Road and Huaqiang Road, the most prosperous in Shenzhen, and under the lane of Shennan Middle Road. The municipal roads in this area are dense, and the traffic volume is large, with the maximum traffic volume reaching 3865 vehicles/h. The main body of the station is a single-column double-deck double-span structure, with a total length of 224.3 m, a standard section width of 18.9 m, a foundation pit depth of about 18.9 m, a shield width of 22.5 m at the west end and a foundation pit depth of about18.7. The construction period of the main structure is 2 years, including 7 months for the envelope structure and temporary pavement. In order to ensure the normal operation of Shennan Middle Road during the construction of subway station, the main structure construction of this section adopts the construction scheme of cover and excavation method.
2.2 Top-down method of cover excavation
The top-down method of cover excavation is to first put the foundation pit retaining structure and the middle column down. Like the top-down cover-and-dig method, the underground continuous wall or curtain column is often used as the foundation pit maintenance structure, and the middle column of the main structure itself is used as the middle support to reduce the project cost. Then the topsoil can be excavated to the ground elevation of the main structure roof, and the roof can be poured with the excavated soil as the soil model. The roof can be used as a powerful cross brace to prevent the deformation of the maintenance structure from entering the foundation pit, and restore the road and traffic after backfilling. The future work will be carried out under the cover of the roof, that is, the main structure will be excavated and built layer by layer from top to bottom, as shown in Figure 3.
If the excavation area is large, the covering soil is shallow, and the surrounding buildings are too close, in order to prevent the adjacent buildings from sinking due to foundation pit excavation as much as possible, or to restore road traffic as soon as possible, but there is no finalized covering soil structure, the top-down construction method is often adopted.
Engineering example: When geological conditions and surrounding environment permit, the construction method of the interval tunnel of the first phase of the South-North Line of Nanjing Metro is selected with the goal of cost, construction period and safety. Among them, Sanshanjie Station, located in the ancient channel of Qinhuai River, is located in silty soil, silty fine sand and muddy clay. As it is the 1 station and located at the crossroads, the retaining structure adopts the underground continuous wall. In addition to the population structure, the top-down construction method is adopted.
2.3 Cover-excavation semi-reverse construction method
The only difference between the semi-top-down method and the top-down method is that after the roof is completed and the pavement is restored, the bottom plate is poured down to the design elevation, and then the side walls and floors are poured up step by step. In semi-top-down construction, it is generally necessary to set up cross braces and apply prestress, as shown in Figure 4.
3. Undercut method
The underground excavation method is a tunnel construction method that under certain conditions, the lining structure is built underground without excavating the ground.
Underground excavation methods mainly include: drilling and blasting method, shield method, boring machine method, shallow buried underground excavation method, pipe jacking method, immersed pipe method and so on. Among them, shallow excavation method and shield method are widely used, so this paper focuses on these two methods.
3. 1 shallow buried excavation method (shallow buried mine method)
Shallow excavation method is a NATM construction method in loose stratum. NATM is to make full use of the self-supporting capacity of surrounding rock and the space constraint of excavation face, adopt anchor rod and shotcrete as the main supporting means, strengthen surrounding rock, restrain the relaxation and deformation of surrounding rock, and guide the design and construction of underground engineering through the measurement and monitoring of surrounding rock and supporting. Shallow-buried excavation method is proposed for the construction of shallow-buried, loose and unstable upper layer and weak and broken rock stratum. For example, most tunnels in Shenzhen Metro adopt shallow-buried excavation method.
The technical characteristics of shallow-buried excavation method are: (1) the deformation of surrounding rock spreads to the surface; Need rigid support or stratum improvement; Guide the design and construction through the test section.
When the tunnel is built by shallow excavation method, the appropriate excavation method and tunneling method should be selected according to the engineering characteristics, surrounding rock conditions, environmental requirements and the construction unit's own conditions.
The commonly used excavation methods in interval tunnel construction include step method, CRD method, glasses method and so on. Multi-span tunnels such as urban subway stations and underground parking lots are mostly constructed by column hole method, hole measuring method or middle hole method.
