Introduction
Pile foundations have evolved significantly in construction technology. Traditionally, precast piles dominated the market, including reinforced concrete square piles, prestressed concrete piles, and steel pipe piles, with some prestressed reinforced concrete piles exceeding 70 meters in length. In recent years, cast-in-place piles have gained prominence, with techniques such as impact drilling, rotary drilling, and manual excavation becoming more common. Large-diameter bored piles are increasingly favored, and new construction methods like the drilled grouting pile technique have emerged. Additionally, advancements have been made in mitigating the environmental impact of pile driving and improving the quality inspection of cast-in-place piles.
1. Pile Foundation Structure and Classification
A pile foundation is a common deep foundation type consisting of foundation piles and a cap connecting the pile heads. If the pile is entirely embedded in the soil with the cap in contact with the ground, it is called a low-cap pile foundation. If the upper part of the pile is exposed and the cap is above ground level, it is referred to as a high-cap pile foundation. Low-cap pile foundations are typical in building construction, while high-cap pile foundations are often used in bridges and wharf engineering.
A. Classification by Load-Bearing Mechanism
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End-Bearing Piles: These penetrate weak soil layers to reach firm strata or bedrock, transferring structural loads primarily through rock resistance. Construction control focuses on penetration depth, with pile tip elevation as a secondary reference.
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Friction Piles: These are entirely set in weak soil layers, compacting the soil to enhance density and load-bearing capacity. Structural loads are shared between tip resistance and side friction. Construction control prioritizes pile tip elevation, with penetration depth as a secondary factor.
B. Classification by Soil Displacement
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Displacement Piles: These include driven (or pressed) solid precast concrete piles, closed-end steel pipe piles, and concrete pipe piles constructed using methods like the casing method or explosive expansion.
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Partial Displacement Piles: Examples include impact-drilled cast-in-place piles, pre-drilled precast piles, prestressed concrete pipe piles, H-shaped steel piles, and open-ended steel pipe piles.
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Non-Displacement Piles: These include dry-process, slurry-supported, and casing-protected cast-in-place piles.
C. Classification by Construction Method
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Precast Piles: Manufactured in factories or on-site, these piles are driven, pressed, or vibrated into the ground. Methods include hammer driving, water jetting, vibration sinking, and static pressing.
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Cast-in-Place Piles: Formed by drilling or excavating holes on-site, placing reinforcement cages, and pouring concrete. Techniques include manual excavation, rotary drilling, impact drilling, cased pile driving, and explosive expansion.
2. Precast Pile Construction Methods
Precast piles include concrete and steel piles. Common concrete precast piles are reinforced solid square piles and prestressed hollow pipe piles. Steel piles include steel pipe piles, H-shaped steel piles, and other specially shaped steel piles.
Before construction, a detailed plan must be developed based on design requirements, pile type, soil conditions, geological surveys, and test pile data. Key considerations include construction methods, pile-driving equipment, sequencing, transportation, and safety measures.
A. Hammer-Driven Piling (Impact Method)
This method uses a pile hammer’s impact force to drive piles into the ground. It is efficient and widely applicable but generates noise and vibration, limiting its use in urban or nighttime operations.
Equipment Components:
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Pile Hammer: Delivers impact force (types include drop hammers, steam hammers, diesel hammers, and hydraulic hammers).
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Pile Frame: Supports and guides the pile during driving (types include tire-mounted, crawler-mounted, and rail-mounted).
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Power System: Provides energy for hammer operation (e.g., air compressors for air hammers, boilers for steam hammers).
Construction Process:
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Preparation: Clear obstacles, level the site, set benchmarks, and conduct trial piles.
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Pile-Driving Sequence: Determined by pile spacing—symmetrical driving for dense piles, unidirectional for widely spaced piles.
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Driving Technique: Use a heavy hammer with low impact, maintain continuous driving, and monitor penetration depth.
B. Static Pressure Piling
This method uses the weight of the pile frame (800–1500 kN) to press piles into soft soil without vibration or noise, making it suitable for urban areas.
Key Features:
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Lower concrete and reinforcement requirements compared to driven piles.
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Minimal soil displacement.
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Joint methods: welding, flange bolting, or sulfur mortar anchoring.
C. Vibration Sinking
A vibrator generates high-frequency vibrations to sink or extract piles with minimal additional force.
D. Water Jetting (Jet Grouting)
Used alongside hammering or vibration, this method involves high-pressure water jets to assist sinking. Final depth (1–1.5 m) is achieved without water jets to ensure load-bearing capacity.
3. Cast-in-Place Pile Construction
A. Dry Process Bored Piles
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Suitable for stable, water-free soils (clay, sand, or artificial fill).
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Uses spiral drills (diameters: 400–600 mm, depths: 8–12 m).
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Clean the hole before placing reinforcement and pouring concrete in layers (50–60 cm each).
B. Slurry-Supported Bored Piles
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Uses circulating slurry to stabilize the hole wall, applicable in water-bearing strata.
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Equipment, impact drills, or grab buckets.
Construction Steps:
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Install a casing (2 m high, steel plate, 4–8 mm thick) to guide drilling and prevent collapse.
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Drill with slurry (density: 1.1–1.5) to remove cuttings.
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Clean the hole using an airlift or slurry circulation.
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Place reinforcement and pour underwater concrete via a tremie pipe.
C. Cased Cast-in-Place Piles
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A steel pipe with a concrete or活瓣 tip is driven into the ground, filled with concrete, and extracted while vibrating to compact the mix.
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Construction Sequence:
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For dense piles, use (skip driving) to avoid soil disturbance.
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Methods: single-order extraction, re-driving, or plug extraction.
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D. Manual Excavation for Large-Diameter Piles
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Workers dig shafts (with支护 like concrete lining or沉井) to bedrock for high-capacity foundations.
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Advantages: Direct quality inspection, adaptable depth, and thorough base cleaning.
Safety Measures:
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Maintain vertical alignment (<0.5% deviation).
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Avoid overloading near shafts.
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Continuous construction with immediate concreting post-excavation.
Conclusion
Modern pile foundations offer diverse solutions for varying soil conditions and structural demands. From high-capacity end-bearing piles to eco-friendly static pressure methods, selecting the right technique ensures stability, efficiency, and minimal environmental impact. Proper execution, including precise drilling, reinforcement placement, and concrete pouring, guarantees long-lasting foundation performance.
