Who can tell me how to build an underground city?

Urban underground geotechnical engineering is a part of geotechnical engineering, which is one of many problems faced by the sustainable development of cities in China, especially big cities, and also a new topic and task faced by rock mechanics workers.

1 Urban underground geotechnical engineering is an important link in urban construction in the new century.

With the rapid development of national economy, the level of urbanization in China is also increasing rapidly, from 1990 to 18.96% to128.9% at the end of 1997. The improvement of urbanization level marks the rapid development of urban engineering construction. However, China's urban construction basically follows the extensive development mode of "spreading the pie", which brings undue losses to the national economy. Mainly:

(1) The urban area has expanded infinitely, resulting in serious loss of cultivated land. Satellite remote sensing data show that during the period from1986 to1996, the actual urban area of 3/kloc-0 megacities in China increased by 50.2%, and some urban areas increased exponentially. It is predicted that by 20 10, the total number of cities in China will increase from 65438+640 in 0996 to 1 000, so a lot of cultivated land will be occupied. By the middle of the next century, China's urbanization level will increase to about 65%, which means that the urban population will increase by more than 700 million people compared with 1990. According to the land used by each urban population, it will occupy more than 100,000 mu of cultivated land1000. Land problem is the key to the sustainable development of China. The rapid growth of urban population and the limitation of regional scale have become prominent contradictions in urban development, and urban development must adopt an intensive development model of saving land.

(2) The high population density in cities leads to the so-called "urban syndrome". First of all, the traffic jam in the city is slow. For example, the average speed of main roads in Beijing is more than 50% lower than that of 10 years ago, and it is decreasing by 2 km/h every year. Shanghai and Beijing have 506 cars and 345 cars per kilometer of roads, which is 1 times and several times that of big cities in developed countries. Secondly, urban infrastructure lags behind the expansion of urban area and the growth of urban population, which leads to the deterioration of urban environment. At present, China's urban environmental situation is becoming more and more severe, and air pollution is becoming more and more serious. The air quality of more than 500 cities in China has reached the first-class standard less than 1%, and the area of acid rain exceeds 40% of the national territory, especially in cities such as Chongqing. 80% of urban sewage is discharged into rivers without treatment; Urban groundwater is polluted; The phenomenon of garbage siege is common; Noise pollution generally exceeds the standard, the building space is crowded, the urban green space is reduced and the ecology is deteriorated.

(3) The overall ability of the city to resist disasters and destruction is low. In the overall urban planning, in addition to flood control and civil air defense, there is also a lack of comprehensive disaster prevention, and there are no disaster prevention measures for urban infrastructure. In order to overcome this disadvantage, solve the three crises of urban population, environment and resources, cure "urban syndrome" and implement urban sustainable development, all developed countries in the world are taking underground space as a new land resource, and the development and utilization of urban underground space has become the guiding principle and development direction of urban construction, which has attracted more and more attention.

Urban functional space can and should be transferred underground in a wide range of fields, including commerce, transportation, some municipal facilities, culture, entertainment and leisure, some industrial production, storage, disaster prevention (refuge) and disaster relief space. Making full use of underground space is the most important part of urban three-dimensional development. It can achieve the multi-functional effects of expanding space capacity, improving development intensity, eliminating hybrid bicycles, smooth traffic, more prosperous business, increasing ground green space, beautiful and open environment, and integrating shopping, leisure and entertainment. Compared with the development mode above the city, it is a more reasonable development mode.

Requiring land and space from underground has become an inevitable trend of urban construction and development, showing unparalleled advantages. Underground commercial city (street), underground garage, underground theater, underground railway and underground civil air defense system in big cities at home and abroad are all well-known urban underground projects. Some countries have begun to implement and plan to adopt underground sewage collection and treatment facilities, underground garbage treatment plants, underground superconducting magnetic direct storage of electric energy, underground heating and refrigeration systems, underground multi-functional public tunnels (in the same ditch) and underground space systems with disaster-resistant functions. They are the development direction of future urban construction.

