Methods to estimate single pile capacity per IS 2911. Static formula uses soil parameters (cohesion c, friction angle φ, SPT N) to compute end-bearing and skin-friction. The result is checked by full-scale load test on test piles. Use the lower of formula and load test capacity, with appropriate factor of safety.
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Pile Capacity — Static Formula (IS 2911 Pt 1/2)
Ultimate capacity Qu = Qb + Qs (end bearing + skin friction)
Parameter Formula / Value Result Note Total ultimate capacity Qu Qu = Qb + Qs kN End bearing + skin friction End bearing Qb Ab × (cb × Nc + γ' × D × Nq) kN Cohesion + overburden contributions Skin friction Qs (cohesive) α × c̄ × As kN α = adhesion factor (0.3–1.0); c̄ = avg cohesion Skin friction Qs (granular) K × σv' × tan δ × As kN K = lateral earth pressure coeff; δ = pile-soil friction Allowable capacity Qa Qu / FoS kN FoS = 2.5 (formula) / 2.0 (load test) FoS — IS 2911 Cl 6.5 Static formula: 2.5 Initial load test: 2.0 Routine load test: 1.5
Bearing Capacity Factors (Terzaghi/Meyerhof)
Parameter Formula / Value Result Note Nc — cohesion factor 9 (deep piles, c-soil) — Skempton; valid for L/D ≥ 4 Nq — overburden factor Function of φ Tabulated See sub-table below Nq @ φ = 0° 1 — Pure cohesive Nq @ φ = 25° 13 — Loose granular Nq @ φ = 30° 20 — Medium dense sand Nq @ φ = 35° 31 — Dense sand Nq @ φ = 40° 50 — Very dense Nq @ φ = 45° 82 — Extremely dense / dense gravel
SPT N to Pile Capacity (Quick Estimation)
Meyerhof method for cohesionless soil
Parameter Formula / Value Result Note End bearing (driven, sand) qb = 40 × N × (Lb/D) ≤ 400 N kN/m² L_b = embedment in bearing layer End bearing (bored, sand) qb = 13 × N × (Lb/D) ≤ 130 N kN/m² Bored = lower than driven Skin friction (driven, sand) fs = 2 × N̄ kN/m² — N̄ = avg N along shaft Skin friction (bored, sand) fs = N̄ kN/m² — Lower than driven Skin friction (clay, undrained) fs = α × cu α = 1.0 (cu < 25); 0.5 (>50) Linear interp between Quick rule (medium dense sand) Qa ≈ 7–10 × N (kN per pile) — For 500 mm dia bored pile
Typical Pile Capacities (Quick Reference)
Parameter Formula / Value Result Note 300 mm dia, bored Length 8–12 m 200–350 kN Light-duty foundations 400 mm dia, bored Length 10–15 m 350–600 kN Residential, low-rise 450 mm dia, bored Length 12–18 m 500–800 kN Mid-rise residential 500 mm dia, bored Length 15–20 m 650–1000 kN G+10 residential 600 mm dia, bored Length 15–25 m 900–1500 kN Mid-rise commercial 750 mm dia, bored Length 20–30 m 1500–2500 kN High-rise, heavy industrial 1000 mm dia, bored Length 25–40 m 2500–4500 kN Bridges, major buildings 1200 mm dia, bored Length 30–50 m 3500–6500 kN Bridge piers, transfer columns
Pile Load Test (IS 2911 Pt 4)
Field verification of computed capacity
Parameter Formula / Value Result Note Initial load test (test pile) Apply 2.5 × design load Increments of 20% Determines ultimate; 2 piles min per project Routine load test Apply 1.5 × design load On working piles 0.5% of total piles, min 2 Maintained load test Each load held until settlement rate < 0.1 mm/30min — Slower, more reliable Cyclic load test Load-unload cycles — Separates skin friction & end bearing Acceptance — settlement at 1.5 × design ≤ 12 mm OR 2% of pile dia Whichever less — Acceptance — settlement at 2.5 × design ≤ 25 mm — Initial load test Acceptance — net residual settlement ≤ 6 mm OR 80% of total — Recovery test
Pile Group Effects
Parameter Formula / Value Result Note Group capacity (sand) ΣQ_individual × η η = 1.0–0.7 Efficiency depends on spacing Min spacing (driven) ≥ 3 × pile dia — Cl 8.4 Min spacing (bored) ≥ 2.5 × pile dia — — Max spacing ≤ 4 × pile dia — Beyond this, pile cap heavy Group settlement (clay) ≈ 5–10 × single pile settlement — Long-term consolidation
Notes
• ALWAYS conduct soil investigation before pile design — formula needs c, φ, N, γ for each layer
• Initial load test (2.5 × design load) is mandatory for projects with > 100 piles
• Routine test on 0.5% of working piles (min 2) verifies workmanship
• Bored cast-in-situ piles are most common in Indian urban work (low noise, limited space)
• Driven piles give higher capacity per cross-section but cause vibration — restricted in cities
• Pile cap depth = max(pile dia + 100 mm, 0.6 m typical)
• Negative skin friction can reduce capacity if pile passes through compressible / fill soil — investigate
• For end-bearing piles in rock, socket length ≥ 1 × pile dia; capacity often governed by rock strength
• Get IS 2911 Pt 1 (driven), Pt 2 (under-reamed), Pt 3 (timber/precast), Pt 4 (load test) — different rules for each pile type
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