The bored pile foundation of mechanical bored pile construction technology is simple and easy to master, and the equipment investment is generally not very large. Therefore, it has been widely used in railway, highway, water conservancy, hydropower and other large-scale construction, as well as various residential and civil buildings. Bored cast-in-place pile is a construction method which uses mechanical drilling to form pile holes and uses conduit method to pour underwater concrete under the condition of mud wall protection. Drilling and pouring concrete are all carried out underwater, and the engineering quality can only be confirmed by scientific process control and instrument testing after completion. Therefore, there are higher requirements for the operating proficiency and technical level of operators. How to effectively avoid the phenomenon of drilling and pile breakage during underwater concrete pouring, prevent uneven concrete quality, and how to properly handle the construction quality problems to ensure the quality of the whole project has been studied by the engineering community, but it has not been completely solved. Therefore, it is necessary to master the construction technology of bored pile correctly.
1, construction preparation
1. 1, survey lofting
First, use total station to measure the axis of foundation pile and pile location mark and fill in the record. The anchor points of foundation pile axis and sample pile position should be set in places that are not affected by bored pile construction, and should be checked frequently during construction. When the anchor points need to be moved, their correctness should be checked and recorded, and all anchor points must be properly protected for quick and accurate measurement and inspection during the whole construction process. Using the original design network, control points are densely arranged to supplement the leveling points, bridge and culvert axes and pier control piles needed for construction. The error of the center line of the pier in the axial direction should not be greater than 65438 05 mm.
1.2, construction platform
In the construction, it is planned to use straw bag cofferdam, fill the earth to build the island, and use it as the construction platform after leveling and tamping. The height of cofferdam should be 0.5 m~0.7 m higher than the highest water level that may occur during construction, and the size can meet the requirements of its own strength and stability. The building materials of weir body should be cohesive soil or sandy clay, which should be compacted after filling to the water level, and the filling should be closed from upstream to downstream. The slope of the side facing water erosion is1∶1.2 ~1∶1
1.3, and the slope of backwater side is within 1: 2.
2. Making and burying the casing, fixing the pile position, protecting the orifice, isolating the groundwater and keeping the water level in the hole higher than the groundwater level or the construction water level to keep the hole wall from collapsing are indispensable equipment for the bored pile construction. 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 cm ~ 20 cm, and drilling with impact cone and grab should be increased by 20 cm~30 cm. The elevation of the top end of the pile casing should meet the requirements of setting the water level in the hole: when the pile casing is on land, it should be more than 0.3 m above the ground; In case of water contact, if the geology is good and the hole is not easy to collapse, it should be higher than the highest construction water level1.0m ~1.5m+0.5m; If the geology is not good and it is easy to collapse, it should be higher than the highest construction water level 1.5 m ~ 2.0 m, and the height of the slag hole should be increased when using the positive circulation drilling rig. The production of the liner should have certain strength and rigidity, and the joints and seams should be tight and watertight.
Generally, it can be made of reinforced concrete or steel plate, and the following requirements should be met when selecting:
1. Reinforced concrete liners are generally poured together with pile concrete and will not be pulled out, so they should be used in the parts where the liners are not ready to be pulled out.
2. The steel lining is easy to assemble and disassemble and can be reused. Generally, it is rolled with 5 mm~6 mm steel plate, each section is 1 m ~ 2 m long, which can be made into a whole circle or two semicircles, and the form adopted can be determined according to the use requirements. The two ends of the shell are connected by flanges. When the pile casing is buried, its center should be aligned with the center of the pile location, and the pile casing should be kept upright strictly, and the displacement of the pile casing should be paid attention to. The liner can be buried by pressing, hammering and vibrating to the necessary depth. The external lining must be higher than the water level 1.5 m, and the internal and external lining should be plugged with yellow mud. The strength and stiffness of the gasket take into account the requirements of hammering and vibration sinking.
3. The preparation of mud, the setting of mud pool and sedimentation tank, the main function of mud is to improve hydrostatic pressure, protect the hole wall, and discharge drilling slag out of the hole through mud circulation to reduce precipitation. Clay with large plasticity index (greater than 18) and low sand content should be selected for slurry making. When mixing slurry, soda ash and chemical slurry can be added appropriately to improve the performance of slurry, and the dosage should be determined through experiments. Use a mud mixer (made by ourselves) to stir the mud. Put the qualified clay soaked for more than 12 h into the mixer and add water to stir it evenly. Put the evenly stirred mud into a pre-dug mud pool, and connect the mud pool with the sedimentation tank with a circulating pool for reuse. Mud ponds and mud sedimentation tanks shall be set up near the piers that will not affect the construction, and their sizes shall be calculated and determined according to the diameter of bored piles.
