Construction engineering pit support quality control?

Deep pit support project is a temporary project, but the reliability of its construction program and the quality of construction will directly affect the structure of the main construction of the basement and the safety of the workers, and the technical complexity of its construction, some of which is far more than the permanent infrastructure or superstructure, a little carelessness, will not only jeopardize the safety of the pit itself, will also be affected by the adjacent structures and various underground facilities, resulting in huge losses. This paper analyzes the current deep foundation pit support system. This paper analyzes the current deep pit support the existence of safety problems, put forward the deep pit support design and construction of precautions and preventive measures.

1 Preface

In recent years, the rapid rise of high-rise buildings has promoted the development of deep foundation pit support technology. Various places have accumulated rich design and construction experience in deep foundation pit excavation and support technology, and new technologies, new structures, and new techniques are constantly emerging. However, nowadays, the distance between urban buildings is very small, and some edges of foundation pits are only a dozen meters or even a few meters away from the existing buildings, which brings great difficulties to the construction of foundation projects, brings great threats to the surrounding environment, and correspondingly increases the construction period and construction costs. In addition, the original deep foundation pit support structure design theory, design principles, arithmetic formulas, construction technology, etc., is no longer in line with the actual situation of deep foundation pit excavation and support structure, resulting in a number of accidents in the foundation pit project, resulting in huge losses. Therefore, the safety of deep foundation pit support engineers and technicians should pay great attention to.

2 Problems of deep foundation pit support

2.1 Improper selection of physical and mechanical parameters of soil in the design of support structure

The size of soil pressure borne by deep foundation pit support structure directly affects its safety, but due to the variable and complex geological conditions, it is still very difficult to accurately calculate the soil pressure, and the Coulomb or Rankine formulas are still used today. Regarding the selection of physical parameters of soil is a very complicated problem, especially after deep foundation pit excavation, the three parameters of water content, internal friction angle and cohesion are variable values, it is difficult to accurately calculate the actual force of the supporting structure.

In the design of deep foundation pit supporting structure, if the value of physical and mechanical parameters of foundation soil is not accurate, it will have a great influence on the result of the design. Soil mechanics test data show that: the difference in internal friction angle value of 5 °, which produces different active soil pressure; original soil cohesion and excavation soil cohesion, the difference is even greater. Different construction techniques and support structure forms also have a great influence on the selection of physical and mechanical parameters of the soil body.

2.2 Sampling of foundation soil is incomplete

Before the design of deep foundation pit support structure, it is necessary to sample and analyze the foundation soil layer, in order to obtain a more reasonable physical and mechanical indexes of the soil, and to provide a reliable basis for the design of the support structure. Generally in the deep foundation pit excavation area, according to the requirements of the national norms for drilling and sampling. In order to reduce the workload of exploration and reduce the cost of the project, it is impossible to drill too many holes. Therefore, the soil samples obtained have a certain degree of randomness and incompleteness. However, the geological structure is extremely complex and variable, and the soil samples obtained cannot fully reflect the reality of the soil layer. Therefore, the design of the support structure may not be completely in line with the actual geological situation.

2.3 Inadequate consideration of the spatial effect of pit excavation

A large number of measured data in the excavation of deep foundation pits show that: the horizontal displacement from the periphery of the pit to the pit is large in the middle and small on both sides. Deep foundation pit slope instability, often to the long side of the center position. This is enough to say that deep foundation pit excavation is a spatial problem. The traditional design of deep foundation pit supporting structure is dealt with according to the plane strain problem. For some elongated pits, this plane strain assumption is more realistic, whereas for nearly square or rectangular deep foundation pits, the difference is greater. Therefore, in the absence of spatial problems and according to the plane strain assumption design, the support structure should be adjusted appropriately to adapt to the requirements of the spatial effect of excavation.

2.4 Support structure design calculation is inconsistent with the actual force

At present, the design calculation of deep foundation pit support structure is still based on the limit equilibrium theory, but the actual force of the support structure is not so simple. Engineering practice has proved that some of the supporting structure according to the limit equilibrium theory design and calculation of the coefficient of safety, theoretically speaking, is absolutely safe, but sometimes the damage occurs; some of the supporting structure, although the coefficient of safety is relatively small, and even can not reach the requirements of the specification, but in the actual project to meet the requirements.