2 characteristics and difficulties of urban underground geotechnical engineering

As we all know, underground geotechnical engineering is a field with a long history. It can be said that geotechnical engineering has existed since human beings, especially after entering the industrial society, but urban geotechnical engineering, in addition to traditional ground engineering, underground geotechnical engineering has developed with the rise of modern cities. After decades of practice, the design, construction, equipment and technology, as well as theory, technology and experience, have all reached a fairly high level, especially the deep underground rock engineering, which has reached a relatively mature level.

However, urban underground geotechnical engineering has different characteristics from general geotechnical engineering, mainly as follows: most of the buried depth. Ground buildings and traffic facilities are dense, and there are many underground pipelines. Excavation has great influence and complicated geological conditions. Mostly soil, there are often expansive soil, sand layer and groundwater, especially in coastal cities along the river. The excavation of silt and soft soil is more difficult. Therefore, there are many special problems to be solved in urban underground geotechnical engineering. Mainly:

(1) Shallow and ultra-shallow excavation construction technology. The buried depth of urban underground engineering not only directly affects the project cost, but also relates to the convenience of the project. Therefore, the buried depth of urban underground engineering is generally shallow. Under the condition of shallow burial, especially ultra-shallow burial, underground engineering needs to cross buildings, lines and streets, and ground protection has become the primary problem in construction technology.

(2) Excavation technology in complex and harsh environment. Such as quicksand layer, expansive soil, high compressibility soft soil silt, weathered broken rocks, high concentration gas stratum, large water inflow, hydrogen sulfide, karst, high stress, underground pipelines, ground truck traffic, large trucks and dense buildings. , are difficult problems in underground geotechnical engineering construction.

(3) Excavation and support technology of large-section tunnel. Mainly subway stations, shopping malls, warehouses, halls and rooms, with a span of more than10 m.

(4) Control technology of excavation influence. With the decrease of the buried depth of the project, the influence of excavation on the ground is increasing. Under the condition of ultra-shallow burial, the control of excavation influence is related to many factors such as excavation mode, construction technology and support mode, which is the most complicated problem in underground engineering construction.

3 urban underground geotechnical engineering excavation technology and its adaptation conditions

The construction of urban underground engineering in China started late. With the construction of a large number of civil air defense, subway, underground shopping malls, warehouses, theaters and other projects. Especially in recent years, the excavation technology of urban underground space has been greatly developed and improved. Open-cut underground geotechnical engineering is a part of geotechnical engineering, which is one of many problems faced by the sustainable development of cities, especially the sustainable development of big cities in China, and also a new topic and task faced by rock mechanics workers.

1 Urban underground geotechnical engineering is an important link in urban construction in the new century.

With the rapid development of national economy, the level of urbanization in China is also increasing rapidly, from 1990 to 18.96% to128.9% at the end of 1997. The improvement of urbanization level marks the rapid development of urban engineering construction. However, China's urban construction basically follows the extensive development mode of "spreading the pie", which brings undue losses to the national economy. Mainly:

(1) The urban area has expanded infinitely, resulting in serious loss of cultivated land. Satellite remote sensing data show that during the period from1986 to1996, the actual urban area of 3/kloc-0 megacities in China increased by 50.2%, and some urban areas increased exponentially. It is predicted that by 20 10, the total number of cities in China will increase from 65438+640 in 0996 to 1 000, so a lot of cultivated land will be occupied. By the middle of the next century, China's urbanization level will increase to about 65%, which means that the urban population will increase by more than 700 million people compared with 1990. According to the land used by each urban population, it will occupy more than 100,000 mu of cultivated land1000. Land problem is the key to the sustainable development of China. The rapid growth of urban population and the limitation of regional scale have become prominent contradictions in urban development, and urban development must adopt an intensive development model of saving land.

(2) The high population density in cities leads to the so-called "urban syndrome". First of all, the traffic jam in the city is slow. For example, the average speed of main roads in Beijing is more than 50% lower than that of 10 years ago, and it is decreasing by 2 km/h every year. Shanghai and Beijing have 506 cars and 345 cars per kilometer of roads, which is 1 times and several times that of big cities in developed countries. Secondly, urban infrastructure lags behind the expansion of urban area and the growth of urban population, which leads to the deterioration of urban environment. At present, China's urban environmental situation is becoming more and more severe, and air pollution is becoming more and more serious. The air quality of more than 500 cities in China has reached the first-class standard less than 1%, and the area of acid rain exceeds 40% of the national territory, especially in cities such as Chongqing. 80% of urban sewage is discharged into rivers without treatment; Urban groundwater is polluted; The phenomenon of garbage siege is common; Noise pollution generally exceeds the standard, the building space is crowded, the urban green space is reduced and the ecology is deteriorated.