4. Before drilling, start the mud pump to transport mud into the casing, and then slowly start drilling. The progress should be properly controlled. For the bottom of casing, drill slowly at a low speed to make the bottom corner have a solid mud wall. After drilling below the casing bottom 1 m, drilling can be carried out at normal speed according to the soil conditions. In the process of drilling, fill in the drilling records in time and pay attention to the changes of soil layer at any time. Every 2 m or soil layer changes, take slag samples, judge the soil layer, and check with the geological slope map. When it is seriously inconsistent with the geological slope map, it should be reported to the supervision engineer in time and handled according to the instructions. Drilling shall be carried out continuously without interruption. 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 1.0 m~ 1.5 m+0.5 m, and prevent metal tools such as wrenches and pipes or other foreign objects 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, check the hole diameter, hole depth and hole inclination. After meeting the design requirements, please ask the supervision engineer to recheck, and clean the hole after passing the test. Other matters needing attention in drilling should be handled in strict accordance with the requirements of the specification.
5. The purpose of hole cleaning and hole cleaning is mainly to remove a large amount of mud mixture remaining in the hole and minimize the thickness of sediment layer at the bottom of the hole. After the borehole reaches the design elevation, it shall be thoroughly inspected immediately, including the hole depth, aperture and inclination rate. After passing the inspection, it should be reported to the supervision engineer for re-inspection and then signed, and the hole cleaning work should be carried out immediately. Hole cleaning method: For the positive circulation drilling rig, the slurry changing method is generally adopted, that is, after passing the inspection, the footage is stopped, the drill bit is slightly lifted from the bottom of the hole 10 cm ~ 20 cm, and the drilling rig is emptied to ensure the normal circulation of mud. Press in fresh high-quality mud at medium speed to replace the mud suspended in the borehole. Note: During hole cleaning and slag discharge, the head height in the hole must always be maintained to prevent hole collapse, and the hole cleaning should not be replaced by deepening the depth of the hole bottom. After hole cleaning, the quality indexes such as the thickness of sediment layer at the bottom of the hole and the specific gravity of mud should be measured, which should meet the specifications and design requirements, otherwise the hole should be cleaned again.
6, steel skeleton production and placement
6. 1 Fabrication of steel skeleton
The fabrication of steel skeleton should strictly follow the relevant requirements of design drawings and specifications. The molding of steel skeleton ingredients is completed by the steel processing plant at the construction site, and the whole skeleton is bound and welded in sections at the construction site to avoid deformation of the whole skeleton due to transportation.
6.2, the placement of steel skeleton
The steel skeleton can be hoisted by drilling rig tower, derrick or crane. After the first section of steel skeleton is hoisted, it is put into the hole vertically and slowly aiming at the center of the casing, and then it is temporarily supported on the beam or casing of the drilling rig with steel pipes. Then lift and weld the other section according to the specification requirements, and then put it into the hole section by section to the design elevation. Finally, hang the lifting ring at the top on the casing, preferably on the beam of the rig chassis.
7, pouring underwater concrete
7. 1 catheter manufacturing:
The inner wall of high-quality catheter is smooth, straight and smooth, without local unevenness. After each catheter is assembled, there should be no water leakage. When necessary, the conduit shall be subjected to water pressure test and ball passing test. After passing the test, the outer wall of the conduit shall be numbered in sections and clearly marked, and the buried depth of the conduit in concrete shall be calculated in proportion.
7.2 When installing the conduit and waterproof bolt, the conduit can be assembled in sections by holes first, and then hoisted by crane section by section. It is suggested to tie two steel wires together. When the conduit is lowered, it should be located in the center of the hole, vertically lowered, and the axis should be straight to prevent the steel skeleton from being stuck and colliding with the hole wall. The fabrication and testing of the conduit shall be completed before the drilling pile starts. Waterproof bolts play a waterproof role in the early stage of concrete pouring, which proves the quality of concrete. The position of waterproof bolt should be set at 20 cm~30 cm above the water level in the hole. At the beginning of concrete pouring, the iron wire or nylon wire hanging the waterproof bolt should be cut off.
Summary:
The quality of bored pile directly affects the quality and safety of the whole subway structure, and its quality should be strictly controlled. This paper expounds the preparation before construction, hole formation, fabrication and hoisting of reinforcing cage in bored pile construction. In order to prevent problems in construction, it is particularly important to strengthen construction quality control. With the maturity and popularization of bored pile construction technology, various problems have appeared in the construction, which requires us to control in advance and strictly control in the construction. The problems in the construction should be deeply analyzed and the treatment measures should be appropriate and effective.
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