Pile Foundations & Pile Groups Notes

Last reviewed 16 Sept 2026 · 8 min read

When piles are used

A pile is a long, slender member that transfers load to deeper, stronger strata or distributes it through skin friction. Piles are used when:

the soil near the surface is too weak or compressible for shallow foundations;
loads are very heavy or concentrated (bridges, tall buildings, industrial plants);
there are uplift or lateral forces (towers, offshore structures, retaining structures);
scour may undermine shallow foundations (bridge piers);
expansive or collapsible soils must be bypassed;
future excavation next to the structure is expected.

Classification

Basis	Types
Function / load transfer	End-bearing (point-bearing) piles on hard strata; friction (floating) piles resisting by skin friction; compaction piles densifying loose sand; tension (uplift) piles; anchor piles; batter (raked) piles for inclined loads; fender piles; sheet piles for retaining
Material	Timber; concrete (precast, cast-in-situ, prestressed); steel (H-piles, pipe piles); composite
Installation	Driven (displacement) piles; bored (replacement/non-displacement) piles; driven cast-in-situ piles (casing driven then concreted, e.g. Franki, Simplex); screw piles

Driven piles densify loose sands (increasing capacity) but cause noise and vibration and may heave clays; bored piles are quiet and suitable near existing structures and in clays, but may loosen sands.

Static capacity

Ultimate capacity = base (point) resistance + shaft (skin) resistance:

Q_{u} = Q_{b} + Q_{s} = q_{b} A_{b} + \sum f_{s} A_{s}

Piles in clay (undrained, total stress)

Q_{b} = 9 c_{u} A_{b}, Q_{s} = α \overset{c}{ˉ}_{u} A_{s}

$α$ = adhesion factor (about 1.0 for soft clays, reducing to about 0.3–0.5 for stiff clays; bored piles lower than driven); $\overset{c}{ˉ}_{u}$ = average undrained strength along the shaft.

Piles in sand (effective stress)

Q_{b} = σ_{v}^{'} N_{q} A_{b}, Q_{s} = \sum K σ_{v}^{'} tan δ A_{s}

$K$ = earth pressure coefficient on the shaft (depends on installation: higher for driven displacement piles); $δ$ = pile–soil friction angle. Field results show that base and shaft resistances do not keep increasing linearly with depth — beyond a critical depth (roughly 15–20 pile diameters) $σ_{v}^{'}$ is usually limited in these calculations.

Safe load: $Q_{s a f e} = Q_{u} / F$ , with $F$ commonly 2.5 (IS 2911), or separate factors on base and shaft.

Dynamic formulas (driven piles)

Based on the energy of the hammer and the penetration (set) per blow at the end of driving:

FormulaEngineering News Record (ENR) formula

Q_{a} = \frac{W H}{6 ( S + C )}

$W$ = weight of hammer; $H$ = height of fall (cm); $S$ = set per blow (cm; average of the last few blows); $C$ = 2.5 cm for drop hammers, 0.25 cm for single- and double-acting steam hammers. The factor 6 is a built-in factor of safety.

FormulaHiley's formula

Q_{u} = \frac{η _{h} W H}{S + C /2} \cdot \frac{W + e ^{2} P}{W + P}

$η_{h}$ = hammer efficiency; $e$ = coefficient of restitution; $P$ = weight of pile (and cap); $C$ = total temporary elastic compression of pile, cap and soil. More rational than ENR.

Dynamic formulas are unreliable in clays (driving resistance differs greatly from static capacity because of pore pressures and remoulding) — use static analysis and load tests.

Pile load test (IS 2911 Part 4)

Initial test — on test piles before the job, to decide design capacity (loaded to about 2.5 times the design load or to failure).
Routine test — on working piles during construction (to about 1.5 times the design load).
Loads applied in increments by a jack against kentledge or reaction piles; settlements recorded.

Code ProvisionIS 2911 — safe load from vertical load test (single pile)

The safe load is the least of:

two-thirds of the load at which the total settlement is 12 mm (unless a different permissible settlement is specified), and
50% of the load at which the total settlement equals 10% of the pile diameter (7.5% for under-reamed piles).

Other tests: cyclic load test (separates base and shaft resistance), lateral load test, pull-out test, dynamic pile testing (PDA), low-strain integrity tests.

Pile groups

Piles are used in groups under a pile cap.

Minimum spacing (centre to centre): commonly about 2.5 to 3 times the pile diameter; wider for friction piles in clay to reduce overlap of stress zones; closer spacing permitted for end-bearing piles on rock.
Stress zones of friction piles overlap, so group capacity may be less than the sum of individual capacities.

Group efficiency

η_{g} = \frac{Q _{g r o u p}}{n Q _{s in g l e}}

FormulaConverse–Labarre formula

η_{g} = 1 - \frac{θ [( n - 1 ) m + ( m - 1 ) n ]}{90 m n}, θ = tan^{- 1} (\frac{d}{s}) in degrees

$m$ = rows, $n$ = piles per row, $d$ = diameter, $s$ = spacing.

Group capacity in clay

Check both:

Sum of individual capacities × efficiency (or simply $n Q_{u}$ ), and
Block failure of the group acting as a single large pier:

Q_{g} = c_{u} N_{c} B_{g} L_{g} + \overset{c}{ˉ}_{u} (2 B_{g} + 2 L_{g}) L

( $N_{c}$ ≈ 9 for deep blocks, $B_{g}$ , $L_{g}$ = plan dimensions of the group, $L$ = pile length).

The smaller value governs. In loose sand driven groups can have efficiency > 1 (densification).

Group settlement

Settlement of a group exceeds that of a single pile under the same load per pile (deeper stressed zone). For friction piles in clay, the group load is assumed to act on an equivalent raft at two-thirds of the pile length below the cap, spreading at 2 vertical : 1 horizontal; consolidation settlement is computed for layers below. For end-bearing groups, the equivalent raft is at the pile tips.

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Pile Foundations & Pile Groups