Abstract: deep foundation pit engineering support technology is a project often encountered in the construction process of special soil roads. Although it has gained a lot of successful experience in different regions and different geological conditions in the country, even some technologies have reached the international level, but 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 supporting types of deep foundation pit, this paper puts forward some problems existing in the supporting technology of deep foundation pit engineering, and looks forward to the development trend of the supporting technology.

Key words: deep foundation pit engineering, support type, construction

Foundation Pit Engineering is an ancient and time-characteristic geotechnical engineering subject. Slope excavation and simple wooden pile enclosure can be traced back to ancient times. In fact, the frequent activities of human civil engineering have promoted the development of foundation pit engineering. Since the 1990 s, under the situation of China's reform and opening up and the sustained and rapid growth of national economy, the national engineering construction has also made rapid progress, and high-rise buildings have sprung up rapidly, it promotes 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 building foundation must meet the requirements of embedded underground depth. The higher the height of the building, the deeper the embedded depth will be, and the requirements for foundation pit engineering will be higher and higher, with more and more problems, which will lead to the construction, especially the construction of the city center brings great difficulties.

I. Main contents of deep foundation pit engineering

1) geotechnical engineering investigation and engineering investigation. Determine geotechnical parameters and groundwater parameters; Measure the working status of adjacent buildings, surrounding underground buried objects (pipelines, cables, optical cables, etc.), urban roads and other engineering facilities, and analyze its limit value along with formation displacement.

2) supporting structure design. Including retaining wall enclosure structure (such as continuous wall, Colonnade-type cast-in-place pile retaining wall), supporting system (such as internal support, anchor rod) and soil reinforcement, etc. The design of supporting structure must be closely combined with the construction scheme of foundation pit engineering, and the main bases to be considered are: local experience, soil and groundwater conditions, the limit of formation deformation allowed by the surrounding environment safety, available construction facilities and construction site, construction period and cost, etc.

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

4) formation displacement prediction and surrounding engineering protection. Formation 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 supporting structure design and construction scheme should be modified, and special protection or reinforcement measures should be taken for the surrounding important engineering facilities when necessary.

5) measurement and monitoring on the construction site. According to the monitoring data and information, conduct feedback design when necessary, and use informatization to guide the next construction.

II. Types of deep foundation pit support

All kinds of buildings and underground pipelines need to excavate foundation pits, and some foundation pits can be excavated directly or sloping. However, when the depth of foundation pits is deep and the surrounding site is not wide, foundation pits are generally used for supporting, in the past, the support was relatively simple, that is, the steel sheet pile and well point dewatering, which can generally meet the safety construction of foundation pit, but for the deep foundation pit, it can no longer meet the requirements. In recent years, with the increase of depth and volume of foundation pit, support Technology has also made great progress. The following are commonly used according to functions [2]:

1) retaining system: steel sheet pile, reinforced concrete sheet pile, deep cement mixing pile, bored pile and underground continuous wall are commonly used. Its function is to form the earth pressure outside the pit by supporting row piles or retaining wall.

2) water retaining system: commonly used are deep cement mixing pile, Jet Grouting pile, Compaction Grouting, underground continuous wall and locking steel sheet pile. Its function is to block water leakage resistance.

3) Support system: commonly used are Steel Pipe and Steel internal support, reinforced concrete internal support, steel and reinforced concrete combined support. Its function is to support the lateral force of the enclosure structure and limit the displacement of the enclosure structure.

The common types of deep foundation pit support mainly include the following:

2.1 steel sheet pile support

Steel sheet pile is made of hot-rolled steel with lock opening or wrench jaw, and the steel sheet pile is interconnected to form steel sheet pile wall, which is widely used for retaining soil and water. At present, the common section forms of steel sheet pile are U-shaped, Z-shaped and straight Web-shaped. Steel sheet pile is widely used due to its simple construction. However, the construction of steel sheet pile may cause deformation of adjacent Foundation and generate noise vibration, which has a great impact on the surrounding environment. Therefore, in areas with dense population and high building density, its use is often limited. Moreover, the steel sheet pile itself is relatively flexible. If the support or anchor system is not properly set, its deformation will be very large. Therefore, when the depth of foundation pit support is greater than 7m, it should not be used. At the same time, because the steel sheet pile needs to be pulled out after the basement construction, the influence of pulling out on the surrounding foundation soil and surface soil should be considered.

