Abstract: As a new type of pile, pipe pile has been widely used for its advantages of convenient construction, high bearing capacity, reliable quality and economy. According to the bearing capacity characteristics and stress status of pipe piles, the factors influencing the bearing capacity of pipe piles and the methods to improve the bearing capacity of pipe piles are analyzed, and the common problems in construction are discussed, and the effective preventive measures are put forward.
Key words: pipe pile; Carrying capacity; Architecture; quality management
1 preface
As a form of foundation treatment and pile foundation, pipe pile has been greatly developed since it came into being at the beginning of last century. It is widely used in various building foundations and plays a huge role. From the development of foreign pipe piles, it has been 890 years since 1920, when Australia invented centrifugal method to make concrete products, and 1925, when Japan introduced this technology for reinforced concrete pipe piles, and 1962 developed prestressed concrete pipe piles (PC pipe piles). At present, the pipe pile has developed towards completely replacing the traditional solid pile. China began to produce concrete centrifugal (RC) pipe piles in 1944. By the end of 1980s, the research on prestressed confined pipe piles was successful, that is, the post-tensioning method was used to prestress the concrete of pipe piles. In recent 15 years, the prestressed concrete pipe piles produced in China have reached the forefront of the world in terms of product performance and output, showing the characteristics of wide layout, complete product varieties and specifications, mature production technology and increasingly perfect supporting application technology. According to the data, the actual usage of precast high-strength concrete pipe piles in Fujian Province reached 25 million meters in 2004. In order to make more rational use of pipe pile technology and effectively promote the use of pipe pile, it is extremely necessary to study pipe pile.
The types of pipe piles are divided into steel pipe piles, precast concrete pipe piles and concrete-filled steel pipe piles. Steel pipe piles and concrete-filled steel pipe piles have the advantages of high strength, good impact fatigue resistance, strong penetration ability, convenient cutting, reliable quality, convenient transportation, fast pile sinking speed and little influence of soil squeezing, but the cost is about 3- 10 times that of prestressed concrete pipe piles. Therefore, it is generally only when it is necessary to cross the sand layer or other pile types and the quality is difficult to guarantee, or when the construction period is urgent, or on the basis of some important special projects, such as offshore drilling platforms, port platforms and other projects. The rapid development and wide application of precast concrete pipe piles mainly benefit from the following advantages:
(a) the construction period is short, the construction is convenient, not limited by seasons, and industrial production:
(b) There is no pollution to the construction site. If static pressure construction is adopted, there will be no noise, which meets the requirements of green construction;
(3) The economic benefits are considerable. The concrete used for the same foundation treatment effect (vertical bearing capacity and horizontal bearing capacity) is 30%-60% less than that of solid piles, which has strong corrosion resistance and basically similar working performance to steel pipe piles.
(d) Strong adaptability to geological conditions with large bearing stratum fluctuation. Generally speaking, formation conditions such as soft soil, cohesive soil, silt, sandy soil and completely weathered rock mass can be adopted. Therefore, such as high-rise buildings, wharf engineering, bridge engineering, expressway, railway engineering and so on. In addition to the special foundation of steel pipe pile, steel pipe pile has been mostly replaced by precast concrete pipe pile in engineering. At present, the use of precast concrete pipe piles in China has been considerable.
Bearing characteristics and bearing capacity analysis of two pipe piles
2. 1 Bearing characteristics of pipe piles
The pile end (shoe) forms of pipe pile bottom pile mainly include cross, cone and open type. The first two belong to the sealed type. The bearing capacity of closed pipe pile with pile tip is mainly composed of side friction around the pile and resistance at the end of the pile. For the pipe pile with pile boots, the inner cavity of the lower part of the pile body (1/3- 1/2) is filled during pile sinking, which has weak soil squeezing effect (compared with pile sinking and static compaction concrete pile), little impact on surrounding buildings and environment, and high environmental protection performance. The soil plug in the inner cavity provides internal friction for the pipe pile, which makes the composition of the bearing capacity of the pipe pile more complicated. There are many factors that affect the bearing characteristics of pipe piles, such as soil properties at the pile side, soil properties at the pile end, pile diameter, wall thickness of open pipe piles, soil depth and construction sequence. Precast concrete pipe piles usually only have the effect of opening piles.
2.2 Force analysis of pipe pile
2.2. 1 There are many methods to determine the vertical bearing capacity of pipe piles and the ultimate bearing capacity of single piles. The most reliable method is the plate load test method. At present, there are two commonly used formulas: first, based on the physical and mechanical indexes of soil and a large number of test pile data, the relationship between pile side and pile end resistance and soil indexes is established through statistical analysis; The other is based on the mechanical properties of soil, such as the standard penetration number of soil. The first formula is adopted in the API-RP2A foundation specifications of China, Europe and the United States. Its expression is
Because of different geological conditions, complex geological structure, diverse pile types and diverse pile sinking techniques, it is difficult to reflect the engineering practice with a single formula.
