CFG piles are high-bonding strength piles formed by mixing cement, fly ash, gravel, stone chips or sand with water, and form a composite foundation together with the soil and cushion between the piles.
1. Project Overview
The subgrade test section of a certain highway is designed to have a total length of 90.0m, a width of 30.0m, and a roadbed fill height of 4.5m. According to the purpose and content of the experimental research and the topographic and geological conditions of the test site, the cfg pile composite foundation pile network structure foundation treatment plan was selected for the work site K040~K090. Among them, K400~K060 are designed as suspended piles with a pile length of 17.5m; K060~K090 are designed as penetrating soft soil piles with a pile length of 27.0m. The pile diameter is 500mm, the distance between piles is 2.0m, and they are arranged in a square shape. A 0.3m thick gravel cushion is laid on top of the pile, with the maximum particle size not exceeding 3cm. A layer of two-way geogrid is laid in the cushion layer, with a designed tensile strength greater than 120kN/m. The construction uses a vibrating pipe sinking pile machine to form piles, and the 28d unconfined compressive strength is not less than 5MPa. The pile material is made of gravel, stone chips, fly ash, cement, and water. The basic ratio is shown in Table 1.
Table 1 CFG pile material mix ratio table
2. Construction technology
2.1 Construction process flow chart
CFG pile construction technology The flow chart is shown in Figure 1.
2.2 Construction technology
1) Construction preparation. a. Construction technical information. Design drawings, geological reports, designed pile location maps and construction organization design, etc. b. The site is level and obstacles are cleared. c. Construction setting out. Use a total station for precise positioning, insert a bamboo stick in the center of the pile position, and sprinkle white ash to mark it clearly. d.Materials. After the raw materials pass the inspection, a mix ratio test is conducted according to the design strength and specification requirements. The mix ratio is selected as cement:sand:gravel (5mm~20mm):gravel (16mm~31.5mm):fly ash:water=1:3.17 ∶2.85∶1.90∶0.7∶0.85. The mixture is mixed in a mixing station and transported to the site. 2) The drilling rig is in place. When constructing CFG piles, after the drilling rig is in place, use the front and rear and left and right vertical benchmarks of the drilling rig tower to check the tower guide rods and correct the position so that the drill rods are vertically aligned with the pile center to ensure that the allowable deviation of the verticality of the CFG piles is not greater than 1%. . 3) Drill into the hole. Close the drill bit valve, move the drill pipe downward until the drill bit touches the ground, and start drilling with the motor. Generally, it is slow at first and then fast. This can not only reduce the shaking of the drill pipe, but also allow the deviation of the drilling to be checked and corrected in time. During the hole making process, if the drill pipe is found to be shaking, the amplitude is large, or it is difficult to drill, the footage should be slowed down, otherwise it will easily lead to serious deflection and displacement of the pile hole, and even damage or twisting of the drill pipe and drilling tools. 4) Perfusion and extubation. After the CFG pile is drilled to the designed elevation, stop drilling and start pumping the mixture. When the core tube of the drill pipe is filled with the mixture, start pulling out the pipe. It is strictly prohibited to lift the pipe first and then pump the material. According to the results of the technical pile test, the lifting speed is controlled at about 2.5m/min, and the pile forming process should be carried out continuously to avoid slow downtime of backstage material supply. The theoretical perfusion volume is 0.196m3/m. 5) Move the machine. When the construction of the previous pile is completed, the drilling rig is shifted to proceed with the construction of the next pile. During construction, the pile positions to be constructed should be reviewed based on the position of the axis or surrounding piles to ensure that the pile positions are accurate.
Figure 1 Process Flow Chart
3. Common construction problems and solutions 1) Neck and broken pile control. For necking and broken piles, the construction plan of skipping piles should be adopted. When constructing in saturated loose silt soil, due to the good vibrating effect of loose silt soil, the density of the soil will increase significantly, and the more piles are driven, the greater the density of the soil. When placing piles all over the hall, it is advisable to proceed from the center outward, or from one side to the other. A long auger drill can prevent the vibration of newly driven piles from breaking the piles that have been driven. 2) Extubation rate control. During construction, the extubation rate should be strictly controlled. The normal extubation rate should be controlled between 1.2m/min and 1.5m/min.
