Abstract: During the construction of prestressed static pressure pipe piles, quality problems such as pile breakage occur from time to time due to various reasons. In engineering practice, exploring how to avoid the fracture of prestressed pipe piles and adopting low-cost and feasible reinforcement treatment methods for broken pipe piles has strong engineering application value.
　　Keywords: prestressed pipe pile, causes of pile breakage, reinforcement treatment, prevention
　　Prestressed static pressure pipe piles are favored by the engineering community due to their advantages of high bearing capacity, low unit price, short construction period, simple construction, and no noise. They have become a widely used foundation form in soft soil areas and have achieved significant technical, economic, and social benefits. This paper analyzes the causes of pile breakage during the construction of prestressed static pressure pipe piles and proposes treatment and prevention measures.
　　1 Prevention of prestressed pipe pile breakage
　　1.1 Causes of prestressed pipe pile breakage
　　From a large number of engineering practices, the breakage of prestressed pipe piles is generally caused by the following three aspects:
　　First, pile body quality problems: such as insufficient concrete strength grade or insufficient curing time before the pipe pile leaves the factory, or insufficient protective measures during lifting, transportation, and stacking of pile materials, resulting in the ultimate structural strength of the pile body not meeting design requirements. Pipe piles with unqualified pile body quality are easily broken during pile driving. Second, design problems, mainly including two aspects: improper selection of pile type, mainly improper selection when the site geology is complex, such as selecting prestressed pipe piles as foundation in soft soil areas with hard interlayers or many boulders and obstacles, it is easy to encounter boulders, etc. During construction, if the bearing layer cannot be reached and the pile length is not adjusted in time, pile breakage is likely to occur.
　　Improper selection of bearing layer, such as failure to correctly select the bearing layer or requiring the pile foundation to penetrate too deep into the bearing layer, making it impossible to drive the pile to the design depth, or large undulations of the bearing layer rock surface and inflexible adjustment of pile length.
　　In addition, improper selection of pile diameter and too dense spacing in design can also easily lead to pile breakage.
　　Construction is also an important issue: mainly including improper pile driving construction and improper foundation pit construction. Improper pile driving construction is reflected in the following aspects:
　　Poor bearing capacity of the site surface soil, the pile machine sinks during pile driving, and the verticality of the pile body cannot be effectively controlled; improper welding of pile joints; improper measures for moving the pile machine, or failure to reasonably arrange the pile driving sequence, pile driving rate, failure to set stress relief holes, and pile driving monitoring. Due to the squeezing effect, subsequent construction causes deviation and breakage of completed piles.
　　Improper foundation pit construction is reflected in the following aspects:
　　During foundation pit excavation, large excavators may accidentally hit the pile head when rotating the bucket, causing pile breakage; the earthwork excavation procedure in the foundation pit is not strictly carried out in layers and sections according to design requirements; in areas with thick muddy soil, the soil itself has high fluidity. In addition, the accumulated pile driving squeezing force and the release of pore water pressure in the soil layer toward the excavation direction aggravate the flow of mud toward the excavation direction. Because prestressed pipe piles have low resistance to horizontal forces, they tilt toward the excavation direction as the soil displaces. If the excavation is too deep at one time, it will cause deviation of the pipe piles and even fracture; improper enclosure causes slope instability. Once the slope is unstable, the side of the foundation pit wall moves laterally, which will seriously damage the engineering piles by tilting and breaking them. This problem is usually prone to occur with support forms such as soil nail support.
　　1.2 Prevention of prestressed pipe pile breakage
　　Through the above analysis and combined with years of engineering practice experience, we believe that measures taken from the following three aspects can prevent pile breakage:
　　First, strictly control the structural strength of the pile body and ensure the quality of engineering piles is qualified, which is the prerequisite for smooth pile driving. To ensure pile body quality, pipe piles are required to have at least 14 days of natural curing time before leaving the factory. During lifting, transportation, and stacking of pile materials, the corresponding construction specification requirements shall be strictly followed. Each batch of piles must be provided with a corresponding quality guarantee certificate upon entering the site. When pile materials enter the site, dedicated personnel of the pile foundation construction enterprise shall cooperate with the supervision unit to inspect the pile body.
　　Second, reasonably select pile type and determine pile foundation parameters in design. On the premise of carefully analyzing geological conditions, test piles must be done before large-scale pile pressing, and the length of pipe piles should be determined according to the test pile situation to avoid forced pile pressing to the specified elevation causing pile breakage. If necessary, thick-walled piles can be used. According to specific engineering geological conditions, some AB-type or B-type pipe piles can be reserved for selective use according to construction conditions and geological conditions. Although the cost of piles increases, from the perspective of reducing pile breakage and shortening the construction period, it is economical.