According to the failure load test in the literature, when the top of the pile is vertically compressed, the vertical stress and elastic deformation first occur in the upper part of the pile body, which are transferred to the lower part of the pile body, and the frictional resistance is gradually established from top to bottom, so that the pile body is in the elastic compression stage. When the load is small, the deformation is small, the pile foundation basically has no displacement, and the resistance at the pile end is zero. With the increase of load, when the vertical stress is transmitted to the pile end, the soil at the pile end is gradually compressed, the relative deformation of pile-soil increases, and the pile side friction is further exerted. In the final stage of loading, with the increase of pile tip resistance, the pile top resistance first reaches the limit (friction resistance tends to be constant), and gradually expands the distribution range of ultimate resistance downward. In this process, compared with the load increment, the proportion of frictional resistance increment as resistance is smaller and smaller, while the proportion of pile tip resistance increment is larger and larger. Finally, the plastic zone appears in the soil at the pile tip and expands rapidly. The pile was destroyed due to the sharp subsidence, and the seepage failure of the soil at the end of the pile preceded the strength failure of the pile. The load on the pile at this time is the ultimate bearing capacity of the pile.
2.2.2 Horizontal bearing behavior of pipe piles and ultimate bearing capacity of single pile
With the rapid development of engineering technology in China, the progress of oil exploration and development technology in shallow sea on continental shelf and the development of high-rise buildings on land, these pipe piles have to bear not only huge vertical loads, but also huge horizontal loads. The analysis of pile under horizontal load is a very important problem in engineering, but it has not been satisfactorily solved. In the literature, horizontal jack is used to apply horizontal force test to measure the horizontal load of single pile. The applied horizontal load is usually classified as110-15 of the estimated horizontal ultimate load. After applying each level of load, measure the horizontal displacement of the pile under constant load for 4 minutes, then unload to zero, and measure the residual horizontal displacement of the pile after stopping for 2 minutes. After multistage loading, the test can be stopped when one of the following situations occurs: 1) the pile body breaks; 2) The horizontal displacement exceeds 40mm or the allowable value of horizontal displacement meets the design requirements. When the pile stress reaches the ultimate strength, the horizontal displacement of the pile top is more than 20-30 mm due to the horizontal force at the top of the pile, or the upper level horizontal load caused by the soil destruction at the side of the pile is the standard value of the horizontal ultimate bearing capacity of a single pile.
2.2.3 Factors affecting the bearing capacity of pipe piles
2.2.3. 1 skew
Deflection pile refers to the passive pile with horizontal movement of soil around the pile. The horizontal pressure between pile and soil leads to horizontal deflection and bending moment of the pile, which leads to the deflection of the pile. The ultimate bearing capacity of prestressed pipe pile after deformation is lower than that of straight pile. The reduction of bearing capacity of skew prestressed pipe piles is not only related to skew degree, but also related to soil properties, buried pile length and pile cap arrangement.
When encountering a pile that exceeds the deflection limit, whether there are cracks or not, rectification and righting should be carried out to control its inclination within the allowable range. Shallow (generally 2-3m) piles can be tilted and reversed, and then righted; Deep soil can be taken out from the opposite side of the pile by drilling and high-pressure water washing, and then righted. Then the pile after rectification and righting is tested to see if there is any abnormality in the rectification construction. If there is no abnormality, the next step of construction can be carried out.
2.2.3.2 crack
Shallow cracks-cracks generally occur at a depth of 4-6m, and some are within 3m. This happens mostly because there is a relatively hard soil layer at the lower part of the pile crack. The deep crack-crack position appears at 8- 10m, which is mainly caused by the deep soft soil layer on the foundation soil. The existence of cracks will inevitably affect the vertical permanent load characteristics of pile foundation. In order to ensure the safe use of pile foundation engineering, it is necessary to reinforce the pile foundation. Such as pile extension and pile repair, etc., need to be determined by calculation under certain circumstances.
2.2.3.3 eccentric load
Vertical load eccentricity is an important reason for bending load of prestressed concrete pipe piles, and load eccentricity will inevitably affect the vertical bearing capacity of piles. Prestressed concrete pipe pile foundation often adopts multi-pile caps under the column. Strictly speaking, most of the piles under the platform are under eccentric stress. How to correctly evaluate the bearing capacity of eccentrically loaded pile, what is the critical value of eccentricity that the pile can bear in the normal service limit state, and the relationship between eccentricity of vertical load and bearing capacity of the pile are problems that need special consideration in the design of prestressed concrete pipe pile foundation.
According to the principle of material mechanics and the current design code of reinforced concrete structures, the calculation method of internal force and displacement (or horizontal displacement of pile top) of prestressed concrete pipe piles under eccentric load is put forward, including the critical value of anti-cracking moment of pile body in pure bending state; Limit value of anti-cracking bending moment of pile under the combined action of axial force and bending moment; The relationship between allowable bearing capacity of pile top and vertical force eccentricity (or horizontal displacement of pile top).
