Prevention and treatment of quality defects of bor

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Prevention and treatment of quality defects of bridge bored pile

with the rapid development of transportation infrastructure construction in China, bored pile, as a foundation form, is widely used in highway bridges and other engineering fields because of its strong adaptability, moderate cost, simple construction and other characteristics. However, cast-in-place piles are concealed works, most of which are carried out underwater. There are many factors affecting the construction quality of cast-in-place piles, and the quality inspection is relatively difficult. Therefore, strict requirements must be made for every link of the construction process. A little carelessness or lax measures will produce quality defects in the grouting. Pile breaking is one of the diseases that often occur and are difficult to deal with. Combined with engineering examples, this paper introduces in detail the treatment methods of broken piles of bored cast-in-place piles, which will have a certain reference for similar projects

1. Project overview

the abutment foundation of a main highway bridge adopts the bored cast-in-place pile design. The design pile length is 21m, the diameter is 1.2m, the geological layer of the pile foundation is shown in Figure 1 (pile foundation profile), and the bottom 8m deep is the medium gravel layer. After the completion of the bored cast-in-place pile, it is found that the 4 pile of 0 platform is 17% long when carrying out the small strain test of the pile foundation There is no pile length response at 0m. According to the small strain waveform, it is determined that the pile is short or broken

after analyzing the length and depth of the drill pipe drilled in the pile and the original record of the length of the buried conduit during concrete pouring, it is found that the actual construction length of the pile has reached 21m, which cannot be a short pile, but a broken pile. As for the pile length below the fault, there is no reflection of the detection strain wave, but the fault is relatively serious. According to the characteristics that the broken position of the pile is close to the pile bottom and has exceeded the zero bending moment point, it is decided to use the grouting reinforcement method for treatment

2. Reinforcement treatment process

2.1 construction preparation

first, clarify the responsibility system of each process during on-site treatment, and appoint professional engineers to continuously control the whole process of the treatment process to ensure the treatment quality

secondly, prepare the required drilling, grouting and other machines and tools. Due to the extremely uneven construction site and the exposed reinforcement of the pile head of the cast-in-place pile, a suspended working platform must be set up on the upper part of the pile head to ensure the drilling and grouting operations

2.2 further inspection and verification of drilling and coring

due to the deep fault near the pile bottom, the drill must constantly correct the verticality of the main shaft of the drill during the drilling process, so as to prevent the drill from leaning out of the pile wall less than 17.5 ~ 18.0m, and the next process cannot be arranged. At the same time, during the drilling and coring process of the drilling rig (concrete coring machine g-210 can be selected), it is necessary to carefully analyze and summarize each section of concrete core sample, and pay close attention to the change of core sample

the field situation shows that the pile is intact when the core is taken to more than 18m, and there is no mud inclusion, and the strength of the concrete is also good. However, there is no complete concrete after 18m, and the core sample is almost medium coarse sand except for the honeycomb mortar with individual stones and no molding. This explains two points. One is that the small strain detection is consistent with the actual situation, which basically reflects the actual situation of the project; Second, it is confirmed that the drilling depth of the pile has reached the design depth of 21m, but the 235 curtain wall support device did not seal the conduit when pouring the first plate of concrete. In addition, the sedimentation was too fast, and the conduit had been silted up by medium and coarse gravel before the opening of the concrete. Later, it may have been mentioned during the operation of the conduit, and it was not found or valued in time, resulting in the bottom pile not forming within 3 meters of the concrete and pile breaking

the main machine of the universal experimental machine adopts imported servo electromechanical and servo speed regulation system, ball screw 2.3 hole bottom desilting

according to the above drilling and coring results, it shows that the pile bottom is silted with medium coarse sand, so it is decided to empty the pile bottom silted medium coarse sand and pour high-grade cement slurry for replacement treatment

however, it is very difficult to empty about 3.5m3 medium coarse sand at the pile bottom with 7cm drill pipe, especially in medium coarse sand geology, which is prone to buried drilling accidents. Therefore, it is necessary to increase the frequency of hole cleaning

in order to prevent the occurrence of buried drilling accidents, according to the hole cleaning experience, first lower the drill bit to 18.5M and keep cleaning the hole according to the prediction data of April group and our estimation. About half an hour later, a large amount of medium and coarse sand flooded along the drill pipe and the hole wall. At the same time, sufficient soda ash (Na2CO3) and polyethylene shall be prepared according to the hole cleaning workload in combination with the construction specifications. The effect of soda ash can increase the pH value, disperse soil particles, increase the negative charge on the surface of clay particles, provide conditions for clay to absorb external positive ion particles, increase the thickness of hydration film, improve the colloidal rate and stability of mud, and reduce water loss; The function of polyethylene is to improve the service performance of mud. The mud prepared with soda ash, polyethylene and a small amount of loess is sticky and dripping by hand, and then the hole can be cleaned

