I. cutting and covering
Usually, when the ground conditions permit, cut and cover should be used in subway tunnels, but it has a great impact on the social environment. Open excavation refers to the construction method of digging the ground, excavating the earthwork from top to bottom to the design elevation, constructing the tunnel from bottom to top, 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. Cut and cover has the advantages of simple, rapid and economical construction technology, and is often the first choice. However, its shortcomings are also obvious, such as traffic jams for a long time, noise and vibration and other environmental effects.
Open cut and cover excavation method includes open cut and cover excavation method, open cut and cover excavation method and foundation pit supporting structure cover excavation method.
(1) Open the incision and cover it. The sections with few buildings on the ground, small traffic volume, large construction site and shallow buried structures, as well as the sections where urban rail transit enters and exits the ground, adopt open excavation and underground excavation.
(2) Open cut and cover plate of supporting structure shall be set in the foundation pit. When cut and cover is used in places with small construction site, poor soil self-bearing, rich groundwater, dense buildings and deep buried depth, supporting structures should be added to the foundation pit.
(3) Cover excavation method. When the buried depth is shallow, the site is narrow, and the ground traffic occupies the road for a long time, the cover excavation method can be used. That is, during the short-term closed ground traffic, continuous wall and bored pile operations are carried out, and the structural roof is excavated and constructed, and then backfilled to restore ground traffic. Then carry out underground operations, excavate foundation pits, build buildings and floors, and use the entrances and exits on both sides of the tunnel for excavation and feeding.
According to the construction sequence of the main structure, it can be divided into three methods: cover-excavation sequence method, cover-excavation top-down method and cover-excavation semi-top-down method. The method comprises the following steps: completing the peripheral retaining structure, the longitudinal and transverse beams arranged on the retaining structure and the pavement slab replacing the original pavement on the existing road, excavating the foundation pit from top to bottom in layers under this cover to the design elevation, then constructing the structure from bottom to top in turn, and finally covering the soil to restore to the cover excavation method; On the contrary, first build the roof, restore traffic, and then construct the structure from top to bottom.
Second, the hidden excavation 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 excavation method, pipe jacking method, new Austrian tunneling method and so on. Among them, shallow excavation method and shield method are widely used. At present, shield method and shallow buried excavation method are mostly used in tunnel construction in China.
1. Borehole blasting method
China has a vast territory and diverse geological types. Cities such as Chongqing and Qingdao are in hard rock stratum, and some sections of Guangzhou Metro are also in hard rock stratum. In this geological condition, the subway construction usually adopts drilling and blasting excavation and bolting and shotcreting support (equivalent to the usual mountain tunnel).
The whole process of drilling and blasting construction can be summarized as: drilling and blasting, ballasting during loading, bolting and shotcreting support, grouting lining, supplemented by ventilation, drainage and power supply. A series of initial support methods such as grouting, steel frame and pipe shed are often used when passing through bad geological sections. According to the engineering geology, hydrological conditions and section size of the tunnel, various excavation methods can be adopted for drilling and blasting tunnel excavation, such as: the upper pilot tunnel arches the wall first, the lower pilot tunnel arches the wall first, the positive bench method, the reverse bench method, the full-face excavation method, the half-face excavation method, the side wall pilot tunnel method, the CD method and the CRD method. For blasting, there are smooth blasting, presplitting blasting and other technologies. The primary support of tunnel includes anchor rod, shotcrete, hanging net, steel arch frame and pipe shed. Timely measurement and information feedback are often used to monitor construction safety and verify whether rock support measures are reasonable.
2. Shield method
In the section where the subway line crosses the river, the surrounding rock structure is loose, saturated, fluid plastic or soft plastic, and the engineering geological conditions are poor, so the shield machine is used for construction. 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 application of shield method in tunnel construction in China began in Shanghai in 1950s and 1960s. Originally used to build urban underground drainage tunnels, relatively old-fashioned shield machines (such as grid format, compressed air type, insert plate type, etc.) were adopted. In the late 1980s and early 1990s, modern shield machines, such as earth pressure type and muddy water type, were used to build subway tunnels. Shield method has the advantages of safety, reliability, rapidity and environmental protection. At present, this method has developed rapidly in the subway construction in China. Most shield machines used in China metro are EPB shield machines.