3 problems in the design and construction of pipe piles and quality control
3. 1 compaction effect
In the process of pile sinking, the surrounding soil is crowded, which causes serious disturbance to the surrounding soil, mainly manifested in radial displacement. The soil at the pile tip and around the pile is subjected to undrained shear and large horizontal extrusion, resulting in large shear deformation, forming a disturbed remodeling area with high pore water pressure, reducing the undrained shear strength of soil and promoting the destruction of adjacent soil around the pile due to undrained shear. Due to the superposition effect in pile group construction, the driven piles will be driven. The denser and larger the pile groups, the greater the displacement of soil.
Preventive measures for soil squeezing effect during construction are as follows:
① Arrange the pile sinking sequence reasonably, control the daily pile sinking amount and reduce the superposition of pore water pressure;
(2) Adopting the construction procedure of excavating the foundation pit first and then sinking the pile can reduce the lateral displacement and uplift of the shallow soft soil of the foundation, which is beneficial to reduce the excess pore water pressure caused by pile sinking, thus reducing the displacement of the deep soil of the foundation.
③ Set bagged manholes or plastic drainage boards in the site to create drainage conditions and reduce pore water pressure.
(4) Pre-drilling to assist pile sinking.
3.2 Floating pile
Floating pile phenomenon is a manifestation of soil squeezing effect of static pressure pipe pile. This problem is very hidden, and it is often discovered when the static load test is done after the pile pressing project is completed. At this time, the pile driver may have left the site. At this time, the handling is very passive. The better treatment measures are: select representative piles for measurement and monitoring in advance, measure and record the pile top elevation with a level immediately after pile construction, and retest regularly during the whole construction process, and check whether the pile body floats through comparison. If floating phenomenon is found, measures such as controlling pile pressing speed, adjusting pile pressing route or drilling soil should be taken to reduce soil squeezing effect, so as to control the floating phenomenon of pile body. If the floating phenomenon of pile body cannot be solved after the above measures are taken, recompression can be used as a remedy.
3.3 Pile sinking can't meet the design requirements.
The main reasons why pile sinking can't meet the design final control requirements are:
① Insufficient exploration points or rough exploration data, unknown engineering geological conditions, especially the fluctuation of bearing stratum elevation, which leads to wrong design consideration of bearing stratum or wrong selection of pile length.
② If the design bearing layer is not properly selected, the bearing layer of prestressed pipe pile should be strongly weathered to achieve higher bearing capacity. However, when the buried depth of the strongly weathered layer is large, considering the limitation of pile length, it is necessary to choose the fully weathered layer as the bearing layer, which will greatly affect the bearing capacity, especially the fully weathered layer is easy to soften when it meets water, and groundwater may infiltrate from the pile tip through the pile pipe, which greatly reduces the bearing capacity of the pile tip.
③ The design does not accurately predict the bearing capacity of a single pile, which leads to the mismatch between the actual pile length and the pile pressing force.
(4) The pile body is broken, and it is impossible to continue to exert pressure.
Preventive measures: first, demonstrate the geological conditions of the project address in detail, make supplementary survey when necessary, and correctly select the bearing stratum or elevation; The pile driver with appropriate tonnage shall be used in the construction; According to the engineering geological conditions, the construction method and piling sequence of piles are reasonably selected to avoid pile breakage and ensure the quality of piles. Reasonable final pressure standard is determined through scientific design of test pile.
3.4 Broken pile
Broken pile is a common problem in precast concrete pipe pile construction, and the main reasons are as follows:
① Use unqualified piles produced by manufacturers;
(2) The pile tip hits an underground obstacle and the pipe pile breaks;
③ The verticality control in pipe pile construction is not good;
(4) The pipe pile suddenly enters the hard soil layer from the soft soil layer, and the pressure of the pile driver rises rapidly, resulting in instantaneous impact on the pile body;
⑤ During the foundation pit construction, due to the uplift of soft soil, the foundation pit wall moves laterally, resulting in broken piles.
If broken piles are found during construction, reinforcement scheme should be adopted. The shallow broken pile of prestressed pipe pile can be connected with pile. For the extension of deep-seated broken piles (including joints after rectification of some dislocated piles), the accumulated water in the piles should be drained, the inclination of the piles should be confirmed within the allowable range, and the reinforcing cage should be released. The reinforcing cage shall extend to 3m below the broken pile and be filled with high-grade concrete. After pile extension, the bearing capacity should be tested. When the dislocation of the broken pile cannot be repaired, the pile should be repaired again. For engineering accidents, it is necessary to analyze the causes of the problems, the possibility of repairing piles and the influence on the built piles, and consider other available conditions, economy and construction period.
4 conclusion
As a new type of pile, pipe pile has been more and more widely used because of its advantages of reliable pile quality, high bearing capacity, fast construction speed, clean site and economy. However, due to the short application time of pipe piles, there are still many problems to be solved in research and application. However, the development of engineering practice has far exceeded the theoretical research level, which seriously restricts the application of pipe piles. This paper summarizes the bearing capacity characteristics and stress analysis of pipe piles, the factors affecting the bearing capacity of pipe piles, the methods to improve the bearing capacity of pipe piles, common problems in construction and preventive measures. However, many problems involved in this paper have not been satisfactorily solved at present, and need to be further discussed through a lot of scientific research and engineering practice.
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