Construction technology of deep foundation pit support in high-rise building engineering

With the continuous increase of urban population, most of the construction projects in the city adopt high-rise or super high-rise buildings to reduce the construction land. The high-rise building foundation pits used at this stage generally belong to the more dangerous sub-projects exceeding a certain scale. Because the foundation pit construction is affected by the surrounding existing buildings, it is impossible to realize the sloping excavation construction. Therefore, in order to ensure the safety of the foundation pit excavation and foundation construction stage, deep foundation pit construction is generally protected by foundation pit support. The foundation pit support is to ensure the safety of the underground engineering construction and the roads around the foundation pit and the existing buildings. Through scientific and reasonable design, the support system is set around the side walls of the foundation pit, thereby improving the excavation construction of the foundation pit and the overall building construction. security.

1. Main forms of deep foundation pit support in construction engineering

In construction engineering, the commonly used support forms for deep foundation pits mainly include 1) underground continuous wall, which can be divided into pile row type, groove plate type, and combined type according to the wall forming method; 2) concrete pouring pile, according to its hole forming method It can be divided into bored cast-in-place piles, immersed pipe cast-in-place piles, artificially dug holes and bottom-excavated cast-in-place piles, etc.; 3) soil nail wall, the slope of the foundation pit is reinforced with soil nails made of steel bars, and the side slope is reinforced with soil nails made of steel bars. Laying a reinforced mesh on the surface of the slope and then spraying a layer of concrete surface layer and earthwork slope combined slope reinforcement support construction method; 4) Water-stop curtains, common water-stop curtains include high-pressure rotary grouting piles, deep mixing piles, etc. water curtain.

Deep foundation pit support is an indispensable part of high-rise building construction. It can ensure the safety and continuity of subsequent foundation pit excavation and underground engineering construction, and is crucial to the success of the entire engineering project. The key points of deep foundation pit support are as follows: First, the support structure of deep foundation pit should not only meet the strength and deformation requirements of the foundation pit wall, but also ensure the safety of existing building structures within and around the foundation pit. The concept of safety first is used as a guide for the design and construction of foundation pit support. In the process of support construction, a complete safety operation regulation and accountability mechanism should be established to ensure the standardization and safety of the support process. In addition, before carrying out the construction of deep foundation pit support, the design and construction drawings should be carefully reviewed, and a special construction plan should be prepared and verified by experts before construction to ensure the construction safety and stability of the support structure. During the construction of deep foundation pits, effective measures must be taken to control and reduce the negative impact of noise, water and air pollution on the surrounding environment, and to ensure the normal life and work of residents around the project. In order to ensure the quality and safety of deep foundation pit support construction, it is necessary to strengthen the monitoring and measurement of the whole process of foundation pit support construction, to detect possible problems in time, and to use targeted technology and equipment in time to prevent and rectify them.

2. Selection and precautions of deep foundation pit support form

With the continuous development of deep foundation pit support technology, the design excavation depth of foundation pit is also increasing, and vigorously developing and utilizing underground space has become an important development trend in the field of high-rise buildings. Therefore, it is of great significance to scientifically and reasonably select the support scheme of deep foundation pit. Since the geological conditions in which most high-rise building projects are located are very complex, this will also have an adverse effect on the stability of high-rise building structures. Before the deep foundation pit excavation of high-rise building projects, the geological conditions of the construction site and the surrounding construction environment should be comprehensively analyzed in order to better understand the construction status and take effective measures to control the construction cost to obtain better economic benefits. The construction of deep foundation pit projects has complex environmental impacts and high risks. During the implementation process, due to the existence of uncontrollable factors, it will have an adverse impact on the construction quality of the construction project, and even cause serious safety accidents. Therefore, when constructing underground works of high-rise buildings, a safe and reasonable deep foundation pit support form should be selected according to the geological survey situation on the site and combined with the economic situation of the project, and standardized foundation pit support construction technology should be adopted to improve the quality of deep foundation pits. The safety and reliability of the pit are stable, ensuring the smooth progress of high-rise building construction.

2.1. Determinants of the choice of deep foundation pit support form

2.1.1 Groundwater impact

The safety of deep foundation pits is significantly affected by groundwater, because during the excavation of foundation pits, the original conditions of groundwater will be changed, especially groundwater will flow into the foundation pit, which will seriously affect the overall stability of the foundation pit wall sex and safety. If the water resistance performance of the foundation pit support is not good, water leakage occurs, it is easy to cause water and soil loss on the foundation pit wall or even quicksand phenomenon, which will affect the stability of the foundation pit wall, and even cause the collapse of the foundation pit support in severe cases. In order to effectively limit the adverse effects of groundwater on deep foundation pits, artificially lowering the groundwater level is usually used to solve such problems, and at the same time, support forms with good waterproof and water resistance performance should be selected for foundation pit support.

