Foundation pit support is an important part of foundation engineering, a scientific method formed by human beings through long-term engineering practice and continuous summarization of experience. Here is a brief introduction:

　　I. Composite Soil Nailing Wall
　　Composite soil nailing wall is a composite support system, which organically combines soil nailing wall with one or several single support technologies or water cutoff technologies. It mainly consists of soil nails, prestressed anchor rods, water cutoff curtains, micro piles, hanging net shotcrete surface layer, in-situ soil and other elements, thus greatly expanding the application range of soil nailing wall support and strengthening the support function.
　　The main features of composite soil nailing wall are: lightweight, flexible, strong applicability, low cost, short construction period, good safety, large support capacity, can be used as advance support, and has both support and water cutoff effects. Therefore, in engineering practice, composite soil nailing wall is a cost-effective and practical foundation pit support technology.

　　II. Larsen Steel Sheet Pile
　　Larsen steel sheet pile is mainly used in bridge construction, pipeline laying, trench excavation, embankment protection, soil and water retaining, sand retaining wall and other engineering fields. Using Larsen steel sheet pile for cofferdam has many advantages, such as green environmental protection, low cost, good waterproof performance and fast construction speed.
　　In construction, cold-formed steel sheet pile and hot-rolled steel sheet pile can be replaced with each other. It is precisely because of the emphasis on the development of steel sheet piles that the state formulated and promulgated the national standard "Hot-rolled Steel Sheet Pile" in May 2007. According to statistics, at present, China's annual consumption of steel sheet piles is only about 30,000 tons, accounting for only 1% of the global application, and there is still a broad market prospect in China.

　　III. Steel Support
　　Steel support generally uses inclined connecting members, such as herringbone and cross-shaped structures, to form a highly stable metal structure. Materials use steel pipes, H-beams, angle steels, etc., and tie bars are round steel bars, i.e., thick steel bars, used to enhance the stability of purlins. The cross-sectional forms of purlins commonly used are H-beams, C-shaped, Z-shaped steels, etc. Steel supports are most widely used in subway construction and foundation pit enclosure.

　　IV. Dynamic Compaction Method
　　Dynamic compaction method is a method of compacting loose soil by the impact force formed by the vertical fall of a heavy free-falling body. Generally, equipment such as dynamic compactors with heavy hammers are used. 　　V. High-pressure Rotary Jet Grouting Pile
　　High-pressure rotary jet grouting pile uses a high-pressure rotating nozzle to spray cement slurry onto the soil layer, forming a continuous cemented cement reinforcement to play a supporting role. The advantages are small land occupation and low noise; the disadvantage is high cost, easy environmental pollution, and some soils are not suitable. For example, under the conditions of bedrock and gravel, block stone, boulder skeleton structure, it will pollute groundwater, which needs attention.
　　This high-pressure jet grouting method is more suitable for foundation reinforcement treatment, foundation pit water cutoff curtain, slope soil or water retaining, foundation pit bottom reinforcement, prevention of piping and heaving, enclosure and reinforcement of large-diameter underground pipelines, soil layer reinforcement or waterproofing of subway projects, reservoir dams, sea dikes, river embankments, dam body and foundation anti-seepage reinforcement, construction of underground reservoir cutoff dams, etc., and its application range is also relatively wide.

　　VI. Underground Diaphragm Wall
　　Underground diaphragm wall technology originated in Europe and was developed by borrowing from the petroleum industry. Underground diaphragm wall uses trenching machinery to first excavate a narrow and deep trench, place a reinforcement cage in the trench, and then use a tremie pipe to pour concrete to build a unit trench section, proceeding section by section, to build a continuous reinforced concrete wall underground, becoming a support structure for soil retaining, seepage prevention, load bearing, and water retaining. It is widely used in enclosure projects such as building basements, underground shopping malls, parking lots, underground warehouses, deep pools, shafts, retaining walls, and deep foundations of high-rise buildings.
　　Due to the rapid development of today's trenching machinery, corresponding trenching methods are also constantly emerging, mainly including SMW method and TRD method. The SMW method uses a special multi-axis mixer to drill into the soil layer, simultaneously injects cement slurry from the drill bit into the soil, mixes and stirs, and then inserts H-beams or other section steels into the mixing pile, thereby forming an underground diaphragm wall as a soil retaining and waterproof structure. Its main features are simple structure, good waterproofing, short construction period, low cost, low pollution, and it is suitable for deep foundation pit engineering.
　　The TRD method is now often called: equal-thickness cement-soil underground diaphragm wall method. It is a complete set of equipment and construction method developed and researched in Japan in the 1990s that can continuously form walls in various soil layers and gravel layers. The basic principle is to use a chain saw-type cutter box to vertically insert into the soil layer, then move horizontally and laterally, driven by the chain to rotate the cutter up and down, stirring and injecting cement slurry to form a wall of a certain thickness. Its main features are continuous wall formation, flat surface, uniform thickness, and good wall uniformity.
　　VII. Vacuum Drainage Preloading Method
　　Vacuum drainage preloading method belongs to a kind of drainage consolidation method. It lays a horizontal drainage sand cushion and vertical drainage bodies set in the soft foundation, then lays an impermeable film sealing device on the sand cushion, and by means of pipes buried in the sand cushion, through a vacuum pumping device, forms a negative pressure in the soil body, extracts water from the soil pores, thereby reducing pore water pressure, increasing effective stress, causing the soil body to consolidate, reducing later settlement, and improving the foundation bearing capacity. This is a relatively new soft soil reinforcement technology, and the construction content consists of 4 parts:
　　1. Construct a vertical and horizontal drainage channel, called construction of plastic drainage boards and sand cushion;
　　2. Construct a sealing layer that isolates the reinforced foundation from the atmosphere to ensure air tightness;
　　3. Have a set of vacuum pumping devices, that is, install vacuum pumps and other equipment;
　　4. Have a set of construction monitoring system to ensure compliance.

　　VIII. Surcharge Preloading Method
　　In order to prevent consolidation settlement of the foundation, fill load greater than or equal to the design load is surcharged, so that the foundation consolidates and settles in advance to increase the strength of the foundation. When the strength meets the standard, the load is removed and normal construction is carried out. This not only improves the foundation bearing capacity but also reduces post-construction settlement.

　　IX. Vacuum Combined Surcharge Preloading Method
　　The vacuum combined surcharge preloading method integrates the above two methods and carries out construction simultaneously. During vacuum preloading, affected by the vacuum preloading load, the reinforced soil body produces lateral shrinkage deformation, while during surcharge preloading, the soil body is affected by the surcharge load, and the reinforced soil body produces lateral extrusion deformation. The above two deformations can offset each other during the construction process, thereby accelerating the surcharge speed.
　　The advantages of this method are: first, it can accelerate the consolidation time; second, the foundation is not prone to shear failure; third, the required equipment and process are relatively simple; fourth, although the construction cost is slightly increased, the construction time can be greatly reduced. Compared with the foundation reinforcement methods of powder jet mixing pile and rotary jet grouting pile, under the same construction period, the cost is reduced by nearly 60%.