The following is the relevant content about new underground engineering construction technologies brought to you by Zhongda Consulting for your reference.
This paper summarizes the new processes and technologies used in the construction of a number of large-scale infrastructure construction projects in my country in recent years, such as the Qinghai-Tibet Railway, Shenzhen Metro, Shanghai Cross-River Tunnel and other underground engineering construction.
The construction and smooth implementation of the Qinghai-Tibet Railway provide a good experimental basis for solving underground engineering construction in plateau permafrost areas; at the same time, the construction of urban subway projects is also helpful for solving underground engineering problems under complex urban geological environment conditions. Engineering construction poses new challenges; and the construction of large bridges, cross-river tunnels and offshore facilities makes underwater underground engineering construction face higher technical requirements. The construction and completion of a series of large-scale infrastructure has greatly promoted the technical level of underground engineering construction. The timely summary and improvement of these new underground engineering construction techniques and other technical achievements will provide good technical support and guarantee for future underground engineering construction, and will promote the construction of underground engineering projects. The construction of underground engineering projects in our country has brought huge promotion effects. This article combines the underground engineering construction technology achievements obtained during the construction of some large-scale infrastructure construction projects in my country in recent years, such as the Qinghai-Tibet Railway, Shenzhen Metro, Shanghai Cross-River Tunnel, etc., and introduces the new technology in order to provide reference for the construction of similar projects in the future. .
1 New technology for underground engineering construction in permafrost areas
The Golmud to Lhasa section of the Qinghai-Tibet Railway is more than 1,100 kilometers long, passing through the world's highest altitude, known as the roof of the world, and has harsh construction conditions. Tibetan Plateau. This is the first time in the world to build a railway in a high-altitude permafrost area. There is no mature construction experience and the technology content is high.
1.1 Construction technology of bored piles in permafrost areas
The key technology is to reduce various heat generated during the construction process, such as friction heat of drilling, heat of backfill material, The heat of hydration of the cast-in-place pile concrete can avoid rapid changes in the temperature field of the foundation soil around the pile, causing the foundation soil around the pile to heat up and melt within a certain range. At the same time, due to seasonal changes in frozen soil areas, the seasonal melting layer on the surface will produce frost heave forces with seasonal changes. Eliminating these frost heave forces is also a focus of bored piles.
In order to reduce the impact of construction heat on the permafrost area and form a new thermal equilibrium state as soon as possible, after the concrete of the bored pile body in the permafrost area is poured, it must go through a stage of heat exchange process before proceeding. For construction above the cap platform, the general heat exchange time is 60 days. After 60 days, the pile foundation can be considered to be basically stable.
During use, the pile foundation will generate frost heave force due to seasonal changes in frozen soil. According to the location and direction where the frost heave force acts on the foundation surface, it can be divided into three types: tangential frost heave force, horizontal frost heave force and normal frost heave force (see Figure 1). Horizontal frost heave forces cancel each other out, and the main damage to the project is the tangential force and normal force generated by frost heave. During project construction, the following measures can be taken to prevent pile foundations from frost heaving: ① In order to avoid the normal frost heaving force on the pile foundations, embed the pile foundations to a certain depth below the natural upper limit of the permafrost soil; ② Bury the steel expansion tubes into the permafrost soil At least 0.5m below the upper limit of the soil, the inner diameter of the casing is 10cm larger than the pile diameter, and residual oil is applied to the outside of the casing. The casing is not removed after the pile is established to reduce the hydrophilicity of the outer surface; ③ Use high pile caps as much as possible, and freeze them In areas with serious swelling, use drilled and expanded bottom piles; ④ Use coarse-grained soil mixed with residual oil to backfill on the outside of the casing and the bottom of the low pile cap. The above measures can effectively reduce the tangential frost heave force and reduce the upward frost heave force of frozen soil on the casing (see Figure 2); ⑤ Use the dry method of drilling with a rotary drilling rig to ensure the correct hole position and drilling accuracy. Verticality; ⑥ Use low-temperature early-strength and durable concrete to avoid the problem of slow strength growth caused by low-temperature concrete pouring.
1.2 Permafrost Tunnel Construction Technology
There is little experience to learn from in the construction of plateau permafrost tunnel projects. The core is to minimize the impact of rising temperatures on frozen soil and avoid Melting frozen soil, compression, subsidence and frost-heaving forces cause construction disasters and operational hazards.
The compressive strength of frozen soil is very high, and its ultimate compressive strength is even equivalent to that of concrete. The compressive strength of frozen soil decreases sharply after melting, and the resulting thermal melt subsidence and frost heave in the next cold season often cause engineering buildings to become unstable and difficult to repair.