(3) The overall ability of the city to resist disasters and destruction is low. In the overall urban planning, in addition to flood control and civil air defense, there is also a lack of comprehensive disaster prevention, and there are no disaster prevention measures for urban infrastructure. In order to overcome this disadvantage, solve the three crises of urban population, environment and resources, cure "urban syndrome" and implement urban sustainable development, all developed countries in the world are taking underground space as a new land resource, and the development and utilization of urban underground space has become the guiding principle and development direction of urban construction, which has attracted more and more attention.

Urban functional space can and should be transferred underground in a wide range of fields, including commerce, transportation, some municipal facilities, culture, entertainment and leisure, some industrial production, storage, disaster prevention (refuge) and disaster relief space. Making full use of underground space is the most important part of urban three-dimensional development. It can achieve the multi-functional effects of expanding space capacity, improving development intensity, eliminating hybrid bicycles, smooth traffic, more prosperous business, increasing ground green space, beautiful and open environment, and integrating shopping, leisure and entertainment. Compared with the development mode above the city, it is a more reasonable development mode.

Requiring land and space from underground has become an inevitable trend of urban construction and development, showing unparalleled advantages. Underground commercial city (street), underground garage, underground theater, underground railway and underground civil air defense system in big cities at home and abroad are all well-known urban underground projects. Some countries have begun to implement and plan to adopt underground sewage collection and treatment facilities, underground garbage treatment plants, underground superconducting magnetic direct storage of electric energy, underground heating and refrigeration systems, underground multi-functional public tunnels (in the same ditch) and underground space systems with disaster-resistant functions. They are the development direction of future urban construction.

2 characteristics and difficulties of urban underground geotechnical engineering

As we all know, underground geotechnical engineering is a field with a long history. It can be said that geotechnical engineering has existed since human beings, especially after entering the industrial society, but urban geotechnical engineering, in addition to traditional ground engineering, underground geotechnical engineering has developed with the rise of modern cities. After decades of practice, the design, construction, equipment and technology, as well as theory, technology and experience, have all reached a fairly high level, especially the deep underground rock engineering, which has reached a relatively mature level.

However, urban underground geotechnical engineering has different characteristics from general geotechnical engineering, mainly as follows: most of the buried depth. Ground buildings and traffic facilities are dense, and there are many underground pipelines. Excavation has great influence and complicated geological conditions. Mostly soil, there are often expansive soil, sand layer and groundwater, especially in coastal cities along the river. The excavation of silt and soft soil is more difficult. Therefore, there are many special problems to be solved in urban underground geotechnical engineering. Mainly:

(1) Shallow and ultra-shallow excavation construction technology. The buried depth of urban underground engineering not only directly affects the project cost, but also relates to the convenience of the project. Therefore, the buried depth of urban underground engineering is generally shallow. Under the condition of shallow burial, especially ultra-shallow burial, underground engineering needs to cross buildings, lines and streets, and ground protection has become the primary problem in construction technology.

(2) Excavation technology in complex and harsh environment. Such as quicksand layer, expansive soil, high compressibility soft soil silt, weathered broken rocks, high concentration gas stratum, large water inflow, hydrogen sulfide, karst, high stress, underground pipelines, ground truck traffic, large trucks and dense buildings. , are difficult problems in underground geotechnical engineering construction.

(3) Excavation and support technology of large-section tunnel. Mainly subway stations, shopping malls, warehouses, halls and rooms, with a span of more than10 m.

(4) Control technology of excavation influence. With the decrease of the buried depth of the project, the influence of excavation on the ground is increasing. Under the condition of ultra-shallow burial, the control of excavation influence is related to many factors such as excavation mode, construction technology and support mode, which is the most complicated problem in underground engineering construction.

