Different deep foundation pit support systems involve different construction techniques and vary in economic efficiency. This article discusses the technical and economic characteristics of several commonly used deep foundation pit support systems in China and provides a comparative analysis.

　　I. Introduction

　　With the rapid growth of the national economy and the continuous development of the construction industry, high-rise buildings have sprung up like mushrooms, such as the Shanghai Jin Mao Tower, Shenzhen Diwang Building, and Guangzhou CITIC Plaza, which rank among the world's top skyscrapers. The heights of these buildings have exceeded 300 meters, and their foundation pit depths have gradually developed from 6m, 8m, 10m to over 20m. The taller the building, the deeper its embedment depth, and the higher the requirements for foundation pit support. The implementation of these projects has promoted the progress of building science and technology and the renewal and development of construction techniques, construction machinery, and building materials, with investments continuously increasing. How to choose an economical and safe support system under existing construction conditions is a problem that needs to be solved in the construction of deep foundation pits for high-rise buildings.

　　II. Types of Deep Foundation Pit Support

　　In recent years, with the increase in depth and volume of foundation pits, support technology has also made significant progress. According to function, the commonly used types can be divided into the following three categories:

　　1. Earth-retaining support systems. Commonly used ones include steel sheet piles, reinforced concrete sheet piles, deep cement mixing piles, bored cast-in-place piles, diaphragm walls, soil anchors, and soil nail walls. Their function is to form support rows of piles or retaining walls to resist earth pressure outside the pit.

　　2. Water-retaining support systems. Commonly used ones include deep cement mixing piles, jet grouting piles, compaction grouting, diaphragm walls, and interlocking steel sheet piles. Their function is to block seepage water from outside.

　　3. Bracing support systems. Commonly used ones include steel pipe and section steel internal bracing, reinforced concrete internal bracing, and composite supports of steel and reinforced concrete. Their function is to support the lateral force of the retaining structure and limit its displacement.

　　III. Comparison of Construction Techniques

　　There are currently six main types of support commonly used in China.

　　1. Steel sheet pile support. Steel sheet piles are widely used due to their simple construction. However, the construction of steel sheet piles may cause deformation of adjacent foundations and generate noise and vibration, and their watertight performance is poor. They can cause significant settlement and displacement of surrounding buildings and underground pipelines, greatly affecting the surrounding environment. Therefore, in densely populated areas with high building density, their use is often limited. Moreover, steel sheet piles themselves are relatively flexible; if the bracing or anchoring system is not properly set, their deformation can be large, so they should not be used when the foundation pit support depth is greater than 7m. At the same time, since steel sheet piles need to be extracted after the basement construction is completed, the impact on the surrounding foundation soil and surface soil during extraction should be considered.

　　2. Deep mixing pile support. As a support structure, cement-soil deep mixing piles are suitable for soil layers such as silt, mucky soil, clay, silty clay, silt, and plain fill, and the foundation pit excavation depth should not exceed 6m. For organic soil and peaty soil, suitability should be determined through testing. In China, the grid form is mostly used, i.e., gravity dam-type retaining walls.

　　3. Row pile support. Generally, when the foundation pit depth h = 8m~14m and the surrounding environment requirements are not very strict, row pile support is often considered. Column-type cast-in-place piles work reliably, but attention should be paid to the overall tying effect of the cap beam; at the corners of the foundation pit, the cap beam should be continuously looped. When the surrounding environment protection requirements are strict, in order to reduce the deformation of row piles, in soft soil areas, sometimes the passive zone along the perimeter of the cast-in-place piles or in some areas at the bottom of the foundation pit is reinforced with cement mixing piles or grouting to improve the resistance of the passive zone and reduce the deformation of the support structure.

　　4. Diaphragm wall. Diaphragm walls are suitable for various stratum conditions such as soft clay and sand below the groundwater level and complex construction environments, especially when there is deep soft soil below the bottom of the foundation pit and the wall needs to be inserted very deep. Therefore, they are widely used in underground engineering both domestically and internationally. With the development of technology and improvements in construction methods and machinery, diaphragm walls have developed to serve as both the earth-retaining structure during foundation pit construction and the side wall of the proposed main structure. If properly supported and combined with correct construction methods and measures, they can effectively control the deformation of soft soil strata. However, excavating trenches in hard soil for diaphragm walls can be difficult, especially when encountering rock layers that require special trenching equipment, resulting in higher construction costs. During construction, the slurry pollutes the construction site, making it muddy. The currently used top-down construction method makes the two walls into one, i.e., used as a retaining structure during construction and also as the outer wall of the underground structure.

　　5. Soil nail support. Soil nail support has been rapidly promoted and applied in China because it is economical, reliable, and quick and simple to construct. Soil nail walls are suitable for second and third grade foundation pits, non-soft soil sites, and foundation pit depths not exceeding 12m. Soil nails are slender rods used to reinforce the in-situ soil on site. Because they are installed as excavation proceeds, they can effectively maintain soil strength and reduce soil disturbance.

