IS 6403:1981 & TERZAGHI CRITERIA

Soil Safe Bearing Capacity (SBC) Calculator

Determine ultimate (q_u), net safe (q_ns), and gross safe (q_s) bearing capacities for shallow foundations considering shear failure modes and ground water table correction.

1. Foundation Geometry

Footing Type

Footing Width B (m)

Length L (m) — Rect only

Depth of Foundation Df (m)

2. Soil Parameters & Water Table

Cohesion c (kN/m²)

Friction Angle ϕ (deg)

Unit Weight γ (kN/m³)

Water Table Depth (m from GL)

Factor of Safety (FOS)

Shear Failure Mode

Safe Bearing Capacity (SBC)

General Shear

-- kN/m²

Ultimate Capacity qu = -- kN/m² | Net Safe qns = -- kN/m²

Factors (Nc, Nq, Nγ):--

Surcharge at Base (q = γ·Df):-- kN/m²

Water Table Factor (W'):--

Safe Soil Pressure in kg/cm²:-- kg/cm²

Subsurface Stress & Rupture Zones

IS 6403

IS 6403:1981 Bearing Capacity Theory & Equations

1. Terzaghi & IS Equation

The general shear equation combines cohesion, surcharge, and soil wedge weight:

qu = c Nc sc + q Nq sq + 0.5 γ B Nγ sγ W'

2. General vs Local Shear

• φ > 36°: General shear failure (dense soils).

• φ < 28°: Local shear failure. Mobilized values c_m = (2/3) c and tan(φ_m) = (2/3) tan(φ) must be adopted.

3. Water Table Effect

When the water table rises within depth $B$ below the foundation base, buoyant unit weight applies, reducing wedge resistance by up to 50% ($W' = 0.5$).