When the temperature of water-containing loose rocks and soil drops to 0°C, ice is produced, which is the main sign of the frozen state. When water freezes into ice, its volume increases by about 9, causing frost heaving in the soil. When the soil freezes, not only does the water in the original position freeze into ice, but also under the action of penetration force (suction force), the water will transfer from the unfrozen area to the freezing front and freeze into ice there, making the frost heave of the soil more intense.
During the freezing process, the volume of soil increases due to water turning into ice, which causes water migration, ice separation, frost heave, and soil skeleton displacement, thus changing the structure of the soil. The melting process is inevitably accompanied by the displacement of soil particles, filling the space discharged by melting ice, resulting in melting and consolidation, which causes downward movement of the local ground, that is, melt subsidence (thermal subsidence).
In order to avoid thermal melt subsidence during tunnel construction, the key process for frozen soil tunnel construction is to take thermal insulation measures.
The tunnel insulation construction technology mainly includes: optimizing the construction of open holes and openings in cold seasons, and adding sunshade and insulation sheds during excavation and construction to block the impact of solar radiation energy on the frozen soil. The main tunnel uses weak blasting and smooth blasting technology to reduce disturbance to the frozen soil and over-excavation. After excavation, the scattered ice cubes in the arch (wall) interlayer are removed, and concrete is sprayed quickly to seal the rock surface; rail transportation is used to reduce exhaust gas in the cave. pollution, reduce the number of ventilation times and air volume; in the warm season, measures such as nighttime cannon ventilation and air cooler ventilation are used to control the temperature of the tunnel face below 5°C and minimize the frozen soil melting zone outside the tunnel excavation section. A "waterproof layer and insulation board waterproof layer" is laid over the entire length of the tunnel to block the disturbance of frozen soil caused by temperature changes in the tunnel after the tunnel is completed, ensuring operational safety.
The main factors affecting soil frost heave are soil type, water content and freezing conditions. Permafrost scientists have proven through long-term experiments that coarse-grained soil has little or no frost heave, while fine-grained soil generally has greater frost heave. When the soil moisture content is large, frost heave will be serious. When the soil moisture content is less than a certain value, the frost heave rate of the soil is zero. In order to prevent the impact of frost heaving on the engineering structure of the open cave and the entrance, the ice-rich frozen soil, ice-saturated frozen soil and soil-containing ice layer within the frost heaving range around the open cave and the upward slope of the entrance were excavated and replaced with coarse-grained soil. Strictly control the moisture content of coarse-grained soil, and provide waterproof and drainage facilities after filling.
Engineering example: The Fenghuoshan permafrost tunnel on the Qinghai-Tibet Railway is 1,338m long and is the highest permafrost tunnel in the world. The permafrost upper limit is 1 to 1.8m and the permafrost layer is 100 to 150m thick. The entire cave body is located in frozen soil. During the construction process, we fully grasped the engineering properties of frozen soil, adopted excavation techniques such as grouting pipe sheds, grouting anchors, and smooth blasting in the hole, and comprehensively used coarse-grained soil to replace the covering layer of the open hole, covering the entire length and cross-section. Set up multiple insulation layers, as well as multiple technologies such as insulation, temperature control, oxygen supply, shotcrete, and information monitoring to minimize the melting circle of frozen soil and rebuild a new heat balance system in the frozen soil tunnel to meet the requirements of safety, high quality, and efficiency. construction requirements.
In addition, the temperature prevention measures in frozen soil areas include the construction technology of filling gravel ventilation roadbed, the construction technology of laying insulation board roadbed in high-temperature fine-grained soil, and the construction technology of hot-rod roadbed in high-temperature fine-grained soil. These measures can all be used Greatly reduce the impact of thermal melting of frozen soil after the roadbed is loaded.
2 New technology for subway and river-crossing tunnel construction
With the rapid development of urbanization in our country, the traffic pressure in big cities is increasing day by day, and large-scale urban subway construction is inevitable. There are also more and more constructions of urban cross-river tunnels planned along the river. This type of engineering construction often involves large scales, harsh construction environments, and complex construction techniques. Here is a brief introduction to several new construction techniques.
2.1 Pile foundation underpinning technology in subway construction
It is inevitable to encounter pile foundation underpinning projects in subway construction. Research on the large axial force pile foundation underpinning technology of Shenzhen Metro Department Store has solved the main key technical problems of large axial force pile foundation underpinning and enriched the construction technology of the pile foundation underpinning project.