3) Concrete slump control. When the extubation rate is 1.2m/min ~ 1.5m/min, generally the slump of the laitance concrete at the top of the pile can be controlled at about 10cm, and the quality of the pile is easy to control.
4. Soil Clearance and CFG Pile Head Treatment
4.1 Soil Clearance
Due to the large area of ??CFG pile treatment, manual cleaning is less efficient. Therefore, a combination of mechanical and manual cleaning is used.
Use an excavator to clean and transport the spoil. It is prohibited for the excavator to walk on the piling working surface when it enters the processing range. When walking, the piling spoil must be used to lay a walking cushion on the piling working surface, and the cushion layer to the top of the pile should not be less than 1m. When the excavator is working, the elevation must be strictly controlled to prevent cutting off completed piles and disturbing the protective soil layer below the piling working surface. Earth-moving vehicles are prohibited from entering the processing area. After the piling waste soil has been cleared and transported, the 50cm protective soil layer below it will be excavated manually. The base soil must not be disturbed when removing the protective soil layer. During construction, the elevation must be strictly controlled, over-excavation is not allowed, and automatic backfilling after over-excavation is not allowed.
4.2 Pile head treatment
After the protective soil layer is removed, the next process is to cut off the pile head above the designed elevation of the pile top. The specific method of cutting piles is as follows: 1) Mark the elevation position of the top of the pile through measurement, and use a pile cutting machine to cut the piles. Before cutting the pile, fix the pipe hoop at the lower part of the pile surface to be cut, and start the pile cutting machine to cut the pile. When cutting, cut in a circular motion, and the general cutting depth is 10cm ~ 15cm. 2) Insert 2 to 4 steel drill bits symmetrically on the cutting surface and hit them simultaneously with a sledgehammer to squeeze out the excess upper part to achieve the purpose of cutting the pile. 3) After the pile head is cut off, use a steel drill and hand hammer to level the top of the pile from all sides to the middle to the design elevation of the top of the pile.
5. Key points of construction quality control
1) Measurement and design of pile positions. According to the design documents, the coordinates are calculated pile by pile, and the coordinate method is used for measurement control. For the convenience of construction, according to the construction scope of the drilling rig (the drilling rig drills in the longitudinal direction, and drills 4 horizontally every time), guard piles are set up in the 5th longitudinal row and on the other side of the drilling range. Use the guard piles to control the direction and distance each time. Place the exact pile position. 2) Control the verticality of CFG piles. When constructing CFG piles, after the drilling rig is in place, use the vertical benchmarks at the front, rear and left of the drilling rig tower to check the tower guide rods and correct the position so that the drill rods are vertically aligned with the pile center to ensure that the allowable deviation of the verticality of the CFG piles is not greater than 1 %, the allowable deviation of the pile position shall not be greater than 10cm, and a horizontal ruler shall be used to detect the drilling, and the support rod shall be adjusted in time. During drilling, two heavy pole hanging balls shall be set up 10m to 15m away from the drilling rig in two different directions. Monitor the verticality of the drilling rig at any time to ensure the verticality of the CFG pile. 3) Control of concrete mix ratio. The mixing of the mixture requires ingredients according to the mix ratio and accurate measurement. The order of loading is: first load gravel, then add cement, fly ash and pumping agent, and finally add sand to make cement, fly ash and pumping agent Sandwiched between sand and stones. The mixing time of each plate of materials should be no less than 60 seconds, and the concrete slump should be controlled between 160mm and 200mm to ensure pumping quality.
6. Conclusion
A qualified unit was commissioned to conduct low strain and composite foundation bearing capacity testing on the CFG piles in the completed section. The results showed: 81% of Class I piles and 19% of Class II piles. %, under the action of loads at all levels, the pile top settlement is uniform and stable, the total settlement is between 21mm and 38mm, and the characteristic value meets the design requirements. In addition, the use of long auger drills can greatly speed up foundation reinforcement construction. CFG piles strengthen the foundation with high efficiency, short construction period, high bearing capacity of the composite foundation and low construction cost. Of course, at different subgrade construction sites, various technical parameters should be appropriately adjusted according to different soil conditions and mechanical equipment configurations.
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