I. Preface With the rapid development of urban construction, many early multi-storey buildings and even high-rise buildings have to be demolished for various reasons, and the foundation construction of new buildings will encounter various pile types left over from the original buildings. Usually, for safety and convenience, designers often abandon the original pile foundation and redesign a new pile type. However, in order to avoid the old pile foundation, it will increase the complexity of the new pile foundation layout, increase the cost of foundation treatment and cause great waste. As for the original pile foundation, as long as it is carefully constructed and reasonably utilized, it can completely turn waste into treasure, greatly reducing the project cost and shortening the construction period. In the renovation of residential building 33-B of Zhebao Dormitory in Hangzhou, we used rammed expanded piles left over from the original building, partially supplemented with anchor static pressure piles, and used the dead load of the new building as the pile pressure reaction force, which saved the foundation cost and accelerated the construction progress. Practice has proved that the new building makes full use of the old pile foundation and adopts anchor static pressure pile to strengthen the foundation, which has high economic value and construction efficiency. Two. Project Introduction The project is located in Zhuganxiang Community, Hangzhou, with a six-story residential building and a semi-basement bicycle garage under it. After the original building foundation was demolished, it was found that the original foundation was designed as φ325 rammed expanded pile, with a pile length of 4 ~ 6 meters, a rammed expanded head of φ500 and a concrete grade of 200 #, which was relatively dense. According to the geological conditions at the site and the characteristics of the buildings nearby, the foundation of the new building is designed as an integral slab foundation, most of which are supported on the original rammed expanded piles, a few of which are static pressed piles with additional anchor rods, and the original rammed expanded piles are used as four test piles. The precast section of anchor static pressure pile adopts 200×200㎜ sections, with each section of pile length 1.5m, welded joints and concrete strength grade C30. Iii. Preparation for construction 1. First, comprehensively inspect the buildings around the site and arrange observation points to confirm that there are no obstacles underground, which will not affect the surrounding environment; Secondly, set two leveling points near the site to check the depth of the pile; Then mark the position of the pile axis. 2. Move the scaffolding that has an impact on the piling construction, dismantle and clean up the formwork and support in the semi-basement, and leave necessary construction operation space for piling. 3. Pump out the accumulated water in the basement and remove the silt on the concrete surface of the bottom plate. Keep the access channel unblocked, which will not affect the transportation of pile segments. 4. Install pile pressing equipment, set up reaction frame and other equipment, and set a scale on the pile frame for debugging first. 5. The piling is equipped with a power supply of 20 kW or more than 380V, and the distribution box is arranged within the range of the farthest pile location 100 m, about 30 meters away from the piling construction section on the north side of the site, and tn-s is used for separate wiring. 6. Check the quality of precast pile segments in the site, and transport the qualified piles to the vicinity of the construction passage, with no more than four stacked layers. Iv. Construction machinery and labor force allocation 1. Equipped with two sets of static pressure equipment and two sets of reaction frames. Hydrostatic equipment adopts high-pressure oil pump with flow rate not less than 4 liters/minute and 500KN hydraulic jack with stroke not less than 20 cm. 2. Equipped with theodolite, level 1, welding machine 1 and stacker 1. 3. Each pile pressing equipment is equipped with two operation technicians, 1 recorder and two general workers; There are also 1 welder, 1 site management builder, quality engineer and safety officer. V. Construction deployment and construction scheme 1. Before piling, four original rammed expanded piles are selected, and the actual bearing capacity of the original rammed expanded piles is tested by static pressure method. According to the respective design requirements of rammed expanded pile and anchor static pressure pile, the pile pressing force of anchor static pressure pile is determined to avoid uneven settlement caused by the great difference of bearing capacity between the two pile types. 2. The length of anchor static pressure pile is divided into two batches, one batch is controlled within 3m, and ②-3 # soil is the bearing stratum; The other group takes ④-3 # soil as the bearing stratum to reduce settlement. The specific data and practices are determined according to the test pile results. 3. First, the foundation and the upper main building are constructed. When the dead weight of the building exceeds the pile pressing force, it can be used as the pile pressing reaction. Then the pile segment is pressed into the soil from the pile pressing hole reserved in the foundation with a jack, and then the pile is connected with the foundation to achieve the effect of strengthening the foundation.