The pile foundation underpinning form is a common form of underpinning technology application in my country. The core technology of pile foundation underpinning lies in the conversion of loads between new piles and old piles, which requires that the deformation of the underpinning structure and new piles be limited to the allowable range of the superstructure during the conversion process. For the control of the above deformation, the underpinning mechanism can be divided into active and passive underpinning.
Active underpinning mainly loads the new piles and the underpinning structure before the old piles are cut, so as to eliminate some of the deformation of the new piles and the underpinning structure, so that the deformation of the piles and structures after underpinning is limited to the allowable range. This technology is used in situations where the axial force is large and the structure has strict deformation requirements. Passive underpinning transfers the load to the new piles during the removal of the old piles. The deformation of the piles and structures after underpinning is difficult to control. This technology is suitable for small tonnage and situations where the control of structural deformation is not strict. The pile foundation underpinning project of the Department Store Plaza Building in the Guomao Old Street District of Shenzhen Metro has many underpinning piles (6), large axial force (18000kN), large pile diameter (2000mm), poor geological conditions, high groundwater head, and deep underpinning location. (2 floors underground), complex usage environment (subway crossing in the middle, vibration influence), etc., there is currently no similar large axial force underpinning construction experience at home and abroad (the maximum axial force of similar underpinning in Japan is 8750kN, and 5900kN in China).
Due to constraints such as direction and minimum radius (Rmin=300m), the Shenzhen Metro Phase I project line must pass under the podium of the Department Store Plaza Building. This creates a pile foundation underpinning problem. The main building of the Department Store Plaza has 22 floors, a podium floor with 9 floors, and a basement with 3 floors. It has a frame-beam shear wall structure and the foundation is an independent pile base end-bearing pile. The standard bearing capacity of the pile end bearing layer (strongly weathered layer) is 2700kPa, and the maximum design axial force of the underpinning pile for a manually dug pile (C25) with a maximum pile diameter of 2000mm is about 18900kN based on the floor.
The section tunnel passes through Department Store Plaza, Shennan East Road, and Huazhong Hotel. Due to the influence of the location of the underground tunnel and its superstructure, some piles are in the tunnel or close to the tunnel, and the Department Store Plaza 9 must be replaced. There are 6 piles in the podium floor (the pile diameter is 2000mm, the pile foundation bearing layer is all in the bedrock below the tunnel structural surface), and the maximum axial force is 18000kN.
According to the structure, foundation form and operating space of the department store plaza, the pile foundation underpinning of the department store plaza adopts the form of beam-type underpinning structural columns, and the new piles for underpinning use manually dug piles. The entire underpinning project is carried out in It is held indoors on the third underground floor.
According to the deformation requirements of high-rise structures, the podium pile foundation adopts active underpinning. During underpinning, a loading jack is installed between the underpinning beam and the new pile. The jack is used to load, so that the upper structure has a slight lifting displacement. At the same time, most of the settlement displacement of the new pile is preloaded during the lifting, thus achieving the goal through active loading. The load acting on the original structural piles is transferred to the new piles through the underpinning girders, and the jacking value of the original piles (columns) and the settlement of the new piles are also effectively controlled. Pile cutting is carried out gradually after digging artificial holes to the bottom of the underpinning beam. After the pile cutting, the tunnel is excavated and the lining deformation is stabilized (the jack device is adjusted in time during this period), the underpinning beam is connected to the new pile to form a permanent structure, and the underpinning is completed. Strict whole-process monitoring and measurement are implemented throughout the pile foundation underpinning and tunnel construction processes to ensure structural safety.
Through strict calculations and construction operations, and through technical research, we have solved technical problems such as excavation and support of pile foundations in soft strata, underpinning beams, pile cutting, and force conversion, ensuring that high-rise buildings such as department stores and shopping malls are safe. Safety and normal use of buildings and underground pipelines.
The pile foundation underpinning principle of this project is shown in Figure 3.
2.2 Horizontal freezing method in the construction of cross-river tunnels
The freezing method of connecting passages between underground tunnels uses artificial refrigeration technology to turn the water in the ground into ice and freeze the natural A special construction method that turns the soil into frozen soil, increases its strength and stability, and isolates the connection between groundwater and underground structures so that communication channels can be constructed under the protection of frozen walls.
The refrigeration technology is completed by three major circulation systems using Freon as the refrigerant. The three major circulation systems are Freon circulation system, brine circulation system and cooling water circulation system. The three major refrigeration circulation systems form a heat pump, which transfers geothermal heat from low-temperature brine to the Freon circulation system through the freezing holes, and then passes it from the Freon circulation system to the cooling water circulation system, and finally discharges it into the atmosphere through the cooling water circulation system. As low-temperature salt water continues to flow in the formation, the water in the formation gradually freezes, forming a frozen soil cylinder with the frozen tube as the center. The frozen soil cylinder continues to expand, and finally the adjacent frozen cylinders are connected into one and form a frozen soil cylinder with a certain thickness. and strength of a permafrost wall or permafrost curtain. The principle of horizontal freezing reinforcement is shown in Figure 4.