　　Third, reasonably arrange the construction plan. Do a good job in preparing for pile driving, lay and roll construction waste on the surface as necessary to ensure it reaches the design bearing capacity; pre-drill stress relief holes (such as pre-drilling, sand wells, plastic drainage boards, etc.) before piling to prevent tilting of engineering piles.
　　Strictly construct correctly according to relevant regulations. It must be ensured that the longitudinal axis of the pile driving rod and the pile body are consistent. The verticality deviation of the first section of pipe pile when lifted and inserted into the ground shall not exceed 0.5%, and it shall be promptly checked with two theodolites in two vertical directions. If necessary, it should be pulled out and reinserted; minimize intermediate rest time; when connecting piles, the upper and lower pile sections should be kept straight, with a misalignment deviation not exceeding 2mm. Welding shall comply with relevant regulations, and the natural cooling time after welding shall not be less than 8min. It is strictly forbidden to cool with water or drive immediately after welding.
　　Take measures to control the pile driving rate and reasonably arrange the pile driving sequence. In the initial stage of pile pressing, the pile driving rate should be controlled at 8-10 piles/day. Pile pressing should be carried out in the direction away from the protected object, and follow the principle of "deep first then shallow, large first then small, middle first then both sides". During construction, according to monitoring results, timely and appropriately adjust the pile driving rate or pile pressing process. If necessary, adopt measures such as skip pressing (interval distance should be greater than 20m) or stop pressing to adjust and control the soil displacement.
　　It is strictly forbidden to excavate while pressing piles. Excavation should be carried out after all pile foundations are completed and at least 15 days later, to facilitate the dissipation of stress in the soil formed during pile pressing. Before foundation pit excavation, the enclosure system should be constructed first, and excavation can only be carried out when the enclosure system meets design requirements.
　　During the excavation of the foundation pit, avoid damaging engineering piles; excavation should be combined with the setting of post-pouring strips and carried out in sections and layers at intervals. The section length should not exceed 20m to fully utilize the "spatial effect". The block positions should be sloped at 1:1.5; the height difference of soil layers between layers should be controlled. The height difference of soil layers in a small range should not exceed 2m, and the height difference of soil around the pile should not exceed 1m, to avoid excessive local stress release leading to deviation and fracture of pipe piles.
　　The excavation sequence strictly follows the principle of "layered excavation, support first then excavation". When constructing the lower layer of soil nails, the upper layer of soil nails must have a curing time of more than 48h.
　　Do a good job in foundation pit monitoring. During foundation pit excavation, if there are abnormal conditions, continuous observation should be carried out, and all relevant units should be notified in time for timely handling. At the same time, emergency preventive measures should be taken to prevent unexpected events.
　　2 Treatment of prestressed pipe pile breakage
　　Although strict preventive measures have been taken against pile breakage during design and construction, due to various reasons, pile breakage problems still occur from time to time in engineering practice. Pile breakage in soft soil areas usually occurs during foundation pit excavation, generally in the upper part of the pipe pile, mainly due to the large pile pressing force or large horizontal displacement in the upper part. For such problems, if methods such as anchor static pressure piles or even backfilling the foundation pit to supplement piles are used, it is time-consuming and uneconomical, and the technical difficulty of supplementing piles is also high.
　　The principles of pile breakage reinforcement are: first, the integrity and verticality of the reinforced pipe pile must meet design requirements. Those not meeting verticality requirements should be corrected by righting methods. Those not meeting integrity requirements should be treated by cutting off the damaged upper part and connecting piles. Second, the bearing capacity of the reinforced pipe pile foundation must meet design requirements.
　　The treatment of pipe pile breakage usually adopts corresponding measures according to different situations. If there is only crack damage, core filling measures can be taken for reinforcement; if the pile body is severely damaged or the upper and lower sections are severely misaligned, pile connection measures can be taken for reinforcement. Below, we illustrate with a specific engineering example.
　　3 Engineering example
　　A project is a small high-rise building with a frame structure. Its pile foundation uses PHC pipe piles. The absolute elevation of the pile top is 1.55m, the relative elevation is -2.9m, the pile diameter is 400mm, the wall thickness is 80mm, the pile length is 26m, and the design value of the vertical bearing capacity of a single pile is Rd=650kN.
　　During foundation pit excavation, 23 pipe piles were found to be tilted and deviated. According to on-site measurement, the maximum tilt of the pile is 1570mm, most of which are 300-1000mm, exceeding the allowable range of the specification. According to the test results, the defect position of the pile body is between 3-5m below the pile top.