3 urban underground geotechnical engineering excavation technology and its adaptation conditions

The construction of urban underground engineering in China started late. With the construction of a large number of civil air defense, subway, underground shopping malls, warehouses, theaters and other projects, especially in recent years, the excavation technology of urban underground space has been greatly developed and improved. Cut and cover method, underground excavation method, cover excavation method, shield method, immersed tube method, freezing method and grouting method are used in urban underground tunnels and boreholes in China, and some of them have reached the international advanced level.

3. 1 Cutting and Covering

Cut and cover has the advantages of simple, rapid, economical and safe construction, and it is the first choice of excavation technology in the early stage of urban underground tunnel engineering development. Its disadvantage is that it has a great influence on the surrounding environment.

The key processes in open cut are: lowering groundwater level, slope support, earthwork excavation, structural construction and waterproof engineering. Among them, slope support is the key technology to ensure safe construction. Mainly includes:

(1) Slope excavation technology. Suitable for open ground and good underground geological conditions. The foundation pit is excavated in layers and sections from top to bottom, and the slope is brushed with the excavation. Cement clay should be used for slope protection when necessary.

(2) Steel support technology. Generally, single-row I-beams or steel sheet piles are used. When the foundation pit is deep, double-row piles can be used, which are connected by tie rods or connecting beams to bear the force together. Multi-layer steel cross braces or single-layer and multi-layer anchor rods and steel beams can also be used to form the support structure.

③ Continuous wall support technology. Generally, wire rope and hydraulic grab are used to form grooves, and multi-head drills and cutting wheels can also be used to form grooves. Diaphragm wall can not only bear large load, but also has waterproof effect, which is suitable for soft soil and loose aquifer.

(4) Supporting technology of concrete cast-in-place pile. There are generally two ways: manual excavation or mechanical drilling. Ordinary concrete and underwater concrete are poured into holes to form piles. Support can be double-row piles and concrete coupling beams, or piles and cross braces or anchor rods can be used to form a stress system.

(5) Soil nailing wall support technology. In the in-situ soil, holes are made manually by mechanical drilling or Luoyang shovel, dense rows of steel bars or steel pipes are added, cement mortar or grouting is injected outside, and concrete is sprayed, so that the soil, steel bars and sprayed concrete surfaces are combined to form a soil nail support system.

(6) Bolt (cable) supporting technology. Grouting will be carried out after placing steel bars or steel cables in the holes. After reaching the strength, it is anchored with the pile wall, and the prestressed anchor bears the same force. Suitable for high slopes and heavy-duty places.

(7) Supporting methods of concrete and steel structure. According to the design calculation, concrete internal support system and steel structure internal support system are poured at different excavation positions to form a frame support system with cast-in-place piles or underground continuous walls to bear lateral earth pressure, and the internal support system should be dismantled when making structures. Suitable for high-rise building dense areas and soft silt strata.

3.2 Underground excavation method

It is suitable for places where open-cut cover construction cannot be used in cities, and also for loose layers and water-bearing loose layers.

Generally, the construction should be designed according to the principle of "New Austrian Tunneling Method", with strong initial support, grouting first and then excavation. The construction principle is: "pipe ahead, strict grouting, short excavation, strong support, quick closure and diligent survey". Generally, 30 ~ 50 mm steel pipe is used to lead out the roof conduit, and then cement or chemical slurry is injected to form a "stone body" to enhance the self-stabilizing ability of surrounding rock. Every time the excavation footage is about 0.75 m, circular excavation shall be carried out first, core soil shall be left, 5 ~ 8 cm concrete shall be sprayed in advance, arch frame and steel mesh shall be erected, and 25 ~ 30 cm concrete shall be sprayed to form initial support, and secondary lining shall be carried out after waterproof layer.

Underground excavation methods include single arch and single span and multi-arch and multi-span underground excavation construction technology. Xidan Station of Beijing Metro is a multi-arch and multi-span. There are also three-arch subway stations, four-arch subway stations, five-arch subway stations, highway tunnels and underground shopping malls. Beijing Tianwaitian underground shopping mall is a five-arch structure. There is also the construction technology of flat wall excavation. The top of the traditional underground excavation method in the world is arched structure, and China has created ultra-shallow underground excavation construction technology with flat top and straight wall, such as crossing the street in Chang 'an Street in Beijing.