In actual construction, freezing holes are drilled horizontally, freezing pipes are set up, and salt water is used as the heat transfer medium for freezing. Generally, freezing equipment is set up at the construction site to cool the antifreeze liquid (usually salt water) to -22~-32°C. Its main features are:
(1) It can effectively isolate groundwater. For water-containing, loose and unstable strata with a water content of >10, the freezing method can be used for construction.
(2) The shape and strength of the frozen soil curtain can be flexibly arranged and adjusted depending on the construction site conditions and geological conditions. The frozen soil strength can reach 4-10MPa, which can effectively improve work efficiency.
(3) Freezing method construction has no pollution to the surrounding environment, no foreign matter enters the soil, and little noise.
(4) There are many factors that affect the strength of frozen soil. Frozen soil is a rheological body. Its strength is related to both the origin of frozen soil and the characteristics of stress. The main factors affecting frozen soil are: Freezing temperature, soil moisture content, soil particle composition, load action time and freezing speed, etc.
The key construction technologies of the freezing method include:
(1) Determine the main technical indicators of freezing, that is, determine the brine temperature and freezing period of the active freezing period and maintenance freezing period based on actual working conditions. Average soil wall temperature and frozen soil strength.
(2) The layout and construction of frozen holes, that is, the frozen holes are designed and arranged according to the plane size of the connecting channel and the stress characteristics of the structure. At the same time, the frozen hole layout should be fine-tuned according to the segment reinforcement diagram and the deflection of the frozen holes. The outward deflection angle of the aperture is controlled in the range of 0.5° to 10°.
(3) Freezing station design, active freezing and maintenance freezing construction, calculation of freezing cooling capacity, and selection of refrigeration units based on cooling capacity needs.
(4) Construction methods and sequences of excavation and construction of connecting channels.
(5)Construction monitoring and control.
The Shanghai Dalian Road Cross-River Tunnel Project consists of two east and west tunnels. There are connecting passages between the two tunnels, both located under the Huangpu River, about 400m apart. The connecting channel (1) located on the Puxi shore, the center distance between the east-west line tunnels is 35.705m, the height difference between tunnels is 3.565m, and the clear distance of the connecting channels is about 25.665m; the connecting channel (2) located on the Pudong shore, the center distance of the east-west line tunnels 27.575m, the height difference between tunnels is 0.345m, and the clear distance of connecting channels is 17.175m. The stratum where the two connecting channels are located is sandy silt and clayey silt, with large permeability coefficient and high pressure head. In order to ensure the construction safety of the channel, the freezing method is used for construction. Engineering practice shows that the freezing construction technology of connecting channels has the advantages of fast freezing speed, high frozen soil strength, good curtain uniformity, high leakage resistance, tight integration with tunnel segments, and safe and reliable construction. For the construction of river bottom connecting channels under long distances, large depths, and high pressure water conditions, the safety and reliability can be guaranteed. As melt settlement is an unavoidable situation in freezing method construction, grouting holes reserved in tunnels and connecting channels can be used to compensate for the grouting of the strata in a timely manner to reduce the amount of melt settlement. In the construction of several connecting channels, its superiority and socio-economic value have been fully demonstrated.
2.3 Construction technology of underground excavation with three-arch and two-column structures in subway stations
With the development of urban subways and rapid transit rails in my country, more and more large cities are building subways. . Since most of the areas that the subway passes through are prosperous commercial districts, and some areas are affected by demolition and reconstruction costs, traffic occupation, underground pipeline protection, ancient cultural relics protection, environmental protection, etc., the open-cut (cover-and-cover) subway stations are restricted. , only the underground excavation method can be used for construction, thus the underground excavation subway station appeared.
The Ciqikou Station, Temple of Heaven East Gate Station, and Chongwenmen Station projects of Beijing Metro Line 5 adopt the three-arch and two-column concealed excavation method of tunneling in the station to comprehensively support the construction technology, ensuring the quality and safety of the project and on schedule. The construction tasks were completed and good social benefits were achieved. This technology is suitable for the construction of large-span double-layer underground subway stations and multi-arch underground parking lots, underground shopping malls, long-span highways, and railway tunnels where the self-stabilizing ability of the surrounding rock is poor.
