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Common Challenges and Quality Control in Underwater Bored Pile Construction

Introduction

Underwater bored piles are widely adopted in construction due to their rapid installation, stable quality, and minimal weather-related disruptions. The construction process involves several key steps: site layout, casing installation, drilling rig positioning, hole cleaning, reinforcement cage placement, secondary conduit installation, underwater concrete pouring, and pile formation.

However, underwater concrete pouring is highly complex, with multiple factors influencing quality. Common issues include:

  • Leakage in conduits, leading to pile fractures

  • Loose concrete layers or slurry inclusions causing weak structural integrity

  • Excessive conduit embedment depth, increasing friction, and hindering extraction

  • Poor concrete workability due to low sand content or slump, resulting in blockages

  • Extended concrete waiting time, reducing fluidity, and impairing proper pouring

  • Improper casing installation, causing water seepage and ground settlement

  • Collapsed boreholes due to geological instability or incorrect drilling

  • Insufficient pile length from measurement errors or excessive sedimentation

  • Faulty ultrasonic testing due to operational errors in inspection tubes

Concrete Mix Design for Optimal Performance

1. Cement Selection

  • Preferred types: Ordinary Portland cement or Portland cement with an initial setting time ≥ 2.5 hours and strength ≥ 42.5 MPa.

  • Testing: Cement must undergo laboratory physical performance tests before use.

  • Dosage: Should not exceed 500 kg/m³, adhering strictly to specifications.

2. Aggregate Selection

  • Coarse aggregate: Crushed stone or gravel (preferred), with particle size ≤ 40 mm (1/6 to 1/8 of conduit diameter).

  • Fine aggregate: Medium or coarse sand with a sand ratio of 45%–50%.

  • Water-cement ratio: Maintained between 0.5 and 0.6.

3. Enhancing Workability

  • Admixtures: Use water reducers, retarders, or hardening accelerators only after experimental validation.

  • Key requirements:

    • Adequate plasticity and cohesion for smooth flow through conduits

    • High durability and strength, exceeding design specifications

    • Uniform mixing to prevent segregation or bleeding

Critical Construction Controls

1. Initial Pour Volume Calculation

The first concrete pour must ensure a minimum conduit embedment depth of 1.0 m to balance external slurry pressure. The required volume is calculated as:

V=π4(d2h1+kD2h2)

Where:

  • V = Initial concrete volume (m³)

  • h1 = Height for pressure equilibrium: h1=(h−h2)γwγc

  • γw = Slurry density (11–12 kN/m³)

  • γc = Concrete density (23–24 kN/m³)

  • k = Concrete filling coefficient (1.1–1.3)

Importance: Proper initial volume ensures:

  • Effective sediment flushing

  • Adequate embedment depth

  • Enhanced pile load-bearing capacity

2. Pouring Speed Optimization

  • Fast pouring: Reduces friction loss, improves concrete uniformity, and minimizes segregation.

  • Slow pouring: Increases side friction, leading to uneven load distribution.

  • Best practice: Continuous, rapid pouring within the initial setting time (retarders may be used if necessary).

3. Conduit Embedment Depth

  • Ideal range: 2–6 m (3–8 m for large-diameter piles).

  • Risks of deviation:

    • Shallow embedment: Slurry intrusion, leading to weak layers.

    • Excessive embedment: Increased friction, conduit blockage, or pile fracture.

4. Hole Cleaning Time

  • Post-drilling delay increases mud cake thickness, reducing pile-soil friction.

  • Solution: Begin concrete pouring within 30 minutes of final hole cleaning.

5. Pile Top Quality Assurance

  • Overpour height: 1× pile diameter above design level to account for slurry removal.

  • Final extraction: Slow conduit withdrawal to prevent slurry entrapment.

Common Issues and Solutions

1. Conduit Slurry Intrusion

Causes:

  • Insufficient initial concrete volume

  • Inadequate conduit embedment (< 2.0 m)

  • Poor conduit sealing

Prevention:

  • Ensure the calculated initial volume

  • Maintain 300–500 mm conduit-to-bottom clearance

Remediation:

  • For minor intrusion: Re-pour with adjusted mix

  • For major intrusion: Redrill and restart

2. Conduit Blockage

Causes:

  • Low slump concrete

  • Excessive sediment at the conduit base

Solutions:

  • Use vibrators to dislodge blockages

  • Extract and clean the conduit if severe

3. Conduit Entrapment

Causes:

  • Prolonged embedment

  • Reinforcement cage interference

Resolution:

  • Apply high-capacity lifting

  • Cut the conduit if extraction fails

4. Under-Pouring (Short Pile)

Causes:

  • Incorrect slurry surface measurement

Fix:

  • For dry conditions, Excavate and extend the pile

  • For wet conditions: Use extended casing to pour

5. Pile Fracture

Primary Causes:

  • Delayed pouring leading to cold joints

  • Slurry contamination

Remedies:

  • Re-drill and replace the reinforcement cage

  • Pressure grouting for minor defects

Conclusion

Successful underwater bored pile construction demands strict adherence to mix design, pouring protocols, and real-time monitoring. By addressing common challenges proactively, engineers can ensure structural integrity, load-bearing capacity, and long-term durability. Continuous innovation in admixtures and quality control techniques further enhances reliability in modern foundation engineering.

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