Abstract: the supporting technology of deep foundation pit engineering is often encountered in the construction process of special soil road. Although it has obtained a lot of successful experience in different regions and different geological conditions of the country, and even reached the international level in some technologies, there are still some problems that need to be further studied or improved to meet the needs of modern economic construction. Based on the summary of the types of deep foundation pit support, this paper puts forward some existing problems of deep foundation pit engineering support technology, and prospects the development trend of support technology.

Key words: Deep Foundation Pit Engineering, support type, construction

Foundation pit engineering is an ancient geotechnical engineering subject with the characteristics of the times. Slope excavation and simple wooden pile enclosure can be traced back to ancient times. In fact, the frequent activities of human civil engineering promote the development of foundation pit engineering. Since the 1990s, under the situation of China's reform and opening up and sustained high-speed growth of the national economy, the national engineering construction has also made rapid progress, and high-rise buildings have mushroomed, which has promoted the progress of building science and technology and the renewal and development of construction technology, construction machinery and building materials [1]. In order to ensure the stability of the building, the foundation of the building must meet the requirements of embedded deep underground. The higher the building height is, the deeper the buried depth is, the higher the requirements for foundation pit engineering are, and more and more problems appear, which brings great difficulties to the construction, especially the construction in the central area of the city.

1、 Main contents of deep foundation pit engineering

1) Geotechnical investigation and engineering investigation. Determine the geotechnical parameters and groundwater parameters; determine the working status of adjacent buildings, underground buried objects (pipelines, cables, optical cables, etc.), urban roads and other engineering facilities, and analyze the limit value of the ground displacement.

2) Support structure design. Including retaining wall retaining structure (such as continuous wall, column cast-in-place pile retaining wall), supporting system (such as internal support, anchor) and soil reinforcement. The design of the supporting structure must be closely combined with the construction scheme of the foundation pit engineering. The main bases to be considered are: local experience, soil and groundwater conditions, allowable stratum deformation limits for the safety of the surrounding environment, available construction facilities and sites, construction period and cost, etc.

3) Foundation pit excavation and support construction. Including earthwork, engineering precipitation and construction organization design and implementation of the project.

4) Stratum displacement prediction and surrounding engineering protection. The ground displacement depends not only on the performance of soil and supporting structure and the change of groundwater, but also on the construction process and construction process. If the predicted deformation exceeds the allowable value, the support structure design and construction scheme shall be modified, and special protection or reinforcement measures shall be taken for the surrounding important engineering facilities when necessary.

5) Construction site measurement and monitoring. According to the monitoring data and information, feedback design should be carried out when necessary, and informatization should be used to guide the next step of construction.

2、 Types of deep foundation pit support

All kinds of buildings and underground pipelines need to excavate the foundation pit. Some foundation pits can be directly excavated or excavated by slope. But when the depth of the foundation pit is deep and the surrounding site is not wide, the foundation pit support is generally used. In the past, the support is relatively simple, that is, steel sheet pile plus well point dewatering, which can generally meet the safety construction of the foundation pit, but the deep foundation pit can not meet the requirements With the increase of degree and volume, the support technology has also made great progress. According to the function, the commonly used ones are as follows [2]:

1) Retaining system: commonly used steel sheet pile, reinforced concrete sheet pile, deep cement mixing pile, bored pile, diaphragm wall. Its function is to form supporting piles or retaining wall to block the earth pressure outside the pit.

2) Water retaining system: commonly used are deep cement mixing pile, jet grouting pile, compaction grouting, diaphragm wall, locking steel sheet pile. Its function is to block water seepage.

3) Support system: commonly used steel pipe and steel inner support, reinforced concrete inner support, steel and reinforced concrete composite support. Its function is to support the side force of the enclosure and limit the displacement of the enclosure.

The common types of deep foundation pit support are as follows:

2.1 steel sheet pile support

Steel sheet pile is made of hot rolled section steel with lock or tongs. The steel sheet pile wall is widely used in soil and water retaining. At present, the commonly used section forms of steel sheet pile are U-shaped, Z-shaped and straight web plate. Steel sheet pile is widely used because of its simple construction. However, the construction of steel sheet pile may cause the deformation of adjacent foundation and produce noise and vibration, which has a great impact on the surrounding environment. Therefore, its use is often limited in densely populated areas with high building density. And the steel sheet pile itself is flexible, if the support or anchor system is not set properly, its deformation will be great, so when the foundation pit support depth is more than 7m, it is not suitable to use. At the same time, because the steel sheet pile needs to be pulled out after the basement construction, the influence on the surrounding foundation soil and surface soil should be considered.

2.2 deep mixing support

Deep mixing support uses cement as solidifying agent and mechanical mixing to mix solidifying agent and soft soil agent compulsively, so that a series of physical and chemical reactions between solidifying agent and soft soil agent occur and gradually harden, forming a cement soil pile wall with integrity, water stability and certain strength as supporting structure. It is suitable for silt, muddy soil, clay, silty clay, silt, plain fill and other soil layers. The excavation depth of foundation pit should not be more than 6m. Organic soil and peat soil should be determined by experiment.