Technical characteristics of the tunnel construction method for underground excavation stations:
(1) The CRD (Cross Diaphragm) construction method is used to complete the tunnel excavation to form a safe tunnel initial support system.
(2) Complete the bottom plate, bottom beam, steel pipe column, middle plate, top beam and middle arch in the middle tunnel to form a stable middle tunnel support system, which can bear the main load of the surrounding rock and provide safe conditions for the excavation of the side tunnel. .
(3) The CRD method is used to symmetrically complete the side tunnel excavation.
(4) Remove the temporary initial support system and complete the construction of the second lining of the side tunnel.
(5) During the system conversion process, the segment lengths should be reasonably determined and steel supports should be added.
(6) Give full play to the role of monitoring and measurement, and guide construction through informatization.
The technical principle of the construction of the underground tunnel in the station: Divide the long-span tunnel with poor geology into three parts, and segment each part into sections to ensure safety during excavation. The initial temporary structure of the middle tunnel is first formed. A permanent lining structure is built within the temporary structure to form a stable support in the middle to bear the main load of the surrounding rock. Then each block of the side tunnel is symmetrically excavated, and finally the overall structure is formed. During the system conversion process, steel supports were added based on monitoring conditions. The process flow is: construction preparation → advanced pipe shed → grouting reinforcement → excavation of each part of the middle tunnel → laying of waterproof layer → middle tunnel floor and bottom beam → column → middle plate of the middle tunnel → top beam and middle arch → advanced pipe shed → Grouting reinforcement → excavation of each part of the side tunnel → removal of temporary partitions → laying of waterproof layer → floor of side tunnel → side wall, middle plate → side arch → grouting behind the secondary lining. The construction of the subway station's three-arch and two-column structure using the underground excavation method is shown in Figure 5.
Ciqikou Station is the transfer station between Beijing Subway Line 5 and the planned Beijing Subway Line 7. The station is 180m long, 21.87m wide and 14.933m high. The construction area of ??the station is 12244.2m2, and the soil covering depth of the main body of the station is 9.8~10.3m. The station is a double-layer island-type structure with three arches and two columns. The first underground floor of the station is the station hall floor, with reserved passages for interchange with Line 7, and the second underground floor is the platform level. The station construction adopted this method to ensure the safety and quality of the project construction and achieved success.
3 Underwater foundation construction technology
3.1 Marine foundation engineering construction
With the construction of infrastructure, offshore projects such as cross-sea bridges have gradually increased, and a number of Bridges planned and under construction, such as the Bohai Bay Cross-sea Project, the Yangtze River Estuary Cross-River Project, the Hangzhou Bay Cross-Sea Project (under construction), the Pearl River Estuary Lingdingyang Cross-Sea Project, and the Qiongzhou Strait Project, have brought new developments to offshore infrastructure construction. challenges. It is an inevitable trend to use large diameter and long foundation piles for the foundation of large-scale cross-sea and cross-river projects. Structural steel pipe piles, temporary steel casings and temporary steel pipe piles for offshore platforms will be widely used. These have put forward new requirements for piling ships. The piling ship equipped with a high pile frame, a powerful pile lifting power system, a high-energy piling hammer and an advanced offshore pile sinking GPS measurement and positioning system can excellently complete the task of hammering and sinking piles at sea.
From a large perspective, the offshore piling system includes a combination of piling ships, pile transporting ships, anchor boats, tugboats and transportation ships. From the perspective of the sinking process of steel pipe piles, the piling ship is the main body of the steel pipe pile sinking, which mainly consists of the following parts: hull system (including hull, anchor system, power system), pile frame and its Pile lifting system, hammer pile sinking system (including pile driving hammer, replacement driver), offshore pile sinking GPS measurement and positioning system, etc. In particular, GPS can realize the positioning of construction ships far away from the shore and the automatic collection and processing of data during the positioning process, and reflect the current and designed position of piling in the form of graphics and numbers, making it easier for operators to adjust the ship position for construction piling, and also It can automatically generate piling reports and playback data, thus bringing convenience to offshore piling.
Offshore pile positioning is achieved using the "Offshore Pile GPSRTK Measurement and Positioning System", as shown in Figure 6.
The three GPS receivers installed on the piling ship receive the fixed frequency data link transmitted by the base station established on land and the reference station at sea as the reference data for positioning. Its working principle: During positioning, the GPS rover fixed on the piling ship controls the position, direction and attitude of the hull in RTK mode, and at the same time, it cooperates with two prism-free rangefinders fixed on the ship to measure the relative position of the pile body at a certain elevation. Based on the position of the hull pile frame, the actual position of the pile body on the design elevation can be calculated and displayed on the system computer screen.
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