2.2 Deep mixing support

Deep mixing support uses cement as curing agent and mechanical stirring to forcibly mix curing agent and soft soil agent, making a series of physical and chemical reactions between curing agent and soft soil agent gradually harden, forming integrity, cement-soil pile wall with water stability and certain strength is used as supporting structure. It is suitable for silt, silt soil, clay, silt clay, silt, plain fill and other soil layers, and the excavation depth of foundation pit should not be greater than 6m. The organic soil and peat soil should be determined through experiments.

2.3 row pile support

Row pile support refers to a supporting form of reinforced concrete digging holes and drilling (punching) bored piles arranged at intervals of colonnade as the main retaining structure. The colonnade-type interval arrangement includes the sparse arrangement form with a certain net distance between piles and the tight arrangement form with the tangent between piles. The colonnade-type cast-in-place pile has good rigidity as the retaining structure, but the connection difference between each pile must be reliably connected by pouring the reinforced concrete cap beam with larger section on the pile top. In order to prevent underground water and entrainment of soil particles from flowing (penetrating) into the pit from the pores between piles, high-pressure grouting should be adopted between piles or at the back of piles at the same time, and measures such as deep mixing piles and jet grouting piles should be set, or specially construct waterproof curtain behind the pile. The construction of cast-in-place pile is simple, and it can be power drill (punching) holes or manually dug holes. During the construction, no large machinery is needed, and there is no damage caused by the noise, vibration and squeezing of the pile, the cost is lower than that of underground continuous wall. Meanwhile, the retaining structure of cast-in-place pile can also be regarded as a part of the external wall to participate in the stress (bearing lateral pressure) when designing the external wall building principal part structure. At this time, there is usually no tension reinforcement between the pile and the main body, and it is separated by waterproof layer. Row pile support can be divided into cantilever type and anchor type, and anchor type can be divided into single point anchor and multi-point anchor. In most cases, cantilever colonnade piles are suitable for third-level foundation pit, while anchor colonnade piles are suitable for first-and second-level foundation pit engineering. Generally speaking, when the depth of foundation pit h = 8M ~ 14m and the surrounding environment requirements are not very strict, row pile support should be considered more. The work of the colonnade cast-in-place pile is relatively reliable, but attention should be paid to the integral binding effect of the cap beam. At the corner of the foundation pit, the cap beam should be connected continuously. When the retaining structure of cast-in-place pile is required to resist water and seepage, The deep waterproof mixing pile or jet grouting pile between piles and the back of piles must be well done (the common drilling and Compaction Grouting method is not easy to ensure water stop, which has caused many major accidents). When the surrounding environment protection requirements are strict, in order to reduce the deformation of row piles, in the soft soil area, sometimes the foundation pit bottom is reinforced in the passive area along the surrounding or part of the cast-in-place pile with cement mixing pile or grouting, in order to improve the resistance of passive zone and reduce the deformation of supporting structure.

