Application of post grouting method for pile end f

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Application of post grouting method for pile end foundation reinforcement of Bored Piles in recent years, China's high-rise buildings have developed rapidly, and the requirements for foundation bearing capacity are higher and higher. The foundation form is generally cast-in-place pile foundation. In order to meet the design requirements, the bearing layer of cast-in-place pile is required to be a relatively complete rock layer, and the pile length and pile diameter are often large, making the cost of underground part occupy a large share in the total project cost. At the same time, Because the pile length is very long, it causes great difficulties in construction. In some areas of China, the landform belongs to the piedmont alluvial plain unit, and the geology is a thick gravel layer after passing through the clay layer. Can this layer be reinforced to meet the design requirements and serve as the bearing stratum of the foundation? Based on this consideration, after years of exploration and practice, the design and construction workers in China have summarized a set of methods of strengthening the foundation at the pile end by post grouting of bored cast-in-place piles, which greatly shortened the pile length and achieved good economic benefits

1. Applicable geological conditions

the construction method introduced in this paper is applicable to the bearing layer of cast-in-place piles. The gravel content should be more than 50%. The filling and gravel are not cemented or slightly cemented. The gravel should be hard, the gravel is evenly distributed, and the thickness of the gravel layer should meet the design requirements

2. Reinforcement mechanism

in the construction of cast-in-place piles, steel pipes will be buried along the outer wall of the pile reinforcement cage. After the strength of the pile concrete meets the requirements, the cement slurry will be pressed into the pores of the gravel layer at the pile end by pressure through the steel pipes, so that the originally loose sediment, gravel, soil particles and cracks are cemented into a high-strength combination. Under the pressure, the cement slurry diffuses from the pile end in the pores of the gravel layer to the surrounding. For the single pile area, the diffusion around is equivalent to increasing the diameter of the end, and the downward diffusion is equivalent to increasing the pile length; All the grout in the pile group area is connected together, making the gravel layer become a whole, so that the gravel layer that did not meet the original requirements can meet the bearing capacity requirements of the structure. During the construction of bored cast-in-place piles, no matter how the hole is cleaned, more or less sediment will be left at the bottom of the hole; During the initial pouring, the concrete falls from the slender conduit, and the concrete at the pile bottom is separated to form "virtual tip" and "dry ballast stone" due to the large drop; The mud skin on the hole wall hinders the combination of the pile body and the soil around the pile, and reduces the friction coefficient. The above points affect the pile end bearing capacity and side wall friction of the cast-in-place pile. After being pressed into the pile end, the slurry is first combined with the sediment at the pile end, the segregated "virtual tip" and "dry ballast stone" to enhance the compactness of this part and improve the bearing capacity; The slurry returns up along the joint layer between the pile body and the soil layer, eliminating the mud skin and improving the pile side friction. At the same time, the horizontal penetration of the slurry into the pile side soil layer also plays a role in increasing the pile diameter. The above points play an important role in improving the bearing capacity of cast-in-place piles

3. Setting of grouting parameters

grouting parameters mainly include grouting water cement ratio, grouting volume and closing pressure. Due to different geological conditions, different parameters should be adopted for different projects. Before the construction of engineering piles, the parameters should be set first according to the practice of previous projects, and then the construction of test piles should be carried out according to the set parameters. After the completion of the test piles, the design strength should be reached, and the static load test of piles should be carried out to finally determine the test parameters

(1) the water cement ratio is generally not too large or too small, which will cause difficulties in grouting. Too small will cause segregation of cement slurry under pressure, generally 015 ~ 017

(2) grouting volume grouting volume refers to the cement consumption of single pile grouting, which is related to the gravel content of the gravel layer and the pile spacing, and depends on the porosity of the gravel layer. When the gravel content of the gravel layer is 50% - 70% and the pile spacing is 4-5m, the grouting volume is generally 115-210t. It is the main parameter to control whether the post grouting construction is completed

(3) closed disc pressure closed disc pressure refers to the control pressure to end grouting. Generally speaking, when to end grouting of a cast-in-place pile should be controlled according to the preset grouting volume, but at the same time, the pressure value of grouting should also be controlled. When the pre-set grouting amount is not reached, but a certain pressure is reached, the grouting shall be stopped. Excessive pressure of grouting will cause segregation of cement slurry and blockage of pipelines on the one hand, and excessive pressure may disturb the gravel layer and may also make the pile float up. Generally, the maximum pressure of closed disc should be controlled at 018mpa

according to the preset parameters, carry out the construction of the test pile, and then according to the static load test results of the test pile, finally determine the grouting parameters of the engineering pile, and then carry out the construction of the engineering pile

4. Post grouting construction process

4.1 construction process

fabrication of grouting pipe for bored pile hole forming reinforcement cage fabrication of grouting pipe for bored pile hole cleaning grouting pipe binding of lower reinforcement cage grouting pile concrete post grouting construction