in the process of hole cleaning, special personnel should be arranged to eliminate and clean the sand flooding constantly. According to the statistics of the total amount of medium and coarse sand cleared, 3.5m3 of medium and coarse sand at the bottom of the hole is basically discharged after 6 hours of hole cleaning, and then there is little sand flooding on the surrounding wall of the hole opening, and a large amount of white mud almost the same as the mud prepared during hole cleaning gushes from the hole opening, which can be judged that the medium and coarse sand is basically discharged at the bottom of the hole. In order to prevent the influx of coarse sand in the wall around the hole bottom, the hole cleaning cannot be stopped before grouting

2.4 grouting treatment

in order to ensure the success of grouting at one time, the amount of 525 cement required for grouting should be calculated first. When preparing cement slurry, it should be prepared according to the high grade as much as possible, but the cement slurry is too thick, which will lead to injection difficulties. Through multiple preparation and injection experiments, it is finally determined that the preparation ratio of cement slurry is 1:1 At the beginning of grouting, the drill bit shall be buried at 20.5M from the pile bottom. During the grouting process, special attention shall be paid to the sealing of the orifice wall to prevent the cement slurry from overflowing, and only a little mud or even clean water can flow out until the grouting is impossible. Then open the closed external wall of the building and force to make the external wall insulation orifice wall. At this time, if a large amount of cement slurry suddenly emerges at the orifice wall, which is basically consistent with the injected prepared cement slurry, the grouting can be stopped. In order to prevent new faults or honeycombs at 17.5m, the drill bit should be positioned at 17.5 ~ 18m for secondary grouting until the grouting fails, and then slowly lift the drill and keep grouting to the pile top. After 2 hours of pressure injection, the pressure reaches 2MPa, and about 2.1m3 of cement slurry is shared

when preparing cement slurry, leave the specimen for sampling. The strength of the specimen reaches 32.5mpa in 28 days, which exceeds the original design strength of the pile

grouting is mainly to ensure the quality of the bottom of the pile through compaction, filling and reinforcement. In order to reduce the pores caused by the shrinkage of cement slurry, expansion agent (in the ratio of 1:10000) can be properly added when preparing cement slurry

2.5 secondary coring verification

grouting cement slurry is an underground concealed project. The specific effect can only be tested by coring again. In order to reduce the difficulty and cost of secondary coring, coring verification shall be carried out at the original hole 12 hours after the final setting of cement slurry. The hole location of the secondary coring verification should be determined according to the specific situation. The verticality of the drill pipe and the core sample quality at 17.5 ~ 18m should also be paid attention to in the secondary coring. After observation, the core samples from the pile top to the pile bottom 21m are intact and dense

after the grouting strength at the bottom of the pile is formed, the small strain test of the pile is arranged again. The test shows that the pile length is 21.0m, and it is judged as a class a pile with good quality

3. Summary

it can be seen from the above treatment process:

(1) during the construction of bored cast-in-place piles, it is normal to have broken piles, short piles or other quality defects, but the treatment must be serious and try to eliminate the accident in the bud. In case of such an accident, the fault location, geology, drilling and hole forming conditions at the pile bottom and the original records at that time must be carefully analyzed and verified after treatment. It is suggested to adopt the three-point grouting method (the three points form an equilateral triangle. If the pile diameter is 1.2m, the side length can be 60cm), so as to ensure that the fault can be fully compacted after coring and grouting to achieve the integrity of the pile. There are at least three advantages to deal with the broken pile according to this scheme: first, the quality is reliable; The second is to ensure the construction period, which can be completed in 10 ~ 15 days normally; Third, the cost is greatly reduced. If a pile is scrapped, the cost will be at least 150000 yuan, and only 150000 yuan will be required for this treatment. However, if the fault occurs in the middle of the pile, the scheme should be determined with caution

(2) in order to prevent pile breaking, mud inclusion, pipe blockage and other phenomena, during the construction process, it is necessary to control the pouring process and operation, pull the tremie to make the concrete surface rise moderately, ensure the programmed pipe pulling and continuous pouring, and the lifting range should not be too large. If the tremie is pulled sharply, it is easy to cause the concrete body to scour the hole wall, causing the hole wall to fall or collapse, and the pile body is mixed with mud, This phenomenon is particularly easy to occur in places with thick sand layers. During the pouring process, the rising height of the concrete surface must be measured every 2m3 or so, and the filling coefficient of each section of pile body must be determined to ensure that the filling coefficient of pile body concrete must be greater than 10

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