2.3 row pile support

Row pile support is a kind of support form of column spaced reinforced concrete bored pile as the main retaining structure. The column spacing arrangement includes the sparse arrangement with a certain clear distance between piles and the dense arrangement with tangent between piles. The column cast-in-place pile has good rigidity as the retaining structure, but the connection between the piles is poor. It is necessary to cast a large section reinforced concrete cap beam at the top of the pile to connect it reliably. In order to prevent groundwater and soil particles from flowing into the pit from the pores between piles, high-pressure grouting should be used between piles or at the back of piles, deep mixing pile, jet grouting pile and other measures should be set, or special waterproof curtain should be built behind piles. The construction of cast-in-place pile is simple, and it can be drilled (punched) by machine or dug by hand. There is no need for large machinery in the construction, and there is no harm caused by noise, vibration of driven pile and squeezing surrounding soil. The cost is lower than that of diaphragm wall. At the same time, the retaining structure of cast-in-place pile can also be regarded as a part of the external wall in the design of the external wall of the main structure of the building to participate in the stress (bear the lateral pressure). At this time, there is usually no tie bar between the pile and the main structure, and it is separated by a waterproof layer. Row pile support can be divided into cantilever type and support anchor type, and support anchor type can be divided into single point support anchor and multi-point support anchor. In most cases, cantilever column pile is suitable for three-stage foundation pit, while anchored column pile is suitable for one-stage and two-stage foundation pit. Generally speaking, when the depth of foundation pit is 8 m ~ 14 m and the surrounding environment is not very strict, row pile support is often considered. The work of column cast-in-place pile is relatively reliable, but the whole tie effect of cap beam should be paid attention to. At the corner of foundation pit, cap beam should be continuous. When the retaining structure of cast-in-place pile is required to play the role of water resistance and seepage prevention, deep water-proof mixing pile or jet grouting pile between piles and pile back must be done well (the general drilling compaction grouting method is not easy to ensure water stop, which has caused many major accidents). When the requirements of surrounding environment protection are strict, in order to reduce the deformation of row piles, sometimes in soft soil area, cement mixing piles or grouting are used to reinforce the passive area around the bottom of foundation pit and cast-in-place piles, so as to improve the resistance of passive area and reduce the deformation of supporting structure.

2.4 diaphragm wall

The underground continuous wall has the characteristics of large overall stiffness and good waterproof and anti-seepage effect. It is suitable for a variety of stratum conditions and complex construction environment such as soft clay and sand below the groundwater level, especially when there is deep soft soil below the bottom of the foundation pit and the wall needs to be inserted deeply. Therefore, it has been widely used in underground engineering at home and abroad. And with the development of technology and the improvement of construction methods and machinery, the underground continuous wall is not only the retaining structure of foundation pit construction, but also the side wall of the proposed main structure. If it is properly supported and combined with the correct construction methods and measures, the deformation of soft soil layer can be better controlled. In the projects with deep foundation pit (generally H > 10m) and high environmental protection requirements, this technology is often used after technical and economic comparison. However, it is difficult to excavate the underground continuous wall in hard soil, especially when special trenching machines are needed in rock stratum, and the construction cost is high. During the construction, the mud pollutes the construction site and makes the site muddy. At present, the top-down construction method makes the two walls in one, that is, it is used as the enclosure structure during construction, and it is also the outer wall of the underground structure. The top-down construction method is generally used in high-rise buildings in the city. The surrounding construction environment is relatively bad, and the adjacent buildings, roads and underground pipelines around the site can not be damaged due to any construction reasons. Therefore, during the foundation pit construction, the underground structure itself can produce the support function to the pit wall (that is, the pile, column, beam and plate of the underground structure itself can be used as the support, and the foundation pit wall can be protected at the same time) It can save the internal support system, reduce the deformation of the support structure, reduce the cost and shorten the construction period, which is one of the new technologies to be popularized and applied. In addition to the cast-in-place diaphragm wall, the precast diaphragm wall and prestressed diaphragm wall have been studied and tried in China. The wall surface of precast diaphragm wall is smooth. Due to the reasonable reinforcement, the wall thickness can be reduced and the construction speed can be accelerated. The prestressed diaphragm wall can increase the rigidity of retaining wall by more than 30%, reduce the wall thickness and the number of internal supports, and reduce the deformation of retaining structure, eliminate cracks and improve the impermeability due to the reverse arch effect caused by the tension of curved reinforcement. These two methods have been used in engineering, and achieved good social and economic benefits.

