A pile foundation is a deep foundation system that transfers structural loads through piles—long, slender structural members—into deeper, load-bearing soil or rock strata. Piles are necessary when surface soils lack sufficient bearing capacity or when structural loads are too large for shallow foundations. The method is widely used in construction on soft soils, near water, or where excavation to competent strata would be economically unfeasible.
Load Transfer Mechanism
Piles transfer loads through two primary mechanisms: end bearing and shaft friction. End bearing occurs when the pile tip reaches a firm layer (bearing stratum) and transfers the load directly through compression. Shaft friction develops along the pile's perimeter where contact with surrounding soil creates resistance to downward movement. Most piles combine both mechanisms; the proportional contribution depends on soil profile, pile length, and diameter.
Driven Piles
Driven piles are preformed members (steel, concrete, or timber) driven into the ground using impact hammers, vibration, or hydraulic pressure. Typical driving equipment includes diesel hammers (energy output 40–300 kJ), hydraulic impact hammers (20–500 kJ), and vibratory drivers (operating frequency 10–40 Hz). Common pile diameters range from 250 mm to 1,220 mm; lengths reach 50 m in practice. Driven piles compact adjacent soil, increasing bearing capacity in granular materials. They generate significant noise and vibration, typically 90–105 dB at 1 m distance.
Bored Piles
Bored (drilled) piles are cast in place by excavating a hole and filling it with reinforced concrete. Auger rigs typically produce holes 300–2,000 mm diameter and depths to 100 m. Temporary casing or slurry (bentonite suspension, density 1,050–1,150 kg/m³) stabilizes the hole walls in weak soils. This method minimizes vibration and noise compared to driven piles, making it suitable for urban sites. Concrete placement requires tremie pipes in slurry-stabilized holes to prevent segregation. Cost per linear meter is generally higher than driven piles but avoids ground displacement effects.
Displacement Piles
Displacement piles (small displacement and large displacement types) move soil laterally during installation rather than removing it. Closed-ended steel tubes (displacement piles proper) displace soil; open-ended tubes remove soil and are technically semi-displacement. Screw piles operate by rotating a helical blade into the ground (torque 50–2,000 kN·m depending on diameter 250–900 mm) and are low-vibration alternatives suitable for sensitive structures. Micropiles (diameter 100–300 mm) use grouted or hollow bars anchored in rock and function under tension and compression; they require minimal headroom and are ideal for retrofitting.
Design and Specification
Pile capacity is determined by site investigation (SPT N-values, CPT cone resistance, or laboratory testing) and geotechnical analysis. Working load capacity typically ranges 500 kN to 10 MN for single piles in standard applications. Pile spacing must prevent overlap of stress bulbs; minimum center-to-center spacing is generally 2.5 to 3.0 times the pile diameter. Settlements under working load are typically 10–50 mm for well-designed systems. Pile group efficiency factors (0.7–1.0) account for load sharing and soil interaction when multiple piles work together.
Quality Assurance
Installation verification includes pile driving records (blow count, set per blow), integrity testing (sonic or ultrasonic methods), and load testing (static or dynamic). Static pile load tests apply 1.5× to 2.0× working load; acceptable settlement limits are typically 10 mm or 5% of pile diameter, whichever is smaller. Non-destructive testing (low-strain integrity testing, cross-hole sonic logging) detects defects in bored piles cast through poor soil conditions.