IS 6403 : 1981Code of practice for determination of bearing capacity of shallow foundations

IS 6403:1981 is the Indian Standard Code of Practice for Determination of Bearing Capacity of Shallow Foundations. It is the calculation methodology that complements IS 1904:1986 — where IS 1904 sets the design philosophy (FoS, settlement limits, presumptive values), IS 6403 gives you the actual formulas to compute ultimate bearing capacity from soil parameters.

Use it whenever you need to: - Calculate ultimate bearing capacity (q_u) from c, φ, γ, foundation geometry, and depth - Apply shape, depth, and inclination factors for square, rectangular, circular, or eccentrically-loaded footings - Handle the water-table effect on bearing capacity - Choose between drained and undrained analysis for cohesive soils

The code is essentially India's adaptation of Terzaghi's, Meyerhof's, and Vesic's bearing capacity equations — well-known formulas from international geotechnical literature, formalized as the Indian computational standard. It is the most-cited foundation engineering code in India alongside IS 1904.

General ultimate bearing capacity formula (Clause 5.1, Vesic-Meyerhof form):

``` q_u = c × Nc × sc × dc × ic + q × Nq × sq × dq × iq + 0.5 × γ × B × Nγ × sγ × dγ × iγ ```

where: - c = effective cohesion of soil (kPa) - q = effective surcharge at foundation level = γ × Df (kPa) - γ = effective unit weight of soil below foundation - B = footing width (smaller dimension for rectangular) - Nc, Nq, Nγ = bearing capacity factors (functions of φ; from Table 1 of IS 6403) - sc, sq, sγ = shape factors (Table 2) - dc, dq, dγ = depth factors (Table 3) - ic, iq, iγ = inclination factors for inclined loads (Table 4)

For undrained clay (φ = 0, only c_u present): ``` q_u = c_u × Nc + γ × Df ``` With Nc = 5.14 (strip footing) or 5.7 (Terzaghi original; IS 6403 still uses this for square/round).

Bearing capacity factors at common φ values (from Table 1):

| φ | Nc | Nq | Nγ | |---|---|---|---| | 0 | 5.7 | 1.0 | 0.0 | | 5 | 7.3 | 1.6 | 0.5 | | 10 | 9.6 | 2.7 | 1.2 | | 15 | 12.9 | 4.4 | 2.5 | | 20 | 17.7 | 7.4 | 5.0 | | 25 | 25.1 | 12.7 | 9.7 | | 30 | 37.2 | 22.5 | 19.7 | | 35 | 57.8 | 41.4 | 42.4 | | 40 | 95.7 | 81.3 | 100.4 |

Net ultimate: q_nu = q_u − γ × Df (subtract the surcharge that was already there before construction)

Net safe: q_ns = q_nu / FoS (typically FoS = 2.5-3.0 per IS 1904)

Allowable: q_a = min(q_ns, settlement-controlled bearing pressure)

Reduction factors (Clause 5.5) — apply to the surcharge and γ terms:

• Case 1: Water table at or above foundation level (Df_w ≤ 0)
• - Use submerged unit weight γ_sub ≈ γ_sat − 9.81 for both surcharge q AND the 0.5γB term
• - Approximately halves bearing capacity

• Case 2: Water table between foundation level and B below foundation (0 < Df_w < B)
• - Surcharge q uses γ (above WT) and γ_sub (below WT) weighted by depths
• - 0.5γB term uses interpolated effective unit weight: γ_eff = γ_sub + (Df_w/B) × (γ − γ_sub)

• Case 3: Water table at depth > B below foundation
• - No correction needed; use γ throughout

Practical rule: if monsoon water table comes within 1.5 × footing width of foundation level, expect significant bearing capacity reduction. Many failures in coastal/alluvial India happen when designers use dry-season soil parameters without considering monsoon WT rise.

Problem: Column DL+LL = 800 kN. Soil: medium-dense sand, c = 0, φ = 32°, γ = 18 kN/m³. Footing depth Df = 1.5 m. Water table at 4 m below ground (well below influence zone).

Step 1 — Bearing capacity factors for φ = 32° (interpolate from Table 1): Nc ≈ 35.5, Nq ≈ 23.2, Nγ ≈ 22.0

Step 2 — Shape factors for square footing (Table 2): sc = 1 + 0.2 × (B/L) = 1.2 (since B = L) sq = 1 + 0.2 × (B/L) = 1.2 sγ = 1 − 0.4 × (B/L) = 0.6

Step 3 — Assume B = 2.0 m and compute (depth and inclination factors all 1.0 for vertical centric load, simple analysis): q_u = 0 + 18 × 1.5 × 23.2 × 1.2 + 0.5 × 18 × 2.0 × 22.0 × 0.6 q_u = 0 + 752 + 238 = 990 kN/m²

Step 4 — Net ultimate: q_nu = 990 − 18 × 1.5 = 963 kN/m²

Step 5 — Net safe (FoS = 3): q_ns = 963 / 3 = 321 kN/m²

Step 6 — Required area: A = 800 / 321 = 2.49 m² → B = √2.49 = 1.58 m. Less than assumed 2.0 m, so try B = 1.6 m.

Step 7 — Re-iterate with B = 1.6: q_u = 0 + 752 + 0.5 × 18 × 1.6 × 22.0 × 0.6 = 752 + 190 = 942 kN/m² q_nu = 942 − 27 = 915, q_ns = 305 kN/m² A_required = 800 / 305 = 2.62 m² → B = 1.62 m