2.4 underground continuous wall

The underground continuous wall has the characteristics of high overall rigidity and good water-stop and anti-seepage effect, and is suitable for various formation conditions such as soft clay and sand below the underground water level and complex construction environment, especially the deep soft soil below the bottom of foundation pit needs to insert the wall deeply, so it is 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 has developed to be not only the retaining structure during Foundation pit construction, but also the side wall of the proposed main structure. If properly supported, and with correct construction methods and measures, the deformation of soft soil formation can be well controlled. In the projects with deep foundation pit (generally h>10m) and high environmental protection requirements, this technology is often adopted after technical and economic comparison. However, it will be difficult for the underground continuous wall to excavate into a groove in hard soil, especially when encountering rock strata, special grooving machines and tools are needed, and the construction cost is relatively high. During the construction, mud pollutes the construction site, causing the site to be muddy. The reverse construction method currently adopted makes the two walls integrated, that is, it is used as the enclosure structure during construction, and at the same time it is the outer wall of the underground structure. Reverse construction is generally used in high-rise urban buildings, and the surrounding construction environment is relatively harsh. Buildings, roads and underground pipelines around the site cannot be damaged due to any construction reasons. Therefore, during Foundation pit construction, by exerting the ability of the underground structure itself to support the pit wall (that is, using the piles, columns, beams and plates of the underground structure itself as the support, and at the same time eliminating the internal support system), reducing deformation of supporting structure, reducing cost and shortening construction period is one of the new technologies for popularization and application. In addition to the on-site pouring underground continuous wall, our country has also carried out the research and trial of prefabricated underground continuous wall and prestressed underground continuous wall. The wall surface of prefabricated assembled diaphragm wall is smooth. Due to reasonable reinforcement, the wall thickness can be reduced and the construction speed can be accelerated. However, prestressed diaphragm wall can improve the rigidity of the enclosure wall by more than 30%, and reduce the thickness of the wall, Reducing the number of internal supports, due to the anti-arch effect caused by the tension of curved cloth reinforcement, the deformation of enclosure structure can be reduced and cracks can be eliminated, thus improving the impermeability. These two methods have been tried in the project and have achieved better social and economic benefits.

2.5 Soil nail support

Soil nail support is a new soil retaining technology used for soil excavation and slope stability. Due to its economy, reliability and fast and simple construction, it has been rapidly popularized and applied in our country. The use of soil nailing support requires the soil to have temporary self-stability ability, so as to give a certain period of time to construct the soil nailing Wall. Therefore, the geological conditions applicable to the soil nailing wall should be limited. "Technical Specification of building foundation pit support (JGJ12021999)" stipulates that soil nailing wall is suitable for second-level and third-level foundation pits, non-soft soil sites, and the depth of foundation pits should not be greater than 12m. The construction speed of soil nailing Wall support is fast, the material usage is low, and the cost is low. 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 closely attached to the construction of existing buildings, so as to save the ground occupied by the pile body or wall. However, from many engineering experiences, the damage of soil nailing wall is almost all due to the effect of water, which softens the soil nailing Wall and causes overall or partial damage. Therefore, it is stipulated that the soil nailing Wall project must be well used for precipitation, and it should not be used as a water retaining structure. Soil nail is a slender rod used to reinforce the in-situ soil on site. It is usually made by drilling, putting deformed steel bars and grouting along the full length of the hole. It relies on the cohesive force or friction force between the soil and passively bears the tensile force when the soil is deformed. It consists of dense soil DingTalk Group, reinforced soil and sprayed concrete surface to form a supporting system. Because the soil strength can be effectively maintained and the disturbance of the soil can be reduced due to the excavation and support. After the 1990 s, the soil nailing Wall technology began to be applied to the southeast coastal area, but the geological conditions in this area belong to the soft soil zone mainly composed of silt and silt soil. In order to adapt to this characteristic, the composite soil nailing support technology has been developed. Reinforced cement earth wall is composed of H-shaped steel (Larsen sheet pile, steel pipe, etc.) inserted into cement soil pile. H-shaped steel bears lateral load, while cement soil has good impermeability, so reinforced cement earth wall has good soil retaining and water sealing impermeability effect. There are generally two types of cement pile and H-shaped steel, and H-shaped steel is inserted into cement pile, It is also very convenient to set support. During construction, H-shaped steel can smoothly sink to the designated elevation by virtue of its own weight. Cement-soil pile construction generally adopts three-axis deep mixer with full depth mixing, and the cement mixing ratio needs to be improved. This technology has been popularized and applied in Shanghai, Jiangsu and Zhejiang areas.

In addition, there are Bolt or spray anchor support, arch ring support and reverse method support, etc.