　　6. Anchor support. Anchor support in China was first used in subway projects and began to be used in deep foundation pit support for high-rise buildings in the early 1980s. In natural soil layers, the anchoring method is mainly drilled and grouted, generally called grouted anchors. In addition to the above six common support forms, there are also arch ring support and top-down construction support, etc. Generally speaking, cement-soil mixing piles and soil nail walls are currently the preferred support forms in China for foundation pits within 5m depth, the latter even within 10m, and when soil conditions are good, they are also often used for foundation pits around 15m. For 5-10m deep soft soil foundation pits, drilled (percussion, excavated) piles, driven cast-in-place piles, or reinforced concrete precast piles are often used, and various layouts can be made. If seepage prevention and water stopping are required, cement-soil mixing piles, chemical grouting, or high-pressure grouting are supplemented to form a waterproof curtain, and sometimes steel sheet piles or H-section steel piles are also used. When the foundation pit depth is greater than 10m, diaphragm walls or SMW method continuous walls can be considered, with bracing or anchors set as needed.

　　IV. Economic Analysis

　　Corresponding economical and reasonable support structure systems should be selected according to different geological conditions and foundation pit depths. First, steel sheet pile support has the lowest cost. It is often used in some simple projects, such as small pipe jacking projects and bridge projects, but due to its high noise, easy deformation, and poor waterproofing, it has gradually been replaced by other support methods. Second, deep cement-soil mixing pile support structures are impermeable and require no bracing, allowing excavation under open conditions, and the material used is only cement, so they have good economic benefits. Third, soil nail wall support has fast construction speed, saves materials, and has low cost. Compared with other pile-wall supports, the construction period can be shortened by more than 50%, and the cost can be saved by about 60%; moreover, soil nail support can be constructed close to existing buildings, thus saving the ground space occupied by piles or walls. Fourth, cast-in-place piles in row pile support are easy to construct, can be mechanically drilled (percussion) or manually excavated, require no large machinery during construction, and have no noise, vibration, or harm from squeezing surrounding soil as driven piles do, and the cost is lower than that of diaphragm walls. At the same time, the cast-in-place pile retaining structure can also be considered as part of the outer wall in the design of the main structure's outer wall to participate in load-bearing (withstand lateral pressure). In this case, usually no tie bars are set between the piles and the main structure, and a waterproof layer is used to separate them, which can relatively save some cost. Fifth, the superiority of diaphragm walls has long been recognized worldwide. They are indispensable for large-depth foundation pits and complex engineering environments. However, their cost is relatively high and requires comprehensive consideration. Since the application of SMW method continuous walls in recent years, they are generally considered to have good performance and suitable cost. However, China still lacks self-made special machinery capable of large-depth construction. Wuhan and Shanghai have introduced special machinery for the SMW method from Japan and are promoting its use. On this basis, anti-friction agents have been developed, which can allow the stiffening steel to be extracted and reused, further reducing costs. China has also conducted research and trial use of prefabricated assembled diaphragm walls and prestressed diaphragm walls. Prefabricated assembled diaphragm walls have smooth surfaces, and due to reasonable reinforcement, the wall thickness can be reduced and construction speed accelerated. Prestressed diaphragm walls can increase the stiffness of the retaining wall by more than 30%, reduce wall thickness, and reduce the number of internal bracings. Because the curved reinforcement produces an arching effect after tensioning, it can reduce the deformation of the retaining structure and eliminate cracks, thereby improving impermeability. These two methods have been tried in engineering and have achieved good social and economic benefits. Sixth, top-down construction is generally used for high-rise buildings in cities where the surrounding construction environment is relatively harsh and the adjacent buildings, roads, and underground pipelines around the site must not be damaged for any construction reason. For this purpose, during foundation pit construction, by utilizing the ability of the underground structure itself to support the pit wall, the deformation of the support structure is reduced, its stiffness is greatly enhanced, the cost of bracing or anchors is saved, and the deformation of the support structure and its impact on adjacent buildings are greatly reduced, thereby lowering the total cost and achieving multiple benefits. It is an advanced construction method. Seventh, closed retaining arch rings sometimes still require cement-soil mixing piles or chemical grouting to form a waterproof curtain. But even so, their cost is still lower than that of general pile-wall support structures. They have been applied in 6-12m deep foundation pits in Guangzhou, Zhuhai, Shenzhen, and other places, reducing costs by about 50% compared with general pile-wall structures. Guangdong Province is located in the southeast coastal zone, and the regional geological conditions belong to a soft soil zone mainly composed of silt and mucky soil. Due to the dense buildings and crisscrossing underground pipelines in the urban area of Guangzhou, when constructing deep foundation pits for high-rise buildings, the combination of internal bracing and support piles is the most economical and reasonable choice for deep foundation pit support schemes; if the soil quality is good and the construction site area permits, composite soil nail support can also be considered, which is not only economical and reliable but also fast and simple to construct; if it is necessary to improve the earth-retaining and water-stopping impermeability effects of the support structure, then reinforced cement-soil walls are a choice that facilitates the installation of bracing and has a reasonable cost-performance ratio.

　　V. Conclusion

　　Foundation pit support engineering is a systematic project with characteristics such as technical complexity, fast progress, and high quality requirements. It must rely on knowledge from structural mechanics, soil mechanics, foundation engineering, and other disciplines, combined with rich construction experience and the soil conditions and surrounding environment of the proposed site, in order to formulate economical and reasonable support structure schemes and construction methods according to local conditions.