When excavating underground in rocks, the drilling and blasting method is usually used. In order to protect the self-supporting capacity of surrounding rock, smooth blasting technology is widely used. In order to reduce the influence of vibration on the ground, millisecond blasting and reasonable design of blasting parameters are also adopted to reduce vibration.

3.3 overburden excavation method

Slope support is a frame structure composed of continuous wall and concrete cast-in-place pile, with cover plate excavation and structure construction method. It has the advantages of rapidity, economy and safety, less impact on the environment than cut and cover, and lower cost than the underground excavation method. Suitable for high-rise buildings in urban areas.

Cover excavation method can be divided into two types: layer-by-layer excavation from shallow to deep, reverse cover excavation method of making structure layer by layer, and positive construction method of making structure after digging to the bottom in turn. The former is suitable for complicated geological conditions and large excavation section, while the latter is the opposite.

3.4 shield method

Refers to the method of full-face advancing cylindrical steel shield excavation. The construction methods are manual, semi-mechanized and fully mechanized. The shield is propelled by a hydraulic jack. Shield method can complete tunnels with a diameter of tens of centimeters to more than ten meters and large-scale projects with double, triple and quadruple shields. Suitable for stable and unstable loose aquifers.

In terms of construction technology, there are many shield methods, such as slurry shield, earth pressure balance method (which can control ground settlement), open mechanized shield, pneumatic shield, knife-inserted shield and hybrid shield. Tunnel boring machine (rock shield) can also be used in rock stratum.

In addition, trenchless technology called "gopher" has been developed at home and abroad, including guided drilling, directional drilling, impact spear, rammed pipe, horizontal pipe jacking and spiral drilling. Is this the way to finish the next jacking of China Capital Airport runway? 273 mm, wall thickness 8 mm, length 1 10 m, used as a steel pipe for installing communication cables. The longest pipe laying length in China can reach 500 m and the largest pipe laying diameter is 800 mm, and the pipe laying equipment has reached the international advanced level.

3.5 Freezing method

Stratum freezing method is a construction method which uses artificial freezing to consolidate unstable loose sandy soil or soft rock and cut off groundwater. A certain number of freezing holes are drilled around the underground project to be excavated, and the low-temperature brine provided by refrigeration equipment circulates through the liquid supply pipe in the freezing pipe, so that a frozen wall with certain ground pressure resistance is formed locally in the stratum, and excavation construction is carried out under its protection. After the project is completed, the frozen wall melts and the stratum rock and soil is restored to its original state. This method is suitable for loose aquifers and has been widely used in coal mines. This method has been successfully applied to Shanghai Metro 1# Line, 2# Line, the foundation pit of Shanghai Yangshupu Water Plant Pump Station, Beijing Metro Dabeiyao Tunnel and other complex and difficult sections, and horizontal freezing technology and liquid nitrogen quick freezing technology have been tried for the first time.

3.6 immersed tube (box) method

Immersed tube (box) method is an underwater geotechnical engineering construction method, which welds precast reinforced concrete structure with head steel plate, and then releases water and floats it to the design position. There are a large number of coal mine construction examples at home and abroad. Guangzhou Pearl River Tunnel adopts this method. It is suitable for shaft construction of underwater parts and shallow soil layers of river-crossing and sea-crossing tunnels.

3.7 Drilling method

Drilling method is a widely used and technologically advanced construction method for geotechnical wells and holes. All the excavation work is carried out on the ground, so workers do not need to "bury" and the labor intensity is low. It uses a special large-diameter drilling rig (the largest drilling diameter in China is 9.3 m) to drive the drill pipe and drill bit to drill, protect the wall with mud, discharge slag with compressed air, float and sink the shaft wall, fill the well behind the wall with cement, etc., and drill ahead at one time and enlarge the hole by stages. This method has been successfully applied to 47 deep vertical shafts in China's coal mines. In addition, there is a bottom-up reverse drilling construction technology. Drilling methods have been successfully applied to complex underwater geotechnical engineering such as mines, railways, transportation, national defense, hydropower and so on.