3. FAQ

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

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

Common Support construction lags behind earthwork construction for a long time, and has to adopt secondary backfilling or setting up shelves to complete support construction generally speaking, earthwork excavation technology content is relatively low, the process is simple, and organization and management are easy. However, retaining support has high technical content, many and complicated procedures, and the construction organization and management are more complicated than earthwork excavation. Therefore, in the process of construction, professional construction teams are responsible for the payment of soil and retaining soil respectively, and most of them are two parallel contracts. In this way, it is difficult to coordinate and manage in the construction process. The earthwork construction unit snatches the progress, delays the construction period, and the excavation sequence is chaotic, especially during the rainy period, even regardless of the working face needed for the retaining support construction, the operation surface left to the supporting construction is almost inoperable, and the supporting work cannot be completed in time, which makes the supporting construction lag behind the earthwork construction. Because the supporting construction has no operating platform to complete drilling, grouting, net distribution, concrete spraying and other work, but have to use earthwork backfilling or build shelves to set up the operating 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 occur, leaving quality hidden dangers.

3.2 Slope repair cannot meet the design and specification requirements

There are often overexcavation and underexcavation phenomena. Generally, mechanical excavation and manual simple slope repair are used for deep foundation excavation, and the concrete initial spraying process of retaining support is started immediately. However, in actual excavation, due to factors such as inadequate construction management personnel, insufficient technical disclosure, different layered and segmented excavation heights, and the operation level of the excavator operator, make the surface roughness and straightness of the slope after mechanical excavation extremely irregular, while it is impossible to dig deeply during manual repair, so we can only trim the roughness of the surface after mechanical excavation, the initial spraying starts without strict inspection and acceptance, so the phenomenon of overdigging and underdigging occurs after the earth retaining payment.

3.3 hole grouting is not in place, soil nail or anchor force can not meet the design requirements

The soil nails or anchor bolts used for deep foundation pit support are used to drill pipes with a straight hole of 100~150. If the hole depth is less than 5 or 6 meters, the depth is more than 10 meters, or even more than 20 meters, the quality of the soil layer passing through the drilling is also different. If the drilling does not carefully study the soil condition, it will often lead to endless slag discharge and residue deposition, which will affect grouting. Some even have difficulty in hole formation and hole collapse, no reinforcement insertion and grouting. Moreover, due to the large randomness of ingredients during grouting, the failure to insert grouting pipe into place, the insufficient grouting pressure and so on, the grouting length and filling degree are insufficient, which make the pullout resistance of soil nails or anchor bolts fail to meet the design requirements, it affects the quality of the project and even needs to be dealt with again.

3.4 The thickness of sprayed concrete is not enough and the strength cannot meet the design requirements

At present, the dry mixing method concrete spraying equipment is commonly used for foundation pit support of construction engineering [4], which is mainly characterized by simple equipment, small volume and long conveying distance, accelerating agent can be added before entering the JET machine, which is easy to operate and can be continuously sprayed. Although dry spraying equipment is simple and convenient to operate, due to the different levels of operators, the operation method and inspection

Incomplete means such as inspection and control, serious rebound of concrete, and factors such as lax quality control of raw materials, inaccurate ingredients, and inadequate maintenance often lead to insufficient thickness of concrete after spraying, the strength of concrete cannot meet the design requirements.

3.5 The difference between construction process and design is too big

The cement content of deep mixing pile is often insufficient, which affects the supporting strength of cement soil. We find that cement-soil cracks occur in the same support, sometimes not in the most stressed area. After checking, the strength is often insufficient, the stacking load of ground construction is often much higher than the design allowable load in local position. The phenomenon of construction quality and cutting corners is not rare. Excavation of foundation pit is a process of significant increase of supporting force and deformation. In the design, there are often requirements for excavation procedures to reduce supporting deformation and make drawings disclosure, however, in actual construction, earthwork bosses often rush for progress regardless of these frames and figure out local benefits.