Provide 1.7 × 1.7 m square footing. Verify settlement separately per IS 8009 (typically not governing for medium-dense sand at this stress level).

1. Confusing total and effective stress parameters — use effective cohesion c' and effective friction φ' for drained (long-term) analysis. Use total cohesion c_u (undrained shear strength) and φ = 0 for undrained (short-term) clay analysis. Mixing the two gives wrong Nc, Nq, Nγ.

2. Skipping the size effect (Nγ × B) — for large foundations (B > 3 m), the 0.5γB × Nγ term becomes dominant for granular soils. Conservatively, use Nγ values for the LOWER reliability bound rather than mean. Some practitioners use Bowles' size-correction: Nγ_corrected = Nγ × (1 − 0.25 × log(B/2)) for B > 2 m.

3. Wrong shape factor for circular footings — IS 6403 Table 2 treats circular as a special case of square (B/L = 1, with slightly different multipliers per Vesic). Don't use rectangular formulas blindly for circular tanks/towers.

4. Forgetting load eccentricity — when column moment + axial load gives eccentricity e > B/6, the linear pressure assumption fails and you must use effective area method (B' = B − 2e). Apply the bearing capacity to the effective area only, then check the maximum corner pressure against q_ns.

5. Using single SPT N for design — bearing capacity from SPT (Terzaghi-Peck or Meyerhof correlations) varies hugely with N. Compute capacity at the lowest N within 1.5× B below foundation level, not the average. A single soft pocket within the influence zone can govern.

6. Ignoring local punching shear — for footings on layered soils where soft layer is within 1.5B below foundation, general bearing capacity formulas don't apply. Use two-layer punching analysis (Meyerhof-Hanna) or carry foundation through to firm stratum.

7. Using c_u from UCS without strain-rate check — Unconfined Compressive Strength (UCS) tests are strain-rate sensitive. UCS / 2 is the standard c_u estimate. Don't use peak undrained strength from a fast-loaded test; field loading is slower and gives lower mobilized strength.

• IS 1904:1986 — design and construction of foundations (uses IS 6403 outputs)
• IS 8009 Part 1:1976 — settlement of foundations on cohesionless soils
• IS 8009 Part 2:1980 — settlement of foundations on cohesive soils
• IS 1888:1982 — plate load test (field validation of computed bearing capacity)
• IS 2131:1981 — Standard Penetration Test (input for empirical bearing capacity in granular soils)
• IS 2720 Part 11/13 — triaxial / direct shear (gives c, φ inputs to IS 6403)
• IS 1498:1970 — soil classification (decides drained vs undrained analysis)
• IS 11385:1985 — code of practice for subsoil investigation for power-house sites
• IS 13094:1992 — design of foundations on expansive soils (different analysis than IS 6403)
• IS 456:2000 — structural design of the footing once q_ns is known
• IRC 78:2014 — limit-state foundation design for bridges (more modern; supersedes IS 6403 for bridge applications)

IS 6403:1981 is 44 years old but remains the everyday calculation reference for shallow-foundation bearing capacity in India. The underlying Terzaghi-Meyerhof-Vesic framework is internationally accepted and timeless — modern updates are incremental refinements rather than paradigm shifts.

For routine RCC buildings: IS 6403 + IS 1904 + IS 8009 is fully adequate. Plan-approval authorities, structural reviewers, PMCs all accept these codes.

For bridge foundations: use IRC 78:2014 instead — it's limit-state, has updated partial factors, and handles complex loading conditions (seismic, vehicular impact, wind) better than IS 6403's allowable-stress framework.

For large infrastructure (metros, ports, power plants): supplement IS 6403 with Eurocode 7 (EN 1997-1, BS EN 1997) for limit-state design and reliability-based partial factors. Most international consultants on Indian mega-projects work to Eurocode 7 with IS 6403 as a sanity check.

Software reality: most Indian design offices use PROKON, STAAD Foundation Advanced, ETABS Foundation, or GeoStudio — these implement IS 6403 + Meyerhof + Vesic + Hansen formulas. Hand calculation is now reserved for verification, preliminary sizing, and academic work. The danger: GIGO — if soil parameters input to software are wrong, IS 6403 output is wrong regardless. Always cross-check software output against hand calculations for a critical foundation.

BIS revision overdue (under CED 43 sectional committee since 2018). When it comes, expect limit-state framework alignment with IRC 78 and Eurocode 7.