4.2 construction points

(1) fabrication of grouting pipe fabrication of grouting pipe while fabrication of reinforcement cage. The grouting pipe is made of black iron pipe with a diameter of 25mm, the joint is connected with screw thread, and both ends are sealed with screw plugs. The length of the grouting pipe is 55cm longer than the length of the reinforcement cage, 5cm longer than the reinforcement cage at the bottom of the pile, and 50cm higher than the concrete surface at the top of the pile, but it shall not be exposed to the ground for protection. The grouting pipe is made into a grouting nozzle (commonly known as flower pipe) at the bottom 20cm. In this part, the drill bit is used to drill 4 rows (4 in each row), with a spacing of 3cm and a diameter of 3mm as the grouting nozzle; Plug the grouting hole with thumbtacks, put the bicycle inner tube with the same diameter on the outside and seal it with adhesive tape at both ends, so that the grouting nozzle forms a simple one-way device: when grouting, the pressure in the grouting pipe bursts the tire, the thumbtacks pop out, and the cement slurry is pressed into the gravel layer through the grouting hole and the hole of the thumbtacks, During concrete pouring, the device ensures that the concrete slurry will not block the grouting pipe, and focuses on the development of special fibers, filter materials, high-temperature functional films, high-performance resin based on thermoplastic polyimide (PI) engineering plastic resin, heteronaphthalene biphenyl polyethersulfone ketone copolymer resin (PPESK), high-end fluoroplastics, high-temperature insulating materials, high-temperature functional coatings, and high-temperature special adhesives

(2) layout of grouting pipe: 2 "polypropylene" products are a uniform mixture of recycled carbon fiber and thermoplastic polypropylene fiber, and the root grouting pipe is symmetrically bound to the outside of the reinforcement cage. After the hole is formed, clean the hole, lift the drill and lower the reinforcement cage. During the hoisting and placement of the reinforcement cage, pay attention to the protection of the grouting pipe. The reinforcement cage shall not be twisted to prevent the grouting pipe from loosening at the screw connection. The nozzle part shall be protected by concrete cushion blocks, and shall not rub the hole wall to prevent the tire from cracking and blocking the grouting hole. Pour concrete according to the specification

(3) selection of grouting pile position according to previous engineering practice, in the gravel layer, the cement slurry has a large influence area under the action of working pressure. In order to prevent the cement slurry, such as liquid, from emerging from the nearby weak place during grouting, the grouting pile should be 3 ~ 7 days after the completion of concrete pouring, and there should be no drilling operation within at least 8m around the pile, and the completion of pile concrete pouring within this range should also be more than 3D

(4) grouting construction sequence it is best to use the whole pile cap group pile for one-time grouting, and the surrounding pile position shall be constructed before the intermediate pile; During grouting, two piles are used for circulating grouting, that is, the a pipe of the first pile is pressed first, and the grouting amount accounts for about 70% of the total amount (111 ~ 114t cement). After that, the a pipe of the other pile is pressed, followed by the B pipe of the first pile and the B pipe of the second pile. In this way, the grouting time interval between the two pipes of the same pile can be guaranteed to be more than 30 ~ 60min, giving the cement slurry a time to diffuse in the gravel layer. During grouting, construction records shall be made, including construction time, grouting start and end time, grouting quantity, abnormal conditions and handling measures

5. Problems and corresponding measures in grouting construction

(1) if the nozzle cannot be opened and the pressure reaches more than 10MPa, the grouting nozzle still cannot be opened, indicating that the nozzle part has been damaged. Do not forcibly increase the pressure, but supplement the grouting quantity in another pipe

(2) in case of grouting, the cement slurry often emerges along the pile side or at other parts. If the cement slurry emerges on other piles or the ground, it indicates that the pile bottom has been saturated, and the grouting can be stopped; If grout emerges from the side wall of the pile, and the grouting volume also meets or approaches the design requirements, grouting can be stopped; If grout emerges from the side wall of the pile and the amount of grouting is small, the grouting pipe can be washed with clean water or pressurized water, and the grouting can be carried out again when the cement slurry originally pressed solidifies and blocks the capillary channels of the grout

(3) when the grouting amount of a single pile is insufficient, it is best to use the whole pile cap group pile for one-time grouting. The surrounding pile is constructed first to form a closed circle, and then the middle is constructed, which can ensure the grouting quality of the middle pile position. If the grouting amount of individual piles fails to meet the design requirements, the grouting amount of adjacent piles can be increased as a supplement according to the situation

6. Conclusion

different engineering geological conditions have great differences, and it is impossible to have the same grouting parameters. The preset grouting parameters often refer to the experience of similar projects. The final determination of grouting parameters depends on the results of test piles, and there is not much experience that can be used for reference in the country, which needs further accumulation, coupled with theoretical discussion, and finally forms a mature technology. Post grouting of cast-in-place piles has the advantages of improving the bearing capacity of single piles, improving productivity and saving construction funds. Therefore, it is of great significance and broad prospects to promote post grouting construction technology in qualified projects

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