Pile Foundation

How a pile carries load

Picture a 25 m concrete column the diameter of a dinner plate, vertical and slender enough that it would buckle if you pulled it out of the ground. Now picture it embedded — surrounded on every side by clay, sand, and gravel that grip the shaft and resist the tip. That's a pile. The surrounding soil does the structural work; the pile is just the conduit.

Two mechanisms develop resistance:

• Skin friction (shaft resistance). Shear stress f_s along the soil-pile interface, integrated over the embedded surface area: Q_skin = Σ f_s · perimeter · ΔL across each soil layer. In clays, f_s is roughly 0.5·c_u (undrained shear strength). In sands, f_s ≈ K · σ'_v · tan(δ), where K is a lateral earth pressure coefficient and δ the wall friction angle.
• End bearing (toe resistance). A unit pressure q_p acting on the cross-sectional area at the tip: Q_tip = q_p · A_tip. In rock or dense gravel, q_p can exceed 10 MPa. In soft clay, it's often less than 0.5 MPa and contributes little.

Total ultimate capacity: Q_ult = Q_skin + Q_tip. Designers divide by a factor of safety — typically 2 to 3 — to get the allowable load. For a 0.6 m diameter, 25 m driven pile in stiff clay (c_u ≈ 120 kPa) overlying dense sand:

Q_skin = 0.5 · 120 · π · 0.6 · 25         ≈ 2,827 kN
Q_tip  = 4000 · π · (0.3)²                ≈ 1,131 kN
Q_ult  = 3,958 kN
Q_allow (FS = 2) ≈ 1,980 kN per pile

Multiply by the number of piles in the group and the foundation can carry tens of thousands of kilonewtons — enough for a tall building.

When piles beat shallow footings

• Surface soils are soft, organic, fill, or expansive.
• Loads are concentrated (column lines on a tower) and a spread footing would be impractically large.
• The water table is high and excavation is impractical.
• Differential settlement under a shallow footing would crack the structure.
• The structure must resist uplift (transmission towers, buoyant tanks) — piles in tension carry shaft friction in reverse.
• Lateral loads from wind, earthquake, or wave action need to anchor below grade.

Pile types compared

	Driven	Bored	CFA	Micro-pile	Sheet pile	Helical
Installation	Hammer/vibrator	Drill, cage, tremie	Continuous auger + grout	Drilled, grouted, reinforced	Driven interlocking sections	Screwed in with torque motor
Diameter	0.25–0.6 m	0.6–3.0 m	0.45–1.2 m	0.10–0.3 m	Sheet, not round	0.05–0.4 m shaft + helices
Length	15–50 m	20–80 m	15–35 m	10–60 m	5–30 m	3–20 m
Capacity per pile	500–3,000 kN	5,000–30,000 kN	1,000–6,000 kN	200–1,500 kN	Lateral retention	50–500 kN
Noise / vibration	High	Low	Low	Very low	High	Very low
Soil displacement	Yes (full)	No (replacement)	Partial	No	Yes	Minimal
Best for	Granular soils, no obstructions	Heavy loads, urban sites	Mixed soils, fast cycle	Restricted access, retrofit	Excavation walls, cofferdams	Light structures, telecoms

Installation methods in detail

• Driven piles — precast concrete, steel H-section, or steel pipe hammered in place. End-of-driving "set" (mm per blow) is recorded against a wave-equation prediction (GRLWEAP) to confirm capacity. Noisy, displaces soil laterally, can heave neighbouring piles. Common for marine works and clean granular sites.
• Bored piles — a rotary rig excavates a shaft, often under bentonite slurry. A reinforcement cage drops in, then concrete fills from the bottom via a tremie pipe. No vibration, no displacement; ideal for dense urban sites. The Burj Khalifa's 192 bored piles (1.5 m diameter, 50 m deep) were placed this way.
• Continuous flight auger (CFA) — a hollow-stem auger drills to depth in one motion. As it withdraws, grout pumps through the stem, filling the bore from the bottom. A cage is plunged in. Fast (one pile in 30–60 minutes), low-vibration, but cage depth is limited by grout set time.
• Micro-piles — slim drilled-and-grouted piles 100–300 mm diameter, often with a high-strength steel core. Used for underpinning, retrofit, and restricted-access sites. Low capacity per pile, installed with small rigs.
• Sheet piles — interlocking Z-, U-, or AZ-section steel members driven edge-to-edge to form a wall. Job is lateral retention — cofferdams, quay walls, basement excavations.
• Helical (screw) piles — a steel shaft with welded helical plates screwed in by a torque motor. Capacity correlates with installation torque (T·k = capacity, k ≈ 9 m⁻¹). Common for telecom towers, solar arrays, residential underpinning.

Variants by load mechanism

• End-bearing piles. Tip rests on bedrock, dense till, or cemented sand. Shaft is mostly a transmission column. Sensitive to tip damage from overdriving.
• Friction piles. No firm bearing layer in reach. Shaft develops capacity through soil-pile shear along its length. Used in deep alluvial valleys (Mexico City, Bangkok, the Mississippi delta).
• Battered piles. Driven at 1:6 to 1:3 from vertical to resist lateral loads — quay walls, transmission towers, offshore jackets.
• Tension piles. Resist uplift from buoyancy or overturning. Capacity is skin friction only (no end bearing in reverse). Used under buoyant tanks and transmission masts.
• Composite piles. Driven steel casing filled with concrete after installation. Drivability of steel plus durability of concrete.

Failure modes & defects

• Drift / out-of-plumb. Pile walks off design position during driving from soft pockets or obstructions. Tolerances ±75 mm horizontal, 1:75 verticality. Severe drift means eccentric loading and head bending.
• Necking. On bored piles, soft layers squeeze inward as concrete is placed. Detected by cross-hole sonic logging or low-strain integrity tests.
• Toe damage. Driven concrete piles split at the tip from over-driving on hard rock; steel pipes can crumple.
• Heave. Displacement piles in saturated clay lift previously installed piles by 10–50 mm. Re-drive or sequence driving so pore pressures dissipate.
• Refusal short of design depth. Boulders or hard strata stop the pile. Accept shorter pile (if load test confirms) or pre-bore through.
• Down-drag (negative skin friction). Settling soil drags the pile down — shaft friction adds load instead of resisting. Bituminous coatings on the upper shaft break the bond.
• Group efficiency loss. Piles closer than 3·D interact; group capacity is less than sum of individuals. Apply efficiency factors (η ≈ 0.7–0.9).

Real-world specs

• Burj Khalifa, Dubai. 192 bored piles, 1.5 m diameter, 50 m deep into weak rock; combined with a 3.7 m raft.
• Petronas Towers, Kuala Lumpur. 104 friction piles up to 120 m — the deepest foundations ever built when completed in 1996, through karst limestone with solution cavities.
• Offshore oil jackets, North Sea. Driven steel pipe piles 2 m diameter, 80–120 m long. Hammers weigh 200+ tonnes.
• Mexico City Cathedral. Hundreds of friction piles in compressing clay; continuous monitoring and shimming have stabilized differential settlement since 2000.