　　To ensure engineering safety, the project department entrusted relevant qualified units to design and consult on the correction and reinforcement of PHC pipe piles.
　　The stratum characteristics of this project are as follows: ① plain fill, loose, ②1 silty clay, plastic to soft plastic, ②3 clayey silt, slightly dense, ③1-1 gray muddy silty clay, plastic, ③1-2 gray muddy clay, soft plastic, ③1-3 gray clay, soft plastic to plastic, ④1 dark green grass-yellow silty clay, hard plastic to plastic, ④3 silty clay, medium dense, ⑤1 gray silty clay, plastic, ⑤2 gray silty clay, slightly dense, ⑤3 gray silty clay, plastic.
　　In view of the characteristics of this project and combined with previous treatment experience, for non-tilted Class III and IV piles, core filling reinforcement is carried out, and the core filling thickness should exceed a certain depth of the crack position; for tilted Class III and IV piles, first correct them, then carry out core filling reinforcement.
　　Core filling mainly involves setting a steel reinforcement cage inside the pipe pile, and densifying the reinforcement within 1.5m above and below the fracture position. The reinforcement cage extends 3m below the fracture position. There are two ways of filling materials: one is to fill C45 micro-expansive concrete in the pipe core; the other is compaction grouting, that is, adding a mixture of sand and gravel in the pipe core and injecting high-pressure cement slurry for reinforcement. To control the amount of filling material, the former requires welding a 5mm thick thin steel plate support plate at the bottom of the reinforcement cage; the latter requires placing a grout stop bag at the bottom of the reinforcement cage, sealing with C45 micro-expansive concrete, and high-pressure grouting can only be carried out after the sealing concrete reaches 70% of the design strength. After grouting is completed, the grouting pipe is pulled out. In comparison, the latter process is more cumbersome and the construction period is longer. After comprehensive comparison, we chose C45 micro-expansive concrete to fill the pipe core.
　　Verification of the compressive bearing capacity of the core filling: due to the existence of cracks, in the most unfavorable case, the core filling bears all the upper loads. According to reference <1>
　　The design value of the compressive bearing capacity of the core filling is calculated as follows:
　　Nμ＝0.7fcac
　　Calculated Nμ＝668kN＞650kN, the compressive bearing capacity meets the requirements. The meanings of symbols in the formula are detailed in reference <1>.
　　For safety, in addition to meeting the compressive requirements, the core filling at the crack should also be compared with the flexural and shear bearing capacity of the original PHC pipe pile.
　　The cracking moment test value of pipe pile PHCAB400 is Nμ=63kN•m. The core filling concrete uses C45, the steel bar is Grade II steel with a diameter of 22mm, and the protective layer thickness is 30mm. Through calculation, the steel bar area AB=3040mm2, n=10 bars. Configuring 12φ22 at the crack can meet the requirements.
　　The main reinforcement of the core filling reinforcement cage is 6φ22, and 6φ22 is densified within 1.5m above and below the fracture position <2>. The reinforcement cage extends 3m below the fracture position, and a 5mm thick thin steel plate support plate is welded at the bottom of the reinforcement cage. Finally, C40 micro-expansive concrete is poured into the pile pipe and vibrated densely.
　　The specific reinforcement construction method is as follows:
　　(1) Core filling in the pile. Clean the pile pipe before core filling, then apply cement slurry on the inner wall of the pipe pile to improve the integrity of the core filling concrete and the pipe pile body concrete. Then lower the reinforcement cage. The main reinforcement is 6φ22, and another 6φ22 is added at the fracture position. The reinforcement cage extends 3m below the fracture position, and a 5mm thick thin steel plate support plate is welded at the bottom of the reinforcement cage.
　　(2) Pour C45 micro-expansive concrete into the pile.
　　(3) After construction is completed, re-measure the deviation of the pile body.
　　(4) Select representative foundation piles for bearing capacity testing, with no less than 3 piles.
　　(5) Add settlement observation points and strengthen monitoring of settlement and settlement difference.
　　The core filling in the pile is shown in Figure 1 below.
　　At present, the building has been topped out and decorated. From the settlement monitoring data, it fully meets the specification requirements.
　　4 Conclusion
　　(1) From the perspective of engineering practice, this paper comprehensively summarizes the causes of prestressed pipe pile breakage in soft soil areas and proposes corresponding preventive measures, which has certain reference significance for guiding actual projects.
　　(2) Practice has proved that the two pile breakage reinforcement methods introduced through a typical engineering example in this paper have intuitive reinforcement mechanisms, are practical and reliable, simple and feasible, economical and reasonable, and have promotion and application value.