Bored cast-in-place piles can pass through various soil layers with complex soil or great changes in soft and hard (such as various cohesive soil, sandy soil, gravel soil, weathered rock, multi-layer rock, etc.) to strengthen the foundation. Its adaptability to bearing capacity is wide (300 ~ 20,000 kN), its construction tools are simple, its construction process has many advantages, such as low noise, little influence on adjacent buildings, good construction safety and so on, and it is widely used in foundation reinforcement engineering. However, due to many construction links, high technical requirements and complex technology, it is necessary to quickly complete the pouring of underwater concrete concealed works in a short time, and it is impossible to directly control the quality, which is greatly influenced by human factors. A little negligence will easily lead to some quality diseases, and even cause major quality accidents such as sick piles and broken piles, which will endanger the safety of pile foundation engineering.
Construction process of bored pile:
The main steps of the construction technology of bored pile are divided into preparation before construction, manufacture and embedding of casing, excavation of mud pit, drilling, manufacture and installation of reinforcing cage, concrete pouring, quality inspection and so on.
1 construction preparation
1. 1 Technology prepares to master the engineering geological and hydrogeological data on site, carefully review the design drawings and technical requirements of bored pile foundation, and prepare detailed construction technology and safety disclosure, so that each operator can master the correct construction technology and construction method and ensure the smooth progress of bored pile. Measure the axis of foundation pile and the center point of pile position with measuring instrument. The control points of the axis of the foundation pile and the center point of the pile position should be located in the position that is not affected by the construction of the bored pile, and all the anchor points must be properly protected.
1.2 during the construction of the construction platform, it is planned to use straw bag cofferdam, fill the earth to build the island, level and tamp it as the construction platform. During the construction period, the highest water level may be as high as 0.5m~0.7m, the elevation of cofferdam shall not be lower than this standard, and the construction size shall meet the requirements of its own strength and stability. Sandy clay or cohesive soil is used as the building material of weir body, which is compacted after being filled to the water level, and the fill should be gradually closed from top to bottom. The slope of the water erosion side is1:1.2 ~1:1.3, and the slope of the rear side is not greater than 1: 2.
2. Manufacture and burial of liners
Casing is an important facility in bored pile construction. Its function is to fix the pile position, protect the orifice, isolate the groundwater, and ensure that the construction water level is always lower than the water level in the hole, thus preventing the hole wall from collapsing. The inner diameter of the casing is larger than the design diameter of the bored pile. Drilling with rotary drilling rig should be increased by 10 ~ 20~30cm, and drilling with impact cone and impact grab should be increased by 20~30cm. The elevation of the top of the pile casing must meet the design requirements of the water level in the hole: when the pile casing is on land, it should be at least 0.3m higher than the ground; When the pile casing is in the water, if the geological conditions allow the construction hole not to collapse easily, the elevation of the pile casing should be about 1.0 ~ 1.5m higher than the highest construction water level. If the geological conditions are not good and the holes are easy to collapse, the casing should be about 1.5 ~ 2.0m higher than the highest water level during construction. When using a positive circulation drilling rig, the height of the slag hole should be increased. The rigidity and strength of the liners must meet the design requirements to prevent water leakage at the joints.
3 preparation of mud and setting of mud pool and sedimentation tank
During the construction, the role of mud is to improve the hydrostatic pressure and protect the hole wall. Drilling residue can be discharged out of the hole through mud circulation, which effectively improves the precipitation phenomenon. Prepare enough bentonite or mud-making clay according to the design requirements, and the mud-making amount is twice the volume of concrete, and adjust the mud amount according to the formation characteristics. In order to make the mud quality meet the design requirements, a proper amount of chemical pulp and soda ash can be added when stirring, and the specific amount depends on the experimental results. Dig the mud pool in advance, pour the mixed mud into the pool, and connect the mud pool with the sedimentation tank through the circulating pool for reuse. Mud ponds and sedimentation tanks should be built near the bridge pier so as not to hinder the construction activities of other links. According to the diameter of bored pile, determine its size.
4 drilling
After the liner is buried and the mud is prepared, the drilling rig can be in place. Before drilling, start the mud pump to inject mud into the casing. After the casing is filled with mud, drill slowly and pay attention to the drilling progress. The bottom of casing must be drilled slowly at a constant speed, so that the bottom corner has a solid mud wall. After drilling below the casing bottom 1m, drill at normal speed according to soil characteristics. Fill in the drilling records completely according to the drilling progress, observe the changes of soil layer in real time, take a slag sample every drilling to the changed part of soil layer or every 2m, analyze the characteristics of soil layer, and compare the analysis results with the geological slope map. If there is a big gap between the analysis results and the geological slope map, it should be reported to the supervision engineer as soon as possible for follow-up treatment. Drilling construction must be carried out continuously without stopping halfway. During the construction period, shift duty should be adopted. During the shift change, the drilling situation of this class and the matters needing attention of the next class should be explained in detail. Drilling mud should be tested frequently, and those that do not meet the requirements should be corrected at any time. Drilling mud should always be higher than the water level outside the hole or the underground water level 1.0m ~ 1.5m, and metal tools such as wrenches and pipes or other foreign objects should be prevented from entering the hole, which will affect the normal drilling construction. The drilling operation must be continuous, and the lifting should be steady, and it is not allowed to collide with the casing or hole wall. When removing and connecting drill pipes, try to be as fast as possible. After the pile hole is drilled to the design elevation, the hole diameter, hole depth and hole inclination shall be checked and recorded. After meeting the design requirements, please ask the supervision engineer to review, and then clean the hole after it is qualified. Other matters needing attention in drilling should be handled in strict accordance with the requirements of the specification.
5 Borehole cleaning
After the completion of drilling construction, carefully check the aperture, depth and inclination according to the design requirements. When both the aperture and depth meet the construction standards, the inclination shall be controlled within 1%.
Clean the hole in strict accordance with the technical specifications, and ensure that the indicators such as sediment at the bottom of the hole, relative density of mud and drilling residue content of mud meet the construction standards. After the hole depth meets the design requirements, check the indexes such as hole diameter, hole depth, inclination and pile diameter deviation, and clean the hole immediately after all the indexes meet the requirements.
Use slag cleaning method to clean holes, and ensure that the thickness of sediment before underwater concrete pouring does not exceed 10cm. Dig slag with a flat-bottomed drill, and check whether it meets the requirements with a hammer (digging slag can't deepen the hole depth). When cleaning the hole, ensure the height of water head in the hole, and continuously inject clean water into the hole to avoid hole collapse.
6. Fabrication and installation of steel skeleton
6. 1 Fabrication of reinforcement skeleton The reinforcement materials used to fabricate reinforcement cages shall be purchased according to the specified diameter, model and specification. In the processing plant on the construction site, the ingredients of the steel skeleton are processed and formed on site, and the whole skeleton is bound and welded in sections on site, which improves the deformation of the whole skeleton during transportation.
6.2 Installation of reinforcement skeleton ① Four reinforcement "earrings" are installed every 2m on a section at the middle and upper and lower ends of the reinforcement cage, so that the thickness of the concrete protective layer outside the reinforcement cage can meet the construction standards. (2) In the process of lifting the reinforcing cage, the reinforcing cage must be lowered vertically slowly. If there is an obstacle in the middle, you can try to twist it slightly or slowly to let it pass. If this method doesn't work, stop lowering immediately and continue construction after finding out the reason. Avoid forced lifting and sudden drop, so as not to hit the hole wall and cause hole collapse. (3) After entering the hole, it can be positioned (the positioning error is less than 5cm), and the reinforcing cage is hung.
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