3.8 grouting method

Grouting method refers to a construction technology of drilling holes with grouting equipment and selected grouting technology for rock and soil reinforcement. It has long been widely used. According to different grouting materials, there are single and double liquid grouting, cement grouting, clay cement grouting and chemical material grouting; According to different grouting machines, there are gravity grouting, pressure grouting, infiltration grouting and jet grouting.

The high-pressure jet grouting method developed in recent years has played a unique role in geotechnical engineering reinforcement and water prevention, such as high-pressure jet grouting pile method, high-pressure fixed jet grouting wall method and horizontal jet grouting method. In the construction of Shanghai Metro 1# line, the triple-pipe high-pressure jet grouting pile method is used for curtain water plugging and seepage control reinforcement of silt stratum, and the effect is very ideal. The method of combining high-pressure jet grouting pile with cast-in-place pile is widely used in foundation pit slope protection engineering of high-rise buildings.

4 urban underground geotechnical engineering excavation impact and environmental protection

The excavation influence in urban underground geotechnical engineering refers to the surrounding rock movement and ground settlement caused by excavation, excluding other disturbance influences. Underground excavation will inevitably cause displacement and deformation of surrounding rock and soil. Because of the small excavation depth, its influence will inevitably spread to the ground, but because of the limited excavation width, its influence can also be controlled. The extent and scope of the impact depend on many factors. For shallow-buried and ultra-shallow-buried tunnel excavation, it mainly depends on the excavation method, section span, pilot tunnel form, machines and tools, support method and timing, member stiffness, backfill, ground load (dynamic and static load), rock and soil properties and groundwater pumping and drainage.

According to the measured research, the cross section of ground settlement caused by tunnel excavation is generally basin-shaped, which is generally described by probability integral curve.

For the ground settlement caused by shallow and ultra-shallow tunnel excavation, the maximum settlement value is roughly composed of the settlement before excavation space support, the settlement caused by groundwater pumping and the settlement after excavation space support. These settlements can be minimized by taking some measures to reduce them. According to the achievements of subway construction in Beijing, Shanghai and Guangzhou, the maximum cumulative settlement of high-rise buildings on both sides of the "Fu Ba Line" of Beijing subway is only 2.5 mm, and the maximum cumulative ground settlement directly above Xidan Station of Beijing subway is no more than 30mm；; ; There is a tunnel in Guangzhou Metro that runs through Tianhe Road, the main road in the city. The top of the tunnel is 7 meters away from the highway. The stratum is a fine sand layer filled with water, and there are water supply pipes, sewage pipes and cables underground. The ground traffic volume is about 6.5438+0.2 million vehicles day and night, and large container trucks with a load of 30 ~ 60 t pass quickly. After excavation, according to the observation of 128, the maximum settlement is 20.7 mm, which is lower than the international standard for land subsidence control.

According to the practice of shallow excavation at home and abroad, the measures to reduce land subsidence are as follows:

(1) Pre-reinforcement of surrounding rock. In order to reinforce soft rock and soil, pilot tunnel or full-face pre-grouting is generally used. For weak or broken rock mass, single liquid or double liquid pressure pre-grouting is adopted; For loose soil, single liquid or double liquid high pressure jet grouting is used.

(2) Strong support. Including pre-supporting, improving the rigidity of supporting members and filling behind the wall. There are two methods of advance support: pipe shed and inserting plate. The drilling depth of pipe shed is limited by the size of pilot tunnel, and it can also be used as grouting pipe, with a wide range of application conditions. The patch panel needs to be jacked in, which is waterproof, but it cannot be used in pebble stratum. Timely support can effectively reduce the settlement before support. The anchor pipe with locking feet is used to stabilize the foundation of supporting members step by step, which creates good conditions for lower excavation and support installation and reduces the settlement of upper supporting members. Increasing the stiffness of support members can reduce the settlement after support. Filling behind the wall is an effective measure to reduce the gap between supporting structure and rock mass. After the primary support and the secondary support, the small pipe grouting method is used for filling.

