What are the forms of foundation pit support structures?

Whether it is high-rise building projects or deep foundation pit works for subways, since excavations are carried out in urban areas, there are usually various structures such as traffic routes, existing buildings, and pipelines around the foundation pits. This raises an important aspect of foundation pit excavation: ensuring the safe use of the structures surrounding them. Moreover, most common foundation pit support systems are temporary structures; the high cost associated with them can lead to waste, while unsafe support structures will inevitably result in construction accidents. Therefore, how to safely and reasonably select an appropriate support structure and conduct scientific design based on the characteristics of the foundation pit project is the main issue that foundation pit engineering needs to address. The following provides a brief introduction to the common types of support structures used in current foundation pit projects, as well as the principles for selecting such support structures depending on different soil conditions. 1 Types of foundation pit support, their characteristics, and application ranges 1.1 Slope excavation: Suitable for areas with open surroundings and no important buildings in the vicinity; stability is sufficient, strict control over displacement is not required; it is the cheapest option, but requires a large amount of backfill soil. 1.2 Deep mixing cement-soil retaining wall: A deep mixing cement-soil retaining wall is created by using a deep mixer to forcefully mix the soil with the injected cement slurry on-site, thereby forming a continuous, overlapping columnar structure of cemented soil that serves as a retaining wall. Advantages of cement-soil retaining walls: Since there is usually no support inside the pit, it facilitates rapid mechanical excavation; it has both soil-retaining and water-blocking functions; it is generally cost-effective; during construction, there is no vibration, no noise, minimal pollution, and slight soil displacement, which makes it particularly advantageous for construction in urban areas. Disadvantages of cement-soil retaining walls: Firstly, the displacement is relatively large, especially when the length of the foundation pit is great; measures such as adding piers in the middle and creating an arch can be taken to limit excessive displacement. Secondly, these walls require a greater thickness, and they can only be used when the designated location permits and the surrounding environment allows it. Additionally, care must be taken during the construction of cement-soil mixing piles to prevent any impact on the surrounding environment. 1.3 High-pressure jet grouting piles: The material used for high-pressure jet grouting piles is also cement slurry. High pressure is utilized to force this cement slurry through rotating nozzles, which injects it into the soil layer where it mixes with the soil to form a cemented soil mass. These masses are arranged side by side to create piles that serve to retain soil and prevent water leakage. The construction cost of high-pressure jet grouting piles is higher than that of deep mixing cement-soil piles. However, their construction equipment is compact, small in size, highly mobile, and requires less space. Moreover, the vibration generated by these construction tools is minimal, and the noise level is low, so they do not cause vibrations or noise pollution to surrounding buildings. They can be used in spaces with limited area, but a large amount of slurry is produced during construction, which can lead to pollution. This method is not suitable for strata with excessive groundwater flow rates, karst areas without fill material, permafrost, or soil types that cause severe corrosion of cement, as the sprayed slurry cannot solidify around the grouting pipes. 1.4 Channel steel sheet piles: This is a simple type of sheet pile retaining wall, constructed by overlapping or arranging channel steels back to back. The channel steel is 6–8 m long, and its model is determined through calculation. Its characteristics are as follows: channel steel possesses good durability; it can be pulled out and reused after backfilling is completed following foundation pit construction. It is easy to construct and requires a short construction time. However, it cannot prevent water from seeping in nor retain fine particles in the soil, so waterproofing or dewatering measures are necessary in areas with high groundwater levels. Its bending resistance is weak, making it suitable for relatively shallow foundation pits or trenches with a depth of ≤4 meters; a support or tie rod should be installed at the top. The stiffness of such supports is low, resulting in significant deformation after excavation. 