Bottom-up method: refers to the process of constructing the surrounding retaining structure, then excavating soil from top to bottom and installing supports, after excavating to the pit bottom, constructing the main structure from bottom to top, and removing supports in a certain order. The bottom-up foundation pit support structure usually consists of a retaining wall, supports (anchors) and their vertical support structures.

　　Top-down method: refers to the construction method that uses the horizontal beam-slab structure of the main underground structure as internal supports, and proceeds alternately from top to bottom by floor and with foundation pit excavation. The retaining wall of the top-down method can be combined with the outer wall of the main structure, or a temporary retaining wall can be used.


　　Advantages of the top-down method: smaller foundation pit deformation, conducive to surrounding environment protection; simultaneous construction above and below ground, which can shorten the construction period;

　　Disadvantages of the top-down method: the foundation pit design is highly related to the structural design, and the communication and collaboration between design and construction are close; high construction technical requirements, such as complex structural component nodes and high verticality control requirements for intermediate support columns;

　　Combination of bottom-up and top-down methods: for certain foundation pits with complex conditions or special technical and economic requirements, a design scheme combining the bottom-up method and the top-down method can be adopted, thereby giving play to the respective advantages of the bottom-up and top-down methods and meeting the specific requirements of foundation pit engineering.
　　In order to achieve advanced technology, reasonable economy, and ensure the safety of the foundation pit and surrounding environment in foundation pit engineering, the selection of support structure forms should comprehensively consider factors such as engineering geological and hydrogeological conditions, underground structure design, foundation pit plane and excavation depth, surrounding environment and edge loads, site conditions, construction season, and service life of the support structure. The selection should consider the improvement of spatial effects and stress conditions, and adopt forms conducive to the stress shape of the support structure materials.


　　Selection of retaining wall:

　　[Gravity cement-soil wall] A gravity retaining structure in which cement-soil piles are interlocked to form a grid or solid body.


　　[Steel sheet pile] Divided into channel steel sheet piles and hot-rolled interlocking steel sheet piles. The advantages are reliable material quality, convenient driving in soft soil areas, fast construction speed, and simplicity.


　　[Steel sheet pile with horizontal lagging] The steel sheet pile with horizontal lagging retaining wall is also called pile-board support structure. It is mostly used in areas with good soil quality and low groundwater level.


　　[Bored cast-in-place pile] The construction of bored cast-in-place piles has no noise, no vibration, no soil squeezing, large stiffness, strong bending resistance, and small deformation, and is applied almost nationwide.


　　[Underground diaphragm wall] The underground diaphragm wall is an underground concrete wall formed by excavating a deep trench with special trenching equipment under mud slurry protection before foundation pit excavation, then lowering a reinforcement cage and pouring concrete.


　　[Steel cement-soil mixing wall] That is, H-shaped steel is inserted into cement-soil mixing piles to make a retaining wall with both stress-bearing and impermeability functions, and supports can also be added. In China, it is mostly used for foundation pits of 8~12m.


　　[Soil nail wall] Soil nail walls are not suitable for mucky soil, mud, expansive soil, and soil with excessively low strength, such as newly filled soil. Adaptability should be comprehensively determined in combination with regional experience.
　
　　Types of internal supports:

　　[Steel support] Steel supports are generally divided into steel pipe supports and section steel supports.

　　[Concrete support] The concrete strength of concrete supports is mostly C30, and they are cast in place on site according to the positions specified in the design as excavation proceeds.


　　[Steel support and concrete support] Under certain conditions, a combination of steel supports and concrete supports can be used for foundation pits.


　　[Support columns] For foundation pits with large plane dimensions, columns need to be set at the intersection points of supports to support the plane supports in the vertical direction.


　　[Layout forms of internal supports] The layout forms of the support system on the plane include orthogonal supports, corner braces, opposite braces, truss type, frame type, circular ring type, etc.