The underground railway is built in the urban area, which requires strict control of surface subsidence. Therefore, it is more important to emphasize the pre-support and pre-reinforcement of strata. The construction methods adopted include pre-grouting with advanced small pipes, deep-hole grouting in excavation face and advanced support with pipe shed. The construction technology of shallow-buried underground excavation method can be summarized as "pipe advance, strict grouting, short excavation, strong support, quick closure and diligent measurement" 18, and its process flow is shown in Figure 5.
Engineering example: dongdan station Southeast Wind Tunnel of Beijing Metro is orthogonal to the main structure of the station, with the north side under Chang 'an Avenue and the middle and south sides passing through residential areas, with a total length of 43.4 m ... It is constructed by shallow-buried and hidden-digging pile method, and smoothly passes under the ground residential areas and civil air defense facilities while basically maintaining the original environment.
3.2 shield method
Shield construction is a kind of construction method to dig tunnels underground with shield construction machinery.
Shield is a kind of movable steel tube structure that can support ground pressure and advance underground. The front end of the steel cylinder is provided with a device for supporting soil excavation, and the middle section of the steel cylinder is provided with a jack required for jacking; Precast or cast-in-place tunnel lining rings can be assembled at the end of steel cylinders. Every time the shield advances a ring distance, a ring lining is assembled (or cast-in-situ) under the support of the shield tail, and cement mortar is pressed into the gap around the lining ring to prevent the tunnel and the ground from sinking. The reaction force of shield propulsion is borne by the lining ring. Before shield construction, it is necessary to build a shaft, install a shield in the shaft, and send the soil dug by the shield out of the ground through the shaft passage. The shield construction technology is shown in Figure 6 below.
According to the different cross-section shapes of shield, it can be divided into four types: circular, arched, rectangular and horseshoe. Circular shape is widely used because of its good resistance to earth pressure and water pressure in stratum, simple lining assembly, universal components and convenient replacement. According to different excavation methods, shield can be divided into three types: manual excavation, semi-mechanical excavation and mechanical excavation. According to the different front structure of the shield, the shield can be divided into two types: open chest type and closed chest type; According to the different ways of removing groundwater and stabilizing excavation face, shield can be divided into: artificial well point dewatering, mud pressurization, earth pressure balance, local air pressure shield, full air pressure shield and so on.
The main advantages of shield method are as follows: except for shaft construction, the construction work is carried out underground, which neither affects the ground traffic, but also reduces the noise and vibration impact on nearby residents; The main processes such as shield propulsion, excavation and lining assembly are carried out in cycles, so the construction is easy to manage and there are fewer construction personnel. Less earthwork; When crossing the river, it will not affect shipping; Construction is not affected by weather conditions such as wind and rain; Shield method has high technical and economic advantages in building deep-buried tunnels in places with poor geological conditions and high groundwater level.
It is characterized by stratum excavation, excavation and transportation, lining assembly, joint waterproofing, grouting to fill the gap at the tail of shield, draining groundwater at any time and controlling ground subsidence. It is a construction method with high technical requirements and strong comprehensiveness.
It can be excavated in various soft soil strata and soft rock strata, and it has little influence on the surrounding dense environment when crossing the area where buildings and underground pipelines are concentrated, especially suitable for urban subway and underwater tunnel excavation.
Engineering example: Beijing Metro Line 5 is constructed by shield method, which is a north-south underground traffic artery running through the center of Beijing. It starts from Songjiazhuang in Fengtai District in the south and passes Puhuangyu, Chongwenmen, Dongdan, Dongsi and Yonghe Palace in Taipingzhuang North Station in Changping District in the north, with a total length of 27.7 km. Due to the dense large-scale buildings on the ground, large traffic flow and complex underground pipe network, in order to reduce the impact on the urban economy and citizens' life, after expert argumentation, it is decided to take the lead in adopting shield construction in the test section of about 700 m from Yonghe Palace to Beixinqiao. The shield is a large diameter EPB shield machine.
4. Immersed tube method
The immersed tube method is to prefabricate the tunnel sections in sections, set temporary water stop heads at both ends of the sections, then float to the tunnel axis and sink into the pre-dug trench to complete the underwater connection between the sections, dismantle the temporary water stop heads, backfill the trench to protect the immersed tube, and lay the tunnel internal facilities, thus forming a complete underwater passage.
