IS 6403:1981 net bearing capacity — Nc, Nq, Nγ, shape, depth and water-table factors, general / local shear.
Reference only. IS 6403:1981 Cl 5.1.2 net bearing capacity. From an SPT N: N60 (energy and borehole), Cn = √(100/σ'v) at Df + B/2, IS 2131 dilatancy for fine sand or silt below the water table, φ by Peck, Hanson & Thornburn for sand and gravel, cu = 4.5·N60 (Stroud) for clay — the same correlations as the Geotechnical module. An inclined load uses ic = iq = (1 − α/90)², iγ = (1 − α/φ)²; an eccentric load the effective width B′ = B − 2e (qnu acts on B′·L; shape factors from the B′ × L rectangle; sliding is not checked). Local shear uses c' = ⅔c, φ' = tan⁻¹(0.67 tanφ); Auto takes general shear for φ ≥ 36°, local for φ ≤ 29° and interpolates between; for φ = 0 (undrained clay) it takes c unreduced — pick Local for a soft clay. Settlement is not checked. Always use a site-specific geotechnical investigation. Size the footing and run settlement checks in Geotechnical.
How the Bearing Capacity Calculator (IS 6403) works
This calculator gives the net ultimate and net safe bearing capacity of a shallow footing by the IS 6403:1981 method. You enter the soil strength (cohesion c and friction angle φ), the unit weight γ, the footing width B, its depth Df, its shape and, if known, the depth of the water table below ground level.
The bearing capacity factors Nc, Nq and Nγ come from the Table 1 expressions for the friction angle. They are then multiplied by the Table 2 shape factors (strip, square, circular or rectangular), the Cl 5.1.2.2 depth factors and the water-table factor W'. W' is 1 when the water table is at least B below the base, 0.5 when it is at or above the base, and varies linearly in between. If the water table is above the base, the overburden pressure q at founding level is reduced by the water pressure.
For loose or soft soils the tool works out a local shear value with c' = ⅔c and φ' = tan⁻¹(0.67 tanφ). You can pick general shear, local shear, or Auto. Auto uses general shear for φ ≥ 36°, local shear for φ ≤ 29°, and interpolates linearly between the two.
The net safe bearing capacity is the net ultimate value divided by the factor of safety, 2.5 by default as in IS 6403. The tool also shows the gross safe value, which is the net safe value plus the overburden q. An inclined load (α to the vertical) applies ic = iq = (1 − α/90)² and iγ = (1 − α/φ)²; an eccentric load uses the effective width B′ = B − 2e, with qnu acting on B′·L. Sliding and settlement are not checked.
- Bearing capacity factors (Table 1)
- Nq = e^(π·tanφ)·tan²(45° + φ/2); Nc = (Nq − 1)·cotφ; Nγ = 2(Nq + 1)·tanφ
- Net ultimate bearing capacity
- qnu = c·Nc·sc·dc + q(Nq − 1)·sq·dq + ½·B·γ·Nγ·sγ·dγ·W'
- Depth factors
- dc = 1 + 0.2(Df/B)·√Nφ; dq = dγ = 1 + 0.1(Df/B)·√Nφ (1 if φ < 10°), where Nφ = tan²(45° + φ/2)
- Shape factors, rectangle
- sc = sq = 1 + 0.2·B/L; sγ = 1 − 0.4·B/L
- Local shear parameters
- c' = ⅔·c; φ' = tan⁻¹(0.67·tanφ)
- Net safe bearing capacity
- qns = qnu / FoS
Follows: IS 6403:1981 Table 1 (bearing capacity factors) · IS 6403:1981 Table 2 (shape factors) · IS 6403:1981 Cl 5.1.2 (net ultimate bearing capacity) · IS 6403:1981 Cl 5.1.2.2 (depth factors)
Worked example: net safe bearing capacity of a 2 m square footing
Given
- Square footing, B = 2.0 m, founded at Df = 1.5 m
- Cohesion c = 10 kPa, friction angle φ = 30°
- Unit weight γ = 18 kN/m³, water table deep (left blank)
- Shear mode: General; factor of safety 2.5
Steps
- Factors for φ = 30° (Table 1): Nc = 30.14, Nq = 18.40, Nγ = 22.40.
- Square footing (Table 2): sc = 1.3, sq = 1.2, sγ = 0.8.