1.5 Reinforced concrete sheet piles: Reinforced concrete sheet piles feature simple construction and a short on-site installation time, which has led to their widespread use in foundation pits. However, since their installation is usually carried out by hammering, this method generates significant vibration and noise; moreover, soil displacement occurs to a considerable extent during installation, which imposes certain limitations on their use in urban projects. Furthermore, its production is generally carried out in factories in advance and then transported to the construction site, resulting in costs that are slightly higher than those of cast-in-place piles and similar methods. However, due to the reasonable stress distribution on the sheet piles resulting from their cross-sectional shape and reinforcement, as well as the possibility of designing them according to specific requirements, it is now possible to manufacture sheet piles with large thicknesses (up to 500 mm or more). Additionally, hydraulic static pile-driving equipment is available, which means that sheet piles remain a viable option for supporting retaining walls in foundation pit projects. 1.6 Bored Piles: Bored pile retaining walls are the most commonly used type among retaining wall systems of the pile type, and they are widely applied in China. It is mostly used in foundation pit projects with depths of 7 to 15 meters; in areas with good soil conditions in northern China, retaining walls made of arm piles have already been used at depths of 8 to 9 meters. The characteristics of retaining walls supported by bored cast-in-place piles include: no environmental hazards such as vibration or noise during construction, no soil displacement, and minimal impact on the surrounding environment; high strength and stiffness of the wall body, good stability in support, and minimal deformation. When the engineering piles are also cast-in-place piles, construction can proceed simultaneously, which facilitates organization, simplifies the process, and shortens the construction time. The gaps between piles can lead to soil erosion, especially in areas with soft clay at high water levels; therefore, construction measures such as grouting, cement mixing piles, or jet grouting must be employed depending on the project conditions to address water retention issues. These walls are suitable for use in areas with soft clay and sand, but construction in gravelly or pebbly layers is difficult, so their use should be approached with caution. The piles are connected together primarily through crown beams and diaphragm beams at the top of the piles, resulting in relatively poor overall integrity; thus, special caution is required when using them in important areas, for special projects, or in foundation pits with large excavation depths. Author: linfan84 2006-3-5 20:21 Reply to this post -------------------------------------------------------------------------------- 2 A brief overview of the types and design principles of foundation pit support structures (reposted) 1.7 Diaphragm walls Typically, the thickness of diaphragm walls is 600mm, 800mm, or 1000mm; thicknesses of up to 1200mm exist as well, but they are less commonly used. The diaphragm wall has high stiffness and excellent water-stopping properties, making it the most effective type of support structure. It is suitable for basins with poor and complex geological conditions, large depths, and high requirements regarding the surrounding environment. However, its cost is high, and its construction requires specialized equipment. 1.8 Soil nailing walls: Soil nailing walls are a type of support system for slope stabilization. Unlike the aforementioned retaining walls that provide passive soil retention, they exert an active anchoring effect to enhance the stability of the slope, thereby keeping the slope surface stable after excavation of the foundation pit. Soil nailing walls are primarily used in areas with good soil quality; they are widely applied in North China and northeastern China. Currently, they are also being used in southern China as well. Some of these walls have been used for foundation pits with depths of over 10 meters. They offer stability and reliability, are easy to construct, require a short construction time, deliver good results, and are cost-effective. Therefore, their use should be promoted actively in areas with good soil quality. 1.9 SMW Method The SMW method, also known as the reinforced cement-soil mixing pile method, involves inserting H-shaped steel members into the cement-soil piles (with H-shaped steel being the most common material; Larsen-type steel sheet piles and steel pipes are also used at times). This approach combines load-bearing capacity with waterproofing properties, resulting in a retaining wall that serves both as a structural support element and as a barrier against water infiltration. The main advantages of SMW support structures are as follows: they generate little noise during construction and have a minimal impact on the surrounding environment; their structural strength is reliable, allowing them to be used in any situation where cement-soil mixing piles are suitable, especially in soft soil layers composed mainly of clay and fine sand; they offer good water-blocking and impermeability properties, eliminating the need for additional water barriers; they can be used in combination with multiple support systems for deeper foundation pits; under certain conditions, this method can serve as a substitute for diaphragm walls as an underground retaining structure. If appropriate construction measures are taken to recover tensile materials such as H-shaped steel, the cost can be **lower than that of diaphragm walls, thus giving this method great potential for development. 