Immersed tube tunnel has low requirements for foundation, especially suitable for soft soil foundation, shallow river bed or coast, and is easily dredged by water facilities outside the foundation trench. Due to the small buried depth, the total length of tunnel line including connecting section is short, and the tunnels built by underground excavation method and shield method are obviously shortened.
The cross-sectional shape of immersed tube can be round and square, which is flexible to choose. Foundation trench excavation, pipe section prefabrication, floating, sinking and internal laying can be operated in parallel, with relatively little interference and easy control of pipe section prefabrication quality. Based on the above advantages, immersed tube method is called the most economical underwater crossing scheme for building tunnels in wide waters such as rivers.
Immersed tunnel can be divided into steel-shell immersed tunnel (which can be divided into single-layer steel-shell tunnel and double-layer steel-shell tunnel) and reinforced concrete immersed tunnel according to pipe materials. Steel shell immersed tube tunnels are widely used in North America, while reinforced concrete immersed tube tunnels are widely used in Europe and Asia.
The main working procedures of immersed tunnel construction: prefabrication of pipe joint → excavation of foundation trench → floating and sinking of pipe section → docking operation → internal decoration.
Example: Guangzhou Pearl River Tunnel is the first river-crossing tunnel in China, which is shared by highway and subway. Total length of highway tunnel1238.5m. The tunnel in the middle of the river is buried under the river bed, which does not affect the navigation on the water surface. The immersed tube in the middle of the river is 457 meters long. The immersed tube is a porous rectangular reinforced concrete structure, including two two-lane motor vehicle holes, a subway hole and a cable duct gallery. The cross-section of immersed tube is a typical rectangular cross-section, with an overall dimension of 33 mx7.956 m (width x height), a bottom plate thickness of 1.2m, a top plate thickness of 1.0m, two outer walls of 0.7 m and 0.55 m respectively, and the longest pipe joint concrete content of 12000 bang. The basement of the pipe section is located on the weathered granite layer of the river bed. When slotting, reef blasting is used. Sand filling method is adopted for foundation treatment.
5.TBM method
TBM is the abbreviation of tunnel boring machine, which is driven by machinery. It is called full-face tunnel boring machine.
Generally, TBM is defined as a shield machine which is driven by rock strata and equipped with a wall-touching anchor when the full-face tunnel boring machine does not have the functions of maintaining the working face such as earth pressure and water pressure, and is propelled by reaction force during propulsion.
Full-face tunnel boring machine (TBM) has been widely used in tunnel excavation abroad because of its advantages such as fast construction speed, good tunnel forming, high degree of mechanization and little impact on the surrounding environment.
5. Mixing method
According to the actual situation of subway tunnel, two or more of the above methods can be used at the same time in the construction process of subway tunnel, which is called mixed method.
Engineering example: Dongsi Station of Beijing Subway is located at the intersection of Chaoyangmennei Street and Dongsi South Street, in the bustling city center, and many buses pass by. The main body of the station runs along Dongsi South Street in a north-south direction. The planned road red line width of Dongsi South Street is 70 m, the current road width is 22 m, Chaonei Street has been reconstructed, the road red line width is 60 m, the two-way passenger flow is balanced, and the traffic is very busy. And the long-term Line 6 runs along Chaochao Inner Street, showing an east-west trend, and you can transfer at this station. Both ends of the station are open-cut sections, and the structural form is a three-story and three-span frame structure; The middle part is an underground excavation section, and the structural form is a single-story structure with three arches and two columns. Total station length 197 m, underground excavation length 96.80 m, open excavation length100.20m. ..
6 concluding remarks
With the development of subway construction in China, the original construction technology is constantly developing and improving. At the same time, new construction methods have been applied to construction, and the level of construction technology has been continuously improved, and some of them have reached the world advanced level. In addition, due to the increase of urban traffic flow, urban roads become crowded and the requirements of urban environment become more and more strict, so it is unrealistic to build road closures in cities. Therefore, shield method, shallow excavation method and other underground excavation technologies will be the main direction of future research and practice.