2.1.2 Surrounding environmental impact

The main function of deep foundation pit support is to ensure the safety of the foundation construction of high-rise buildings, the surrounding environment of the foundation pit and the existing buildings. But theoretically speaking, the excavation construction of deep foundation pit will cause certain damage to the soil and environment around the foundation pit, especially in the case of large soil deformation, the adverse effects of foundation pit excavation will be more serious. obvious. The purpose of foundation pit support is to reduce the adverse impact of construction on the surrounding soil and environment of the foundation pit by selecting safe and reasonable support forms, so as to ensure the stability and safety of the foundation pit wall. At the same time, the load around the foundation pit should also be controlled to avoid excessive force on the support and cause the deformation to exceed the limit.

2.1.3 Foundation Pit Support Monitoring and Measurement

Due to the immature design theory of deep foundation pit support, the large volume of underground engineering, and complex geological conditions, the excavation of deep foundation pits has brought serious safety hazards to people's personal and property safety. The monitoring of foundation pit support is not in place and dangerous situations Inaccurate forecasting is the main reason for frequent safety accidents in foundation pit construction. Secondly, the accuracy of the geological survey report can be verified through the monitoring and measurement of the foundation pit, and the correctness of the parameters used in the theoretical calculation of the design can be verified at the same time to guide the construction of deep foundation pit excavation and foundation pit support. Thirdly, ensure the safety of foundation pit excavation construction, surrounding existing buildings, municipal facilities and underground pipelines during the construction stage, and ensure the safe and smooth implementation of the project construction process. Finally, through monitoring and measurement, real-time real-time measurement data such as displacement deformation, settlement, strain, and groundwater level of foundation pit support can be grasped at any time, and potential safety hazards that may occur in foundation pit engineering can be predicted and warned to avoid major safety hazards. ACCIDENT.

2.2   Matters needing attention in the application of deep foundation pit support technology

2.2.1 Innovative Design Concept

When designing deep foundation pit support structures, designers must constantly explore new design concepts and choose more realistic, reasonable and comprehensive support forms to ensure that the effect of foundation pit support meets the requirements of specifications and construction safety , optimize the design scheme of deep foundation pit support structure.

2.2.2 Strengthen the design theory of deep foundation pit support structure

In order to ensure the safety and rationality of deep foundation pit support design in high-rise building engineering, the theoretical research of deep foundation pit support structure must be paid attention to. Reasonable calculation methods and professional theories should be adopted, combined with a large number of experimental data, to simulate and analyze the mechanical model of deep foundation pit support through computer construction, and obtain scientific and reasonable calculation results, so as to provide support for the design of deep foundation pit support structures for high-rise buildings. and construction to provide more accurate data and theoretical support.

3. Application of deep foundation pit support construction technology in high-rise housing construction projects

In order to ensure the construction quality and safety of the underground engineering of high-rise buildings, it is necessary to effectively control the construction of deep foundation pit excavation and support, and improve the construction quality of deep foundation pit support structures. In the foundation pit support construction project, it is necessary to master the construction points of different forms of support structures.

3.1 Steel sheet pile support construction

The core of steel sheet pile support is the combination of hot-rolled section steel and steel sheet piles, and supports the outer soil layer of the foundation pit in the form of steel sheet walls. The steel sheet piles have good integrity and can effectively block groundwater, but the support of steel sheet piles The protection depth is generally not greater than 10m, which is widely used in soft soil foundation support, and the steel sheet pile can be reused, thereby reducing the cost of steel sheet pile support. During the construction process, in order to ensure the effectiveness of the steel sheet pile support, technical staff need to conduct a comprehensive study of the foundation soil and surface soil environment around the support to avoid excessive deformation of the support.

3.2 Row pile support

This form of foundation pit support has extremely high flexibility and a wide range of applications, and can be applied to various soil types of foundation pits, especially in soft soil foundation pit support. During the construction of row pile support, special attention must be paid to the following points: First, the cast-in-situ piles with appropriate pile diameter and depth should be constructed in advance, and arranged according to the arrangement required by the design; second, the dewatering of the foundation pit should be focused on , to limit the adverse effects of changes in groundwater on pile row support construction; third, in the construction process of densely packed bored piles, technical personnel are required to take effective control measures according to the depth of the foundation pit, and reasonably formulate the requirements for drilling piles. order to ensure that the depth and integrity of the row pile support meet the requirements of the code.