4. Process flow: positioning → digging and pressing pile hole → drilling anchor hole → planting anchor. 5. After pile pressing is completed, clear the reserved connecting reinforcement, weld the anchoring reinforcement, and carry out capping construction. Quality requirements and technical measures of intransitive verbs 1. Precast pile joints are manufactured in the factory, the cross-sectional dimensions and accessories meet the design requirements, and the thickness of longitudinal reinforced concrete protective layer is 30㎜. The end face is required to be flat and perpendicular to the axis of the pile. Pile concrete is dense, the deviation of bending height is less than11000l, and the depth of falling angle is less than10mm. 2. The bearing platform should be flat, the pile hole should be kept straight, and the deviation of the middle distance of the anchor rod (250×650) is not more than 20mm. 3. When piling, the pile body must be vertical, and the inclination shall not be greater than 65438 0%. The pile frame must be fixed and stable, and the nut should be tightened at any time when shaking occurs during pile pressing. 4. When connecting piles, the four corners shall be fixed by spot welding first, and then welded symmetrically. The joint between pile segments must ensure that there are two completely welded opposite sides. Low carbon steel is used for welding steel plates, and E43 is used in covered electrode. The surface of iron parts should be clean, and the gap between upper and lower piles should be filled with iron parts and welded firmly. The shutdown time after welding is more than 65438 0 minutes. The deviation of bending height of pile extension joint is less than11000l. 5. Record the pile pressing in time, and write down the reading of the oil pressure gauge at the end of each stroke of the jack. 6. The distance from the pile top to the cap shall not be less than 40㎝, generally not more than 50㎝. 7. During the construction of pile cap concrete, the water in the hole should be drained as far as possible, and the gap around the pile body should be filled with stones or yellow sand, so that the top of the pile is exposed by about 5㎝, and the strength of pile cap concrete should be greater than or equal to that of pile body concrete. 8. When piling, two machines should be careful not to get too close together, and the distance should not be less than 3m. 9. Pile foundation is a concealed work, and the static pressure process of each pile should be observed and recorded in detail as the basis for acceptance visa. 10. After the pile pressing construction is completed, check whether the pile position deviation meets the specification requirements, and test the pile bearing capacity, which should meet the design requirements. Seven. Safety measures 1. Strictly implement the operating rules for safe production, adhere to safe and civilized construction, and make safety technical disclosure before taking up the post. 2. Operators should strictly abide by the post responsibility system and be familiar with the safety technical operation procedures of this type of work. 3. Before using all kinds of mechanical equipment, check the safety performance first, and install leakage protection devices for electrical equipment. 4. Arrange the working procedure reasonably, control the construction progress, and observe the settlement and inclination during the construction. 5. The piling equipment shall be operated, maintained and maintained by designated personnel. 6. In order to avoid and reduce the harm of piling to the surrounding environment, take the following preventive measures: (1) Control the piling speed. That is, controlling the number of piles pressed every day can avoid serious soil squeezing and hydrostatic pressure. (2) Choose a reasonable pile pressing sequence. The method of pushing the pile forward on the back of the protected object is adopted to reduce the influence of squeezing the soil layer after the pile is buried on the foundation of the original nearby building. (3) Add vertical drainage channels. Bury bagged manholes between the newly pressed pile and the original rammed expanded cast-in-place pile, with a diameter of 80mm, a spacing of 1.5m and a depth of 10m, to avoid horizontal displacement of foundation soil during pile pressing, resulting in deviation or broken pile of the original pile. Eight. Experience summary Through practical application, new buildings make full use of the original pile foundation and adopt anchor static pressure pile to strengthen the bearing capacity of building foundation. As long as the design and construction are carefully carried out, the construction steps are well mastered, and the bearing capacity of the new and old foundations and the overall stiffness of the superstructure are well handled, the civil construction cost can be greatly reduced and the construction period can be shortened. Hangzhou Zhebao Dormitory 33-B residential building project, first, saves the cost and time limit for original pile foundation treatment and re-piling; Secondly, the foundation construction is carried out first, and then the anchor static pressure pile construction is carried out, which speeds up the construction progress of the superstructure and avoids the delay of pile foundation construction; Thirdly, anchor static pressure pile construction reduces the impact on surrounding buildings and residents' living environment. After the completion of the project, after fourteen times of settlement observation, the maximum settlement is 18㎜, the minimum settlement is 14㎜, and the settlement difference is only 4㎜, which meets the specification and design requirements and achieves the expected effect. Compared with abandoning the original pile foundation and piling again, the pile foundation treatment cost can be directly saved by more than 50% (about 500,000). The treatment scheme has achieved good economic and social benefits, and can be widely used in the process of old city reconstruction. Refer to [1] Building Construction Manual, China Building Industry Press, 2002. [2] Technical Specification for Building Pile Foundation, China Building Industry Press. [3] Code for Acceptance of Construction Quality of Building Foundation Engineering, GB50202-2002. [4] Technical Specification for Foundation Reinforcement of Existing Buildings. JGJ 123-2000。 [5] edited by Wu. Lecture on the application of construction quality acceptance specification of building engineering, China Building Industry Press. [6] Ye Editor. Handbook of Pile Foundation Engineering, China Building Industry Press.
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