With the deepening of the research on shield method and the increase of engineering application, the shield construction technology and the supporting technology of shield machine construction have also been developed and improved: Shanghai subway tunnel is basically built by shield method, and at present, two parallel section tunnels are being built by double-circle shield at one time, and underground passages are also built by square-section shield; The slurry shield with the diameter of 1 1.2m is used in the highway tunnel crossing the river in Dalian, which is also the largest shield machine in China at present. A new type of composite shield machine with earth pressure balance, air pressure balance and half earth pressure balance mode has been successfully applied to subway tunnel construction in complex strata such as soft soil, hard rock and broken zone, which greatly expands the application scope of shield method. Shenzhen, Nanjing, Beijing, Tianjin and other cities have different geological and hydrological conditions, but shield method has been successfully used to build interval tunnels.
The main advantages of shield method are: except for shaft construction, construction work is carried out underground, which will not affect ground traffic, and reduce the impact of noise and vibration 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.
3. Roadheader method
It is used when the buried depth is shallow, but the site is narrow and the ground traffic environment does not allow blasting vibration disturbance, so it is not suitable for shield method to break soft rock stratum. This method mainly adopts boom roadheader, which is greatly influenced by geological conditions.
4.NATM
When the urban rail transit line crosses the bedrock, the surrounding rock has a certain self-stabilizing ability. NATM construction is generally adopted, that is, shotcrete and anchor are the main supporting means, and the self-supporting function of surrounding rock is exerted at the same time, so that it and the supporting structure become a complete tunnel supporting system, and information design can be adopted, that is, the original design can be adjusted at any time according to the construction monitoring data to make the design more reasonable.
NATM is the abbreviation of NATM tunnel construction method. The original text is NATM. The concept of NATM was put forward by Austrian scholar Professor ICZ in 1950s. It is a construction method based on tunnel engineering experience and rock mechanics theory, combined with anchor rod and shotcrete as the main supporting means. After many times of practice and theoretical research in some countries, it was obtained in the 1960s. Since then, this method has developed rapidly in many underground projects in western Europe, northern Europe, the United States and Japan, and has become one of the new technical signs of modern tunnel engineering. NATM was introduced into China in 1960s, and developed rapidly in the late 1970s and early 1980s. So far, it can be said that NATM is indispensable in all key and difficult underground projects. NATM has almost become a basic method to build tunnels in soft and broken surrounding rocks.
In China, NATM is often called "anchor spraying method". When this method is used to build underground tunnels, the interference to the ground is very small and the engineering investment is relatively small. Accumulated mature construction experience, the project quality can be well guaranteed. When this method is used in construction, for rock strata, one-time or full-face excavation, anchor-shotcrete support and anchor-shotcrete support composite lining can be adopted, and secondary lining can be carried out if necessary; For the soil stratum, it is generally necessary to reinforce the stratum before excavation, support and lining, and the construction can only be carried out after dewatering under the condition of groundwater. NATM is widely used in underground engineering, such as mountain tunnels, urban subways, underground warehouses, underground workshops and mine tunnels.
In our country, using NATM principle to build subway has become a main construction method, especially in the case of limited construction site, complex and changeable stratum conditions and complex underground engineering structure.
The supporting principles of NATM are: the surrounding rock is both a load-bearing object and a load-bearing structure; Surrounding rock bearing ring and supporting body form the unity of tunnel, which is a mechanical system; The purpose of tunnel excavation and support is to maintain and improve the self-supporting capacity of surrounding rock.
NATM takes shotcrete and bolt support as the main support means, because bolt shotcrete support can form a flexible thin layer, which is closely bonded with the surrounding rock, allowing the surrounding rock to have a certain coordinated deformation without making the supporting structure bear excessive pressure.
The construction sequence can be summarized as: excavation → primary support → secondary support.
The excavation work includes: drilling, charging, blasting, ventilation, slagging, etc. Excavation and initial support operations are carried out simultaneously. The first support operation includes: primary shotcreting, bolt support, net hanging, erection of steel arch and re-spraying. After the initial support, when the deformation of surrounding rock tends to be stable, secondary support and bottom sealing, that is, permanent support (or shotcreting or pouring concrete arch), can improve the safety and enhance the bearing capacity of the whole support, and this support opportunity can be obtained from the monitoring results.