Bearing Capacity Predictor (IS:6403-1981)
Conforming to Indian standard1. Foundation geometry (intelligent)
2. Soil & load parameters
3. Environmental & safety
Bearing Capacity Predictor as per IS 6403:1981
Detailed documentation of methodology, inputs, and premium enhancements
This web application implements the Indian Standard IS:6403-1981 "Code of practice for determination of bearing capacity of shallow foundations". It computes the ultimate bearing capacity, net safe bearing capacity, and gross safe bearing capacity for strip, square, rectangular, and circular foundations. The calculator incorporates the standard's recommendations for bearing capacity factors, shape, depth, inclination, and water table corrections, along with the special failure‑mode adjustment for soils with friction angle φ < 29°.
The current version introduces intelligent dimension handling, dynamic input locking based on foundation shape, real‑time factor display, and a refined water‑table correction logic that strictly follows IS:6403 guidelines.
- Shape‑aware dimension logic: For strip foundations L is set to ∞ (input disabled); square → L = B automatically; circular → L = 0, B treated as diameter.
- Interpolated bearing factors: Nc, Nq, Nγ are linearly interpolated from the standard IS table (0° to 45°).
- Water table correction (W′, W″): Separate reduction factors for surcharge and density terms as per IS:6403 clause 5.1.3.
- Failure mode adjustment: For φ < 29°, cohesion and friction angle are reduced (c' = ⅔ c , tan φ' = ⅔ tan φ) to account for local shear failure.
- Transparent intermediate factors: All shape, depth, inclination, and bearing factors are displayed before final results.
- Inclination factors (ic, iq, iγ): Computed from load inclination α (degrees) as per IS:6403.
| Field | Symbol | Unit | Description / premium notes |
|---|---|---|---|
| Foundation type | – | – | Strip, square, rectangular, circular. Changes the availability of L and the shape factors. |
| Width (B) / Diameter | B | m | For strip/square/rectangular = width; for circular = diameter. |
| Length (L) | L | m | For strip: disabled (effectively ∞). Square: locked = B. Circular: = 0. Rectangular: user defined. |
| Depth (Df) | D | m | Depth of foundation below ground surface. |
| Cohesion (c) | c | kN/m² | Effective cohesion intercept. If φ < 29°, reduced automatically (c' = ⅔c). |
| Friction angle (φ) | φ | ° | Angle of internal friction. For φ < 29° reduced angle φ' = atan(⅔ tan φ) is used for factor calculation. |
| Unit weight (γ) | γ | kN/m³ | Bulk unit weight of soil. |
| Load inclination (α) | α | ° | Inclination of resultant load from vertical. Affects ic, iq, iγ. |
| Water table depth (Dw) | Dw | m | Depth of water table from ground surface. Used to compute reduction factors r₁ (surcharge) and r₂ (density). |
| Factor of safety (FOS) | F | – | Global factor applied to net ultimate capacity to obtain net safe capacity. |
The gross ultimate bearing capacity qu is given by:
where q = γ·D is the overburden pressure at foundation base. The net ultimate capacity is qnu = qu – q. The gross safe capacity qs = q + qnu / F and net safe qns = qnu / F.
🔹 Bearing capacity factors Nc, Nq, Nγ
Obtained from linear interpolation of the standard IS table (reproduced below) using the reduced friction angle φ' (if φoriginal < 29°, otherwise φ' = φ).
| φ (°) | Nc | Nq | Nγ |
|---|---|---|---|
| 0 | 5.14 | 1.00 | 0.00 |
| 5 | 6.49 | 1.57 | 0.45 |
| 10 | 8.35 | 2.47 | 1.22 |
| 15 | 10.98 | 3.94 | 2.65 |
| 20 | 14.83 | 6.40 | 5.39 |
| φ (°) | Nc | Nq | Nγ |
|---|---|---|---|
| 25 | 20.72 | 10.66 | 10.88 |
| 30 | 30.14 | 18.40 | 22.40 |
| 35 | 46.12 | 33.30 | 48.03 |
| 40 | 75.31 | 64.20 | 109.41 |
| 45 | 133.88 | 134.88 | 271.75 |
🔹 Shape factors (sc, sq, sγ)
- Strip: sc = sq = sγ = 1.0
- Square: sc = 1.3, sq = 1.2, sγ = 0.8
- Circular: sc = 1.3, sq = 1.2, sγ = 0.6
- Rectangular: sc = 1 + 0.2·(B/L), sq = 1 + 0.2·(B/L), sγ = 1 – 0.4·(B/L)
🔹 Depth factors (dc, dq, dγ)
Using nφ = tan²(45° + φ'/2):
- dc = 1 + 0.2·(D/B)·√nφ
- dq = dγ = 1 + 0.1·(D/B)·√nφ (for φ' > 10°), otherwise 1.0
🔹 Inclination factors (ic, iq, iγ)
- ic = iq = (1 – α/90)²
- iγ = (1 – α/φ')² (if φ' > 0 and α>0, otherwise 1.0)
🔹 Water table correction (W′ , W″)
W′ (for surcharge term): if Dw ≤ D → W′ = 0.5·(1 + Dw/D) ; else 1.0.
W″ (for density term): let d = Dw – D (distance from base to WT).
- If d < 0 (WT above base) → W″ = 0.5
- If 0 ≤ d ≤ B → W″ = 0.5·(1 + d/B)
- If d > B → W″ = 1.0
Input: Rectangular foundation (B=2 m, L=3 m, D=1.5 m), c=10 kPa, φ=30°, γ=18 kN/m³, α=0°, Dw=1.0 m, FOS=2.5.
- Failure mode adjustment: φ=30° ≥ 29°, so no reduction: φ' = 30°, c' = 10 kPa.
- Bearing factors (interpolated): Nc=30.14, Nq=18.40, Nγ=22.40.
- Shape factors (rectangular): sc = 1+0.2*(2/3)=1.133, sq=1.133, sγ=1–0.4*(2/3)=0.733.
- Depth factors: nφ=tan²(60°)=3.0, √nφ=1.732; dc=1+0.2*(1.5/2)*1.732=1.26; dq=dγ=1+0.1*(1.5/2)*1.732=1.13.
- Inclination (α=0): all i =1.0.
- Water table: Dw=1.0 ≤ D=1.5 → W′=0.5*(1+1/1.5)=0.833. d = Dw–D = –0.5 (<0) → W″=0.5.
- Term1: c·Nc·sc·dc·ic = 10*30.14*1.133*1.26*1 = 430.2 kPa
Term2: q·(Nq–1)·sq·dq·iq·W′ = (18*1.5)=27 * (17.4)*1.133*1.13*0.833 = 27*17.4*1.133*1.13*0.833 ≈ 27*18.51 = 499.8 kPa
Term3: 0.5·γ·B·Nγ·sγ·dγ·iγ·W″ = 0.5*18*2*22.4*0.733*1.13*0.5 = 0.5*18*2*22.4*0.733*1.13*0.5 ≈ 167.4 kPa - qnu = 430.2 + 499.8 + 167.4 = 1097.4 kPa
q = 27 kPa → qu = 1124.4 kPa - Safe capacities: qns = 1097.4/2.5 = 438.96 kPa; qs = 438.96 + 27 = 465.96 kPa.
- IS:6403 – 1981 (Reaffirmed 2002) “Code of practice for determination of bearing capacity of shallow foundations”, Bureau of Indian Standards.
- Bowles, J.E. (1996) “Foundation Analysis and Design”, McGraw-Hill.
- Murthy, V.N.S. (2007) “Geotechnical Engineering: Principles and Practices”, CRC Press.