Limit equilibrium theory is a kind of static design of deep foundation pit supporting structure, while in fact the soil after excavation is a dynamic equilibrium state, is also a process of gradual relaxation of the soil, with the growth of time, the strength of the soil body gradually decline, and produce a certain deformation. Therefore, this must be fully considered in the design.

3 Precautions in the design and construction of deep foundation pit support program

3.1 Thoroughly change the traditional design concept

In the last decade or so, our country has accumulated a lot of practical experience in deep foundation pit support technology, collected some technical data in the process of construction, and has initially mapped out the law of the actual stress on the support structure of geotechnical changes to establish deep foundation pit support It has laid a good foundation for the establishment of new theories and new methods of deep foundation pit support structure design. However, for the design of deep foundation pit supporting structure, there is no accurate calculation method at home and abroad, most of them are in the stage of exploration and exploration, and there is no unified design specification for supporting structure in China. The distribution of soil pressure is still determined according to the theory of Cullen or Rankine, and the supporting pile is still calculated by the "equivalent beam method". Its calculation results and deep foundation pit supporting structure of the actual force disparity is large, neither safe nor economical. It can be seen that the design of deep foundation pit supporting structure should no longer use the traditional "structural load method", and should completely change the traditional design concepts, and gradually establish the construction monitoring as the leading information feedback dynamic design system. This is the designers need to strengthen the direction of scientific research.

3.2 Establish a new engineering design method for deformation control

At present, the limit equilibrium principle used by designers is a simple and practical common design method, and its calculation results have important reference value. However, this design method is used for deep foundation pit support structure, can only purely meet the strength requirements of the support structure, but can not guarantee the rigidity of the support structure. Numerous engineering accidents are caused by excessive deformation of the supporting structure, so it can be seen that the evaluation of the design of a supporting structure is not only to see whether it meets the strength requirements, but also to see whether it produces environmental problems, the key lies in the size of its deformation. In view of the above reality, when establishing the new deformation control design method, it should focus on the research of the standard of deformation control of the supporting structure, the spatial effect is transformed into plane strain and the determination of the ground overload and its influence on the supporting structure.

3.3 Great efforts to carry out experimental research on supporting structure

Correct theory must be based on a large number of experimental research. However, in the deep foundation pit supporting structure, China has not yet carried out scientific and systematic experimental research. Some of the support structure project success, but also can not tell the specific merits; some of the support structure project failure, but also can not tell the real reason for failure. In the process of supporting engineering construction of the accumulated technical information is very rich, but the lack of scientific test data, can not be scientifically analyzed, can not rise to the height of the theory, which is a great defect.

To carry out the experimental research of the supporting structure (including laboratory simulation test and engineering site test), although it costs some money, but because of the huge investment in deep foundation pit support project, such as after scientific test and then when the design, it will surely save sizable amount of money. Therefore, engineering field test is very necessary. Accumulation of a large amount of test data through engineering practice can lay a good foundation for the success of similar projects, and provide reliable first-hand information for theoretical research and the establishment of new calculation methods.

3.4 Explore the calculation method of new type of support structure

The rapid development of high-rise buildings has brought a technological revolution to the deep foundation pit support structure. After the successful application of steel sheet piles, reinforced concrete sheet piles, bored piles retaining walls, diaphragm walls and other supporting structures, new supporting structure types such as double-row piles, soil nails, combined arch curtains, rotary spray soil anchors, pre-stressed reinforced concrete porous slabs and other new supporting structure types have also come out one after another. However, how to establish the calculation model of these support structure type, how to select the calculation sketch, how to design method tends to be scientific, is still the current new support structure design of the urgent need to solve the problem.

The excavation and support structure of construction pit is a systematic project involving engineering geology, hydrogeology, engineering structure, construction materials, construction technology and construction management. It is a set of soil mechanics, hydraulics, materials and structural mechanics is equal to a comprehensive discipline. The support structure is a whole composed of several systems with independent functions. Because of this, both structural design and construction organization should start from the overall function, the components of the coordination, in order to ensure its safety, reliability, economic and reasonable.

The above is collected and organized by Zhongda Consulting

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