Detailed Construction Process of Secant Piles

Secant piles are a specialized construction technique widely used in deep foundation pit support and diaphragm wall construction. By interlocking individual piles, they form a continuous pile wall, achieving water stoppage, seepage prevention, and load-bearing purposes. The construction of secant piles requires precise positioning and strict adherence to the process. This article details the construction process of secant piles, focusing on preparation, execution, and quality control.

1. Construction Preparation

Before beginning secant pile construction, thorough preparation is necessary, including the following:

1. Site Preparation

• Clear the site to ensure it is level, facilitating equipment setup and operation.

• Build drainage systems to prevent water accumulation from affecting construction quality.

2. Equipment Preparation

• Choose suitable rotary drilling rigs or impact drilling equipment based on design requirements, along with slurry circulation systems to stabilize the borehole walls.

3. Technical Preparation

• Use accurate surveying instruments, such as total stations or theodolites, to mark the centerline and boundaries of the pile locations.

• Develop a detailed construction plan to ensure smooth coordination between all stages of the process.

4. Material Preparation

• Ensure that rebar cages, concrete, and slurry meet quality standards and are available in sufficient quantities.

2. Construction Process

The construction process of secant piles involves both primary and secondary piles. The detailed steps are as follows:

(1) Borehole Drilling

• Positioning:
  Determine the center points of the primary and secondary piles based on the layout, and accurately position the drilling rig.

• Borehole Formation:
  Depending on the geological conditions, use appropriate drilling techniques (e.g., slurry or casing support) to prevent borehole collapse. Maintain strict control over verticality and diameter to ensure precise interlocking of primary and secondary piles.

• Cleaning the Borehole:
  After drilling, use slurry circulation systems to clean the borehole, ensuring it is free of debris for proper concrete placement.

(2) Rebar Cage Fabrication and Installation

• Fabricate rebar cages in advance according to design specifications, ensuring their length matches the depth of the borehole.

• Slowly lower the rebar cage into the borehole using cranes or winches, ensuring vertical alignment and proper positioning.

• Secure the cage with ties and positioning brackets to maintain rigidity.

(3) Concrete Placement

• Use tremie pipes or pumping methods to place concrete, ensuring a continuous flow to fill the borehole completely.

• Control the workability and slump of the concrete to avoid defects such as segregation or discontinuity.

• Once the concrete sets, promptly remove the casing if used.

(4) Secondary Pile Construction

• Secondary pile construction follows a similar process to primary piles but requires precise adjustment of the secondary pile’s position relative to the primary pile to ensure interlocking.

• Special care must be taken to avoid damaging the previously completed primary piles.

(5) Interlocking Adjustment

• After both primary and secondary piles are formed, inspect the interlocking to ensure full connection.

• For areas with excessive gaps, apply grout or other reinforcement measures to maintain watertightness and load-bearing capacity.

3. Quality Control

Secant pile construction demands high accuracy and quality. The following measures should be implemented:

1. Verticality Control of Boreholes

• Monitor verticality using built-in sensors in rotary drilling rigs or external measuring devices.

2. Slurry Performance Monitoring

• Regularly check the density, viscosity, and sand content of the slurry to stabilize borehole walls and maintain clean drilling conditions.

3. Rebar Cage Installation

• Verify the diameter, length, and positioning of rebar cages against design specifications, ensuring proper alignment during installation.

4. Concrete Placement

• Monitor the slump and strength of the concrete to meet design requirements. Maintain detailed records of the placement process.

4. Key Points in Secant Pile Construction

• Accurate Positioning: Precise control of the center points for primary and secondary piles is crucial to prevent excessive gaps or insufficient overlap.

• Construction Sequence: Primary piles should be constructed first, followed by secondary piles, ensuring full interlocking.

• Borehole Stability: Use slurry or casing support techniques for collapsible or water-bearing strata.

• Concrete Purity: Thoroughly clean the borehole before concrete placement to avoid contamination that could compromise pile quality.

END