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.
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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