Do you have any requirements for the subway route? Like those with tall buildings on them, do you dig tunnels at will?

Generally, the layout of subway lines is first planned by the Municipal Planning Institute, and then sampled and analyzed to determine the lines.

After analysis and finalization, the construction will be started through bidding.

Generally, there are high-rise buildings on the ground that will be deliberately staggered.

Subway construction has no influence on low sea level and high-rise buildings. The slab foundations of high-rise buildings are all underground to a certain depth, depending on local geological conditions. They are all built on rocks, so they are not affected by water. For example, in mountainous areas, the foundation is sometimes very shallow, only a few meters, because a few meters are all rocks, and the slab foundation is directly placed on it without piling.

Supplement: 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.