3.6 The design differs greatly from the actual situation.

Because the Earth pressure of the deep foundation pit support is different from that of the retaining wall of the traditional theory, under the guidance of the current imperfect earth pressure theory, it is usually calculated by the traditional theory, so it is normal to have errors, many scholars have done a lot of researches on this, and on the basis of traditional theoretical earth pressure calculation, combined with necessary experience correction can meet practical requirements. The problem is that for such a very complicated subject, separating from the actual engineering situation will often cause the consequences of excessive deformation. For example, if some designs do not consider the differences of geological conditions and ground loads, copy the supporting designs with the same pit depth. The lateral pressure on the supporting structure must be estimated correctly according to the load that may occur on the actual ground, including the influence of building stacking, automotive truck, temporary facilities and nearby residential buildings.

3.7 project supervision is not in place

According to the regulations, the deep foundation pit of high-rise buildings, major municipal buildings and so on must be subject to engineering supervision. Most accident projects do not carry out engineering supervision according to the regulations, or although there is supervision, the work is not in place, only the on-site projects, regardless of off-site influence, there are fewer people who implement the whole process supervision including design. Objectively speaking, the supervision of deep foundation pit engineering requires the supervisor to have a higher professional level. At the present stage of our country, it mainly monitors the quality, construction period and progress of supporting structure engineering, however, there is still a certain gap between the design supervision and the monitoring of residence and surrounding environment, and Wu needs to be perfected and improved.

3.8 construction monitoring does not pay attention

The main reason is that the construction unit does not require construction monitoring in order to save money, or although some measuring points are set up, the data is insufficient, the detection of residential buildings near the pit is ignored, or the monitoring data is ignored, which is useless. There is no monitoring scheme in the support design, so the situation can not be timely warned, and it is not easy to analyze the cause after the accident, which is not conducive to the early treatment of the accident and saves a lot of money.

In order to reduce the supporting accidents, it needs to be carefully designed, elaborately constructed, strengthened supervision, protected the residence and environment at the pit edge, and improved the supporting technology and management level of deep foundation pit.

IV. Development trend of deep foundation pit technology

1) The foundation pit develops towards the direction of large depth and large area, the surrounding environment is more complex, and the excavation and support of deep foundation pit are more and more difficult. Therefore, from the perspective of construction period and cost, the reverse method of combining two walls will be the main direction of future development. However, the reverse construction method is greatly limited by the bearing capacity of piles. When the reverse construction method is adopted, one column and one pile can not be adopted, but multiple piles per column, which increases the cost and construction difficulty. How to improve the bearing capacity of a single pile, reduce the settlement, reduce the middle column pile (middle bearing columns), reach one column and one pile, so that the construction speed of the upper structure can be relaxed, thus speeding up the progress and shortening the total construction period, this will become the future research direction.

2) the massive implementation of soil nailing support scheme enables the full application and development of sprayed concrete technology. In order to reduce the rebound of sprayed concrete and the need to protect the environment, wet sprayed concrete will gradually replace dry sprayed concrete.

3) at present, in the deep foundation pit engineering with support, the excavation of foundation pit is mainly artificial excavation, and the efficiency is not high. In the future, we must vigorously research and develop small, flexible and special underground excavation machinery, in order to improve work efficiency, speed up construction progress and reduce the influence of time effect.

4) in order to reduce the deformation of foundation pit, controlling the deformation by applying prestressed method will be promoted gradually. In addition, deep mixing or grouting technology is adopted to reinforce the soil at the bottom of foundation pit or in the passive area, it will also become an effective means to control deformation and be promoted.

5) in order to reduce the environmental effect brought by foundation pit engineering (such as additional ground settlement caused by precipitation), or for the need of protecting groundwater resources, sometimes the foundation pit adopts curtain type for supporting. In addition to the underground continuous wall, the construction methods such as jet grouting pile or deep mixing pile are generally used to build the waterproof curtain. At present, there is a tendency to introduce the construction method of anti-seepage wall in water conservancy projects into foundation pit projects.

6) in the soft soil area, in order to avoid the bulge at the bottom of the foundation pit, the horizontal displacement of the supporting structure increases and the adjacent buildings (structures) sink, the deep mixing pile or grouting technology can be adopted to reinforce the soil at the bottom of the foundation pit, that is, to improve the strength of the soil in the passive area of the supporting structure.