The Construction Process and Common Accidents of Bored Piles in Bridge Engineering

With the continuous development of science and technology, bored piles are widely used in high-rise buildings, super high-rise structures, and bridges. The quality of construction significantly affects the bearing capacity of the piles, making bored pile construction a critical step in bridge engineering. What are the construction techniques involved? And what common accidents might occur during the process?

Construction Flowchart

1. Site Preparation

• Dry Land: Level and compact the ground.
• Shallow Water: Use the island construction method (sand blowing for island formation).
• Deep Water: Utilize cofferdam methods (double-wall steel cofferdam, caisson island, or sheet pile cofferdam).

2. Pile Position Marking

Pile positions are marked using a total station coordinate method. After setting the casing, markings are made around it.

3. Casing Installation

Functions of Casing:

• Fixes the pile position.
• Protects the hole opening.
• Prevents surface water inflow.
• Increases internal water pressure to avoid collapse.
• Guides the drill bit direction.

Requirements:

• Made of reinforced concrete or steel (4–8 mm thick for steel, 8–10 cm for concrete).
• The inner diameter should be 20–40 cm larger than the pile design diameter.
• Installation depth: 2–4 m (minimum 1.0 m in clay, 1.5 m in sandy soil).

4. Mud Preparation

Functions of Mud:

• Stabilizes the whole wall.
• Suspends drill cuttings.
• Lubricates drilling tools.
• Facilitates circulation for debris removal.

Composition:

• Water (pH 7–8, impurity-free).
• Clay (plasticity index >25).
• Additives (e.g., soda ash for dispersion).

Key Mud Parameters:

• Density: 1.03–1.1
• Viscosity: 17–20 s
• Sand content: <2%

5. Drilling Rig Setup

Requirements:

• Must support the weight of drilling tools.
• Maintain stability to prevent displacement or tilting.
• Proper alignment with the pile center.

6. Drilling

Common Drilling Methods:

1. Impact Drilling:
   • Suitable for various soil types.
   • Uses a cross-shaped drill bit with varying stroke heights.
2. Full-Section Casing Grab Drilling:
   • No noise or vibration.
   • Ideal for visual soil inspection.
3. Direct Circulation Drilling:
   • Simple operation, but slower.
   • Best for clay, silt, and gravel layers.
4. Reverse Circulation Drilling:
   • Efficient debris removal.
   • Higher risk of hole collapse.
5. Rotary Drilling:
   • Suitable for sandy and cohesive soils.
   • Maximum hole diameter: 1.5–4 m.

7. Final Hole & First Cleaning

Purpose: Remove sediment to ensure proper bonding between concrete and bedrock.
Methods: Slurry pumping, mud replacement, or manual debris removal.

Inspection Criteria:

• Hole depth, diameter, and verticality.
• Mud properties (density, viscosity, sand content).

8. Reinforcement Cage Fabrication & Installation

Key Points:

• Use reinforcing hoops (14–18 mm diameter) every 2–2.5 m.
• Ensure proper cover thickness.
• Avoid deformation during transport.
• Weld seams must be full-length (10d for single-side welding, 5d for double-side).

9. Secondary Hole Cleaning

After installing the reinforcement cage, sediment may settle, requiring a second cleaning using a vacuum circulation system.

10. Underwater Concrete Pouring

Requirements:

• Concrete strength should be one grade higher than the design.
• Good workability and proper mix design.
• Use the tremie method for pouring.

Process:

• Fill the tremie pipe before pouring.
• The initial concrete volume must submerge the pipe by >0.8 m.
• Maintain a pipe embedment depth of 2–6 m during pouring.

11. Prevention & Handling of Drilling Accidents

1. Hole Collapse:
   • Causes: Weak mud, low water head, improper casing installation.
   • Solutions: Increase mud density, refill collapsed sections, stabilize water levels.
2. Drilling Deviation:
   • Causes: Uneven strata, misaligned drill rig.
   • Solutions: Adjust drilling speed, use guide frames, and correct bent drill pipes.
3. Drill Bit Drop or Tool Loss:
   • Causes: Broken wires, poor welding, operational errors.
   • Solutions: Use retrieval tools (hooks, loops), clear debris first.
4. Bit Clogging & Burying:
   • Occurs in: Soft clay layers.
   • Solutions: Reduce drilling speed, clean the bit, and adjust mud viscosity.
5. Hole Enlargement or Shrinkage:
   • Causes: Wall collapse or worn drill bits.
   • Solutions: Repair drill bits, use high-quality mud, and ream the hole.
6. Mud Leakage:
   • Causes: Permeable strata, poor casing sealing.
   • Solutions: Thicken mud, reinforce the hole wall, and seal gaps.

By following these procedures and precautions, the quality and safety of bored pile construction in bridge engineering can be significantly improved.