- Depth factors with √Nφ = tan 60° = 1.732: dc = 1 + 0.2 × (1.5/2) × 1.732 = 1.260; dq = dγ = 1 + 0.1 × 0.75 × 1.732 = 1.130.
- Overburden at founding level: q = 18 × 1.5 = 27.0 kPa. The water table is deep, so W' = 1.
- Cohesion term: 10 × 30.14 × 1.3 × 1.260 = 493.6 kPa.
- Overburden term: 27 × (18.40 − 1) × 1.2 × 1.130 = 637.0 kPa.
- Width term: 0.5 × 2 × 18 × 22.40 × 0.8 × 1.130 × 1 = 364.5 kPa.
- Net ultimate: qnu = 493.6 + 637.0 + 364.5 = 1495.1 kPa.
- Net safe: qns = 1495.1 / 2.5 = 598.1 kPa. Gross safe = 598.1 + 27.0 = 625.1 kPa.
Result: Net safe bearing capacity 598.1 kPa (net ultimate 1495.1 kPa) in general shear.
Enter the same values in the calculator above to check each step.
Frequently asked questions
How is safe bearing capacity of soil calculated as per IS 6403?
Work out the net ultimate bearing capacity from c, φ, γ, the footing size and depth using the IS 6403 factors Nc, Nq and Nγ with shape, depth and water-table corrections, then divide by a factor of safety, normally 2.5. For a 2 m square footing at 1.5 m depth in soil with c = 10 kPa, φ = 30° and γ = 18 kN/m³, general shear gives a net ultimate of 1495.1 kPa and a net safe value of 598.1 kPa.
What is the difference between general and local shear failure in bearing capacity?
General shear applies to dense or stiff soil that fails along a full slip surface; local shear applies to loose or soft soil that compresses before it fails. For local shear the strength is reduced to c' = ⅔c and φ' = tan⁻¹(0.67 tanφ). In the example above, φ = 30° in Auto mode is only 14% of the way from local to general shear, so the net safe value drops from 598.1 kPa to 254.0 kPa.
How does the water table affect the bearing capacity of a footing?
A high water table reduces the effective unit weight of the soil, so the width term is multiplied by W', which falls from 1 (water table at least B below the base) to 0.5 (water table at or above the base). Water above the base also lowers the overburden pressure q. For the 2 m square footing example, a water table at the founding level of 1.5 m reduces the net safe value from 598.1 kPa to 525.2 kPa.
What is the difference between net and gross safe bearing capacity?
Net safe bearing capacity is the extra pressure the soil can take beyond the overburden already acting at founding level. Gross safe bearing capacity adds that overburden back: qns + γ·Df. In the worked example the net safe value is 598.1 kPa and the gross safe value is 598.1 + 27.0 = 625.1 kPa.
Does this bearing capacity calculator check settlement?
No. It gives the shear-failure bearing capacity only, including the IS 6403 inclination factors and the effective width B′ = B − 2e for an eccentric load; sliding is not checked. The allowable bearing pressure must also meet settlement limits, and the soil parameters should come from a site-specific geotechnical investigation.
Step-by-step guides
- Mat Foundation Design (IS 456 & ACI 318) — GuideMat foundation design with drop panels for punching shear: step-by-step calculation per IS 456 Clause 31.6 and ACI 318-19 Section 22.6, critical perimeter, drop panel sizing, and a worked example.
- Basement Wall Design (IS 456 & IS 3370) — Worked ExampleBasement wall design to IS 456 and IS 3370, step by step: at-rest earth and water pressure, propped-wall moments, base slab uplift, the 1.2 flotation check and 0.2 mm crack width, with a full worked example run on the CivilTech Studio engine.
- Isolated Footing Design (IS 456) — Worked ExampleIsolated footing design IS 456 guide, step by step: plan size from net SBC, depth from one-way shear, punching and dowel anchorage, steel by Annex G. A full worked example for a 900 kN column, hand-checked against the CivilTech Studio engine.
- SBC from SPT N Value (IS 6403) — Worked ExampleHow to find safe bearing capacity from SPT N value: N corrections, the IS 6403 shear check, Meyerhof, Bowles and Burland & Burbidge settlement methods, water table effects and IS 1893 liquefaction screening, with a full worked example.
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