3.3 Soil nail wall support

The soil nail wall is to set soil nails on the soil wall of the foundation pit by drilling holes, weld and fix it with the steel mesh bound on the surface of the soil wall, and then spray a layer of early-strength fine stone concrete panels to form a structure similar to a gravity retaining wall to balance The earth pressure behind the wall; thereby maintaining the stability of the foundation pit wall. The soil nail wall can effectively control the deformation of the soil wall, and has the advantages of strong applicability, simple construction, and convenient operation, and is especially suitable for the support of cohesive soil foundation pits. Before construction, construction technicians must determine the depth of the drill hole through the soil nail pulling test to ensure that the anchorage length of the soil nail can meet the design pull-out strength requirements. Then carry out large-scale soil nail drilling and anchor grouting operations. Strictly control the water-cement ratio of cement grout during grouting to ensure that the grout can be effectively integrated with the soil after setting, thereby improving the strength and stability of deep foundation pit support.

3.4 Cement-soil gravity type retaining wall support

In the form of deep foundation pit support, the cement-soil gravity retaining wall is a continuous integral support structure, which is drilled into the soil layer to the design depth through the mixing machine, and then reinforced materials such as cement and the drilled loose soil The body is forced to stir evenly, and a continuous lapped cement-soil columnar reinforced solid retaining wall is formed in the soil. Relying on the self-weight of the cement-soil wall and the frictional resistance of the bottom of the wall to resist the pressure of the soil behind the wall and effectively control the deformation of the wall. This support form has good environmental protection and stability during the construction process, and its anti-seepage ability and support effect are also excellent. In practical engineering application, it is necessary to plan scientifically and fully consider various influencing factors in order to improve the construction performance and construction efficiency of the cement-soil gravity type retaining wall. Specific measures include the following aspects:

One is the measurement and setting-out: In order to ensure the accuracy of the measurement and setting-out, the measurement is divided into three stages during construction: first, the basic control line and control points are determined by measurement, which will be reviewed and confirmed by the supervisor and the construction unit; Axis, measuring and setting the axis of the gravity-type retaining wall of cement-soil mixing pile; finally, according to the confirmed axis of the retaining wall, measuring and setting the specific sideline position of the retaining wall of cement mixing pile.

The second is the test pile: before the construction of the cement-soil mixing pile, the test pile must be carried out to verify the construction technology of the cement-soil mixing pile. Determine the sinking speed of the drilling equipment, and adjust the matching relationship between the spraying speed of the nozzle and the sinking and lifting speed of the drill bit. If it is found that the slurry pipeline is blocked, the pumping should be stopped immediately, and the drilling tool of the mixing equipment should be raised or lowered by 1m immediately after the treatment is completed, and then the spraying and mixing operation should be continued to ensure the integrity and continuity of the cement-soil mixing pile.

3.5 Underground Diaphragm Wall Support

In the process of foundation pit construction of high-rise building projects, due to the large differences in geological conditions in different regions, special geological conditions will be encountered during construction. Especially when encountering weak alluvium, medium-hard stratum, dense gravel layer and rock geological conditions, and the groundwater level is high, the most commonly used foundation pit support method is to use the underground diaphragm wall support structure. During the construction of deep foundation pits of high-rise buildings with strict requirements on surrounding settlement, the underground diaphragm wall support structure is used extremely frequently, and its application value far exceeds other support methods. The underground diaphragm wall is suitable for a variety of soil conditions, with little vibration and noise pollution during construction; it is also friendly to the surrounding environment, and can effectively ensure that the foundation pit excavation and structural construction of high-rise buildings are in a safe and stable state. Of course, in practical applications, this process has also exposed some defects and limitations. Once the hardness of the soil in the construction area is high, the technical requirements for the support of the underground diaphragm wall will be more stringent, resulting in an increase in the overall construction cost. Due to the large amount of mud produced by the supporting structure of the underground water diaphragm wall during construction, it is easy to pollute the surrounding water bodies. Therefore, an effective mud treatment and discharge plan must be formulated to minimize groundwater pollution in the construction area. If the underground diaphragm wall is only used as a temporary support structure during construction, it will lose its use value after the construction of the underground structure is completed. Therefore, when the excavation depth of the foundation pit is not large, it is not as economical as using other support forms.