2.5 soil nailing

Soil nailing support is a new retaining technology for soil excavation and slope stability. It has been rapidly promoted and applied in our country because of its economy, reliability, fast and simple construction. The use of soil nailing requires that the soil has the ability of temporary self stabilization in order to give a certain time for the construction of soil nailing wall, so the applicable geological conditions of soil nailing wall should be limited. "Technical specification for building foundation pit support (jgj12021999)" stipulates that soil nailing wall is suitable for class II and III foundation pit, non soft soil site, and the depth of foundation pit should not be greater than 12m. Compared with other pile wall support, the construction period can be shortened by more than 50% and the cost can be saved by about 60%; moreover, the soil nailing support can be constructed close to the existing buildings, so as to save the ground occupied by piles or walls. However, from many engineering experiences, the damage of soil nailing wall is almost due to the action of water, which makes the soil nailing wall soften, causing overall or local damage. Therefore, it is stipulated that the soil nailing wall project must do a good job in dewatering, and it is not suitable to be used as a water retaining structure. Soil nailing is a slender member used to reinforce in situ soil. It is usually made by drilling, placing deformed steel bars and grouting along the whole length of the hole. It depends on the cohesive force or friction force between it and the soil and passively bears the tensile force when the soil is deformed. It is composed of dense soil nailing group, reinforced soil and shotcrete surface. Due to the support with excavation, it can effectively maintain the strength of soil and reduce the disturbance of soil. Since the 1990s, soil nailing wall technology has been applied in the southeast coastal area, but the geological conditions in this area belong to the soft soil zone dominated by silt and muddy soil. In order to adapt to this characteristic, composite soil nailing support technology has been developed. Reinforced cement wall is composed of H-shaped steel (Larsen sheet pile, steel pipe, etc.) inserted into cement soil pile. The H-shaped steel bears the lateral load, while the cement soil has good impermeability, so the reinforced cement wall has good soil retaining and water sealing impermeability. There are two forms of cement soil pile and H-shaped steel, and it is very convenient to insert H-shaped steel into cement soil pile to set support. In order to make the H-shaped steel sink to the designated elevation by its own weight, the deep mixer with triaxial full depth mixing is generally used in the construction of cement soil pile, and the cement mixing ratio needs to be increased. The technology has been applied in Shanghai, Jiangsu and Zhejiang.

In addition, there are bolt or shotcrete bolt support, arch ring support and reverse method support.

3、 Common problems

Although the supporting technology of deep foundation pit engineering has achieved a lot of successful experience in different regions and different geological conditions in China, and even reached the international level in some areas, there are still some problems that need to be further studied or improved to meet the needs of modern economic construction. The main problems in the process of deep foundation pit support construction are as follows:

3.1 soil excavation and slope support are not matched [3]

The common support construction lags behind the earthwork construction for a long time, and has to take the secondary backfill or set up a shelf to complete the support construction. Generally speaking, the earthwork excavation technology content is relatively low, the process is simple, and the organization and management is easy. The technology of retaining support is high, the process is more and more complex, and the construction organization and management are more complex than earthwork excavation. Therefore, in the construction process, large-scale projects are paid by professional construction teams to complete earthwork and retaining work respectively, and most of them are two parallel contracts. In this way, it is difficult to coordinate the management in the construction process. The earthwork construction unit scrambles for the progress, delays the construction period, and the excavation sequence is chaotic, especially in the rainy season. Even regardless of the working face required by the retaining support construction, the operation face left for the support construction is almost impossible to operate, and the support work can not be completed in time, so that the support construction lags behind the earthwork construction, because the support construction has no operation In order to complete the drilling, grouting, net layout and shotcrete, the platform has to be backfilled with earthwork or set up a shelf to set up the operation platform to complete the construction. In this way, it is not only difficult to ensure the progress, but also difficult to ensure the quality of the project, and even safety accidents, leaving quality risks.

3.2 the slope repair can not meet the design and specification requirements

There are often over excavation and under excavation phenomena. Generally, mechanical excavation and manual simple slope repair are used in the excavation of deep foundation, and then the initial concrete spraying process of retaining support is started. In the actual excavation, due to the construction management personnel are not in place, the technical disclosure is not sufficient, the excavation height is different in layers and sections, and the operation level of the digging manipulator and other factors, the flatness and straightness of the slope surface after mechanical excavation are extremely irregular. However, it is impossible to excavate in depth during manual repair, so the flatness of the mechanical excavation surface can only be trimmed without strict inspection The initial shotcreting started immediately after the closing, so the overbreak and underbreak appeared after the retaining payment.

3.3 the hole forming grouting is not in place, and the stress of soil nail or anchor can not meet the design requirements

The soil nail or anchor used in deep foundation pit support is usually drilled with 100 ~ 150 drill pipe. The hole depth is less than five or six meters, more than ten meters, or even more than 20 meters. The quality of the soil layer through which the hole passes is also different. If the soil condition is not carefully studied, the slag is often endless, and the residue deposition affects the grouting. Some even have difficulty in drilling, and the hole collapses Pulp. In addition, the grouting materials are too random, the grouting pipe is not inserted in place, and the grouting pressure is not enough, resulting in the insufficient grouting length and filling degree, which makes the anti pulling force of soil nails or anchor rods fail to meet the design requirements, which affects the engineering quality, and even needs to be dealt with again.

3.4 the thickness of shotcrete is not enough and the strength can not meet the design requirements

At present, dry mixing shotcrete equipment is commonly used in foundation pit support of construction engineering [4]. Its main characteristics are simple equipment, small volume, long conveying distance, accelerator can be added before entering the shotcrete machine, convenient operation, and continuous shotcrete construction. Although the dry spraying equipment is simple and convenient to operate, due to the different level of operators, the operation methods and inspection methods are different

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