(3) Excavation step by step and timely support. Practice has proved that step-by-step excavation and timely support can effectively reduce the settlement of surrounding rock and ground. For example, when Beijing Chang 'an Street crosses the street, it has large span (excavation span 1 1.6 m), ultra-shallow burial (topsoil thickness is only 0.6 ~ 1.0 m), large dynamic load and many underground pipelines. In order to reduce the ground settlement, the "middle hole method" was used to construct step by step, and the ground settlement was reduced to 24 mm, with good results.

(4) Precipitation-recharge technology is an effective method to control groundwater and reduce land subsidence, which has been widely used in Beijing subway construction. Generally, it is "shallow pumping and deep irrigation" or "pumping first and then irrigation". According to the actual measurement of "Fu-Ba Line" of Beijing subway, the settlement of high-rise buildings on both sides is less than 2.5 mm after adopting this method.

It is worth studying that the "high water and quick setting material" successfully tested in China in recent years has the performance of quickly consolidating water-bearing sand layers. If it can be tested in underground engineering, it will have a good application prospect for preventing groundwater from infiltrating into building space.

5. New development of foreign underground geotechnical engineering excavation technology

(1) full mechanization. From slope protection, earthwork excavation and structural construction, including the mechanization of arch installation, shotcrete, mud preparation and treatment. Computer technology is used for monitoring, thus ensuring safe and rapid construction and excellent engineering quality.

(2) The shield method has made great progress. In recent 30 years, Britain, the United States, France, Japan and other countries have adopted a large number of shield construction technologies. Japan has produced nearly 10,000 shields for subway, railway, highway, water conservancy project and pipe network construction, and there are double, triple and quadruple shields, which can complete the excavation of a three-span subway station with a width of 17m. Japan is conceiving to design a shield with a diameter of 80 m to build an artificial sun and a residential area underground.

(3) Micro shield and trenchless technology are widely used. It is mainly used to build rainwater, sewage, tap water pipelines and cable pipelines with various diameters. Miniature shield is a shield with a diameter of less than 2 m. Cutter head excavation, remote control and satellite positioning control the direction and slope, and then install segments. Trenchless technology is to use a micro-drill, punch holes through the cutting wheel, and install pipes or cables after returning to the drill pipe.

(4) Precast block construction technology. After earthwork excavation, install arch ring with assembling machine, and leave grouting hole on segment. After the lining assembly is completed, grouting will be carried out from the grouting hole to the back of the wall to plug the gap and enhance the * * * interaction between surrounding rock and lining. The largest single arch span built by this method in France is 24.48 meters.

(5) Pre-slotting construction technology. Italy, France and other countries have made pre-slotting machines for strata, which use chains to cut the strata into grooves with a width of 15 cm and a length of 4 ~ 5 m along the arch ring, and then spray concrete into the grooves, and excavate earthwork under its protection to make waterproof layer and secondary lining, thus forming a tunnel.

(6) Pipe jacking and large pipe shed method. When building a subway station, concrete is poured into the pipe jacking to form a large pipe shed, and then underground excavation is carried out under its protection.

(7) Micro-pressure underground excavation method. The construction is carried out in the compressed air environment of 1 atmospheric pressure according to the principle of "NATM". The advantage is that it can discharge groundwater and ensure the working face to be dry; Because of the existence of air pressure, land subsidence can be reduced; But also can reduce that cost of the lin.

(8) Digital excavation, also known as computerized drilling, is suitable for the excavation of hard rock engineering. In the digital tunnel, the advance of drill pipe is programmed, and it is also automatic to go from one hole to another. The boring machine can manage three sets of drill pipes at the same time, and its function is to supervise the movement of drill pipes and make adjustments when necessary. In the computer software of the roadheader, the hole position, hole depth and tunneling sequence have been preset, and the tunneling direction is controlled by the laser beam, thus realizing the strict positioning of holes, thus realizing the optimization of tunneling technology and the tunneling of curved roadway. The advantages of digital tunnel are: controlling the overbreak of tunnel excavation; Realize the optimization of tunneling scheme; The manual measurement on the working face is eliminated. , underground excavation method, cover excavation method, shield method, immersed tube method, freezing method and grouting method, etc. Some of these technologies have reached the international advanced level.