Overview of the Construction Process
Tri-axial mixing piles involve a series of key construction procedures, which are crucial for achieving the desired pile strength and stability. The main construction steps include equipment assembly, drilling machine positioning, grout mixing and pumping, drilling, and extraction.
1. Equipment Assembly
Tri-axial mixing piles can be constructed using either walking-type or crawler-type equipment. In both types, a cement silo (for storing cement) and a rear system (for mixing the grout) are essential components. The number and capacity of the cement silos should be configured based on the daily cement demand on-site, ensuring sufficient cement supply.
2. Positioning and Verticality Adjustment
The positioning line should be as long as possible to avoid frequent resetting of control lines along the same straight line. This ensures accurate pile placement. To verify verticality, a plumb bob is used to check the vertical alignment of the pile.
3. Grouting System and Pumping
The mixing system uses three pumps: two for grout (for the outer holes) and one for air (for the middle hole). This ensures an efficient mixing and pumping operation during pile construction.
4. Drilling and Extraction
Various parameters, such as drilling speed and verticality, should be closely monitored during the drilling and extraction phases to maintain pile quality and consistency.
Process Parameters and Quality Control
1. Cement Calculation
Cement quantity is calculated based on the following guidelines:
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According to the Technical Regulations for Steel-Cement Soil Mixing Walls (JGJ/T199-2010), the water-to-cement ratio should be between 1.5 and 2.0.
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The Construction Technical Regulations for Building Pit Support (JGJ120-2012) state that the water-to-cement ratio should range from 0.6 to 0.8.
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The Technical Regulations for Foundation Treatment of Buildings (JGJ79-2012) allow a water-to-cement ratio of 0.45 to 0.55.
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The Hubei Province Technical Regulations for Building Pit Projects (DB159-2012) suggests a ratio of 1.5 to 2.0 for cement-soil mixing piles.
Example Calculation:
For a pile with a diameter of 850mm and a length of 20 meters:
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The cement content is calculated as follows:
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The volume of the actual pile (20 meters long) is 11.34 m³.
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The volume of the empty pile (3.2 meters long) is 1.81 m³.
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The cement required for the actual pile is 8.6 tons (standard section).
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The cement required for the empty pile is 12.9 tons (first section).
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The total cement required for mixing is 600 kg per 900 kg of water, with a water-to-cement ratio of 1.5.
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2. Grout Volume, Flow, and Pressure Control
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Grout Pressure: The grout pressure should not exceed 0.8 MPa, according to the Hubei Province Technical Regulations (DB159-2012).
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Grout Volume and Flow Rate: The cement quantity calculation includes 8.6 tons of cement and a water-to-cement ratio of 1.5. The total grout volume will be 15.8 m³.
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The total time for drilling and extraction is calculated at 46.4 minutes.
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The grout flow rate is 340 L/min, with each of the two pumps required to provide 170 L/min.
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3. Drilling Speed Control
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According to the Technical Regulations for Steel-Cement Soil Mixing Walls (JGJ/T199-2010), the sinking speed should be between 0.5-1 meter per minute, and the extraction speed should be between 1-2 meters per minute.
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The Hubei Province Technical Regulations (DB159-2012) suggest controlling sinking speed at 0.5-1 meter per minute and extraction speed at 1-2 meters per minute. For high environmental protection areas, the sinking speed should range between 0.5-0.8 meters per minute, and extraction should be less than 1 meter per minute.
4. Site Control
The construction method for tri-axial mixing piles typically involves two spray and four mixing or two spray and two mixing operations. In normal geological conditions, controlling cement quantity is straightforward. However, when encountering harder soil layers, drilling becomes more challenging, and pile formation time may increase, leading to excessive cement loss.
In such cases, the flow rate of the pump must be dynamically adjusted. If the formation process is too slow, the pump flow should be reduced. Once the hard soil layer is penetrated, the pump flow can be resumed. In softer geological conditions, the sinking speed should remain within the prescribed limits to ensure optimal grout mixing and quality control.
Key Control Points in Construction
1. Pre-construction Equipment Inspection
Before starting construction, check that all equipment functions are operational. Tri-axial mixing pile machines should have indicators for current work, verticality adjustment, and winch speed regulation. Additionally, the mud pump should have a speed regulation function, along with a meter and pressure gauge, where the pressure gauge rating should not be less than 2.5 MPa.
2. Cement and Grout Quality Control
Ensure that all cement used on-site is tested and complies with quality standards. During the mixing process, random checks of the cement slurry’s specific gravity should be performed. Additionally, a specific gravity meter and a plumb bob should be used to verify the verticality and slurry parameters.
3. Excavation of Guide Ditches
Before excavation, the guide ditch should be carefully checked to meet the design requirements. The guide ditch is primarily used to prevent grout from spilling out during pile construction.
4. Drilling and Verticality Control
Before drilling, verify the center position of the drill bit and ensure that the deviation of the drill rod’s center is less than 20mm. The position deviation of each pile should be controlled within 50mm. Verticality deviation during drilling should be controlled to within 1/250, with regular checks and corrections to ensure pile accuracy.
5. Grout Mixing and Spray Control
During drilling, it is important to spray and mix the grout at regular intervals. In sandy soil layers, grout mixing should be repeated within a 2-3 meter range to ensure even cement distribution. Furthermore, ensure that the slurry is stirred according to the required design proportions and is screened before being pumped into the mixing tank to prevent segregation.
6. Handling Cold Joints
If there is a gap of more than 24 hours between consecutive pile sections, the mixing speed should be reduced. Any unqualified overlap should be recorded as a cold joint and communicated with the design team to take remedial action.
Conclusion
The tri-axial mixing pile construction method is an efficient and reliable technique for deep foundation work, particularly in challenging soil conditions. By following the correct procedures for cement quantity, grout flow, drilling speed, and verticality control, this method can ensure the successful and cost-effective completion of foundation piles. Continuous monitoring and adjustment during the construction process are key to achieving high-quality results that meet design specifications.
