CFG (Cement Fly ash Grave) pile, also known as cement fly ash gravel pile in Chinese, is a high bonding strength pile formed by uniformly mixing cement, fly ash, gravel, stone chips or sand and water in a certain mix proportion. It forms a composite foundation together with the soil between the piles and the cushion layer. It can fully utilize the potential of pile materials, fully utilize the bearing capacity of natural foundations, and adapt local materials according to local conditions. It has the advantages of high efficiency, low cost, small post construction deformation, and fast settlement stability. CFG pile foundation treatment consists of several parts: CFG pile body, pile cap (plate), and cushion layer. Structural type: pile+slab, pile+cap+cushion layer (this form is adopted in this section)
1、 CFG pile construction technology
1. The selection of equipment and the installation of CFG piles can be carried out using vibration immersed tube drilling machines or long spiral drilling machines. The specific type and model of pile forming machinery to be used depend on the specific situation of the project. For cohesive soil, silt soil, and silty soil, the vibration sinking tube pile forming process is adopted. For areas with geological conditions of hard soil layers, the use of vibration sinking machines for construction will cause significant vibration to the already formed piles, resulting in pile cracking or fracture. For soils with high sensitivity, vibration can cause structural strength damage and a decrease in bearing capacity. Spiral drills can be used to pre drill holes, and then the vibration sinking tube can be used to form piles. For areas where high quality drilling is required, the long spiral drilling pipe is used to pump and form piles. This section is designed to be constructed using a long spiral drilling rig. There are also two types of construction machinery for pumping concrete inside long spiral drill pipes: walking type and crawler type. Crawler type long spiral drilling machines are equipped with walking type long spiral drilling machines. According to the schedule and process tests, the equipment configuration is implemented and maintained in a timely manner to keep all machinery in normal condition, meet the needs of construction, and not affect the progress and quality of construction.
2. The selection of materials and mix proportions for raw materials such as cement, fly ash, crushed stone, and additives should meet the requirements and relevant standards for quality acceptance of raw materials, and be randomly inspected according to regulations. Conduct indoor mix proportion tests according to design requirements and select appropriate mix proportions.
2、 Quality control measures for CFG piles
1. Strictly follow the design mix ratio during construction, randomly select a group of concrete specimens from each drilling rig and shift, and use compressive strength as the standard for determining the strength of the mixture;
2. After the drilling rig enters the site, first use a steel ruler to check the diameter of the drilling rig’s drill rod. The diameter of the drill rod should not be less than the design pile diameter, and the height of the drilling rig’s main tower should be about 5 meters greater than the pile length;
3. Before drilling, release the control pile positions and provide technical briefing to the drilling personnel. The drilling personnel will use a steel ruler to release each pile position based on the control pile positions.
4. Before drilling, make clear markings at the main tower position of the drilling rig based on the designed pile length and the thickness of the pile head protective layer, as a basis for controlling the drilling depth of the drilling rig.
5. After the drilling rig is in place, the commander commands the drilling rig to adjust its position, and uses the two vertical markers hanging on the frame to determine whether the verticality of the drilling rig meets the requirements;
6. At the beginning of CFG pile construction, there is concern that pile by pile construction may cause cross hole drilling. Therefore, the construction method of interval pile jumping is used. However, when interval pile jumping is used, the second pass of pile driver in place may easily cause compression and damage to the already constructed piles. Therefore, jumping and pile by pile driving should be selected according to different geological conditions.
7. When pouring concrete into CFG piles, the pressure on the upper 1-3 meters of the concrete decreases, and fine bubbles in the concrete cannot be released. The main load-bearing part of CFG piles is in the upper part, so the lack of compactness of the upper pile body can easily cause damage to the pile during engineering use. The solution is to use a vibrating rod to compact the upper concrete after construction and before it solidifies, in order to strengthen the compactness of the concrete; The second is to strengthen the control of concrete slump, as a small slump can easily cause honeycomb phenomenon.
8. The control of pipe pulling rate: If the pipe pulling rate is too fast, it will cause the pile diameter to be too small or the pile to shrink and break, while if the pipe pulling rate is too slow, it will cause uneven distribution of cement slurry, excessive floating slurry on the pile top, insufficient strength of the pile body, and the formation of mixed material segregation, resulting in insufficient strength of the pile body. Therefore, during construction, the pulling speed should be strictly controlled. The pulling speed is generally controlled at 2-2.5m/min, which is more suitable. The pulling speed here is a linear speed, not an average speed. If encountering silt or silty soil, the pulling speed should be slowed down appropriately. Reverse insertion is not allowed during the process of unplugging.
9. The analysis and treatment of pile breakage refers to the discontinuity of the concrete surface of the CFG pile after it is formed, with cracks or gaps perpendicular to the central axis of the pile in the middle. Pile breakage is the biggest quality accident of CFG piles. There are many reasons for pile breakage, mainly including: 1) insufficient construction protection, large construction machinery operating in CFG pile areas with insufficient strength, causing the pile to be crushed or the pile head to be crushed; 2) The exhaust valve of the long spiral drilling rig is blocked; 3) When pouring concrete, the supply of concrete pouring is not timely; 4) Geological reasons, abundant groundwater, and easy occurrence of pile breakage; 5) Inconsistent coordination between pipe pulling and pumping concrete; 6) Improper operation during the removal of the pile head resulted in damage.
Post time: Oct-17-2024