1.10 Summary of Foundation Pit Support Selection The proper selection of the foundation pit support type is the primary task in the design of such supports; it should be determined based on geological conditions, requirements of the surrounding environment, as well as the characteristics and costs of different support types. Generally, when the geological conditions are favorable and the requirements for the surrounding environment are less stringent, flexible support methods such as soil nailing walls can be used ； When high requirements are imposed by the surrounding environment, a more rigid support system should be used to control horizontal displacement, such as retaining piles or diaphragm walls. Similarly, regarding the type of support, when the surrounding environment requires higher standards but the geological conditions are poor, the use of anchor rods can easily cause disturbance to the soil in the vicinity and affect the safety of that environment; it is better to use internal support systems in such cases ； When the geological conditions are particularly poor, the foundation pit is deep, and high requirements exist regarding the surrounding environment, the strongest form of support, namely diaphragm walls combined with the top-down construction method, can be employed. The most important aspect of foundation pit support is to ensure the safety of the surrounding environment. 2. Design requirements for foundation pit support: As a structural system, foundation pit support must meet the requirements regarding stability and deformation, that is, the requirements of the two extreme states specified in relevant codes: the ultimate bearing capacity state and the serviceability limit state. The so-called ultimate limit state of bearing capacity, in the context of foundation pit support, refers to the failure, collapse, sliding of the support structure, or damage to the surrounding environment, resulting in large-scale instability. The general design requirement is that the support structure should not reach such an extreme state. The limit state for normal use refers to a situation in which the deformation of the support structure, or the deformation of the surrounding soil caused by excavation, is excessive enough to affect normal use, but does not result in the structural instability. Therefore, the design of foundation pit support must include a sufficient safety factor with respect to the ultimate bearing capacity, to prevent instability of the support system. Meanwhile, while ensuring stability, it is also necessary to control displacement levels so as not to affect the safe use of surrounding buildings. Therefore, as a computational theory for design, it must not only be able to calculate the stability of support structures but also their deformation, and control this deformation within a certain range based on the surrounding environmental conditions. The displacement control of general support structures focuses primarily on horizontal displacement, as it is more intuitive and easier to monitor. Horizontal displacement control is related to the requirements of the surrounding environment; this is what is referred to in standard specifications as the classification of foundation pit safety levels. When there are important structures around the foundation pit that need to be protected, small deformations should be controlled – these are the displacement requirements for what is typically considered a grade 1 foundation pit ； In areas with open surroundings where there are no structures that need protection, the displacement can be larger; theoretically, as long as stability is maintained, that is sufficient. This is what is referred to as the displacement requirements for grade III foundation pits ； Those falling between level 1 and level 3 represent the displacement requirements for level 2 foundation pits. For the maximum horizontal displacement of primary foundation pits, it is generally advisable that it not exceed 30 mm; for deeper foundation pits, it should be less than 0.3% of H, where H is the depth of excavation of the foundation pit. For ordinary foundation pits, the maximum horizontal displacement should also not exceed 50 mm. Generally, when the maximum horizontal displacement is within 30 mm, no significant cracks in the ground occur. However, when this displacement ranges from 40 to 50 mm, visible cracks in the ground appear. Therefore, it is advisable to keep the maximum horizontal displacement for foundation pits at no more than 50 mm; otherwise, noticeable cracks and settlement in the ground will occur, creating a sense of insecurity. Generally, more rigid support structures such as retaining piles and diaphragm walls with internal support systems exhibit smaller displacements, which can be kept within 30 mm. In the case of soil nailing support, displacements are usually greater than 30 mm, unless the geological conditions are favorable and reinforcement measures such as advance support and prestressed anchors are employed to limit displacement. Conclusion: Foundation pit support is a special structural method with many functions. Different support structures are suitable for various hydrogeological conditions; therefore, a specific analysis must be conducted based on the particular situation in order to select an economical and appropriate support structure!!!