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Shallow Foundation Bearing Capacity

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  • The bearing capacity has three terms. The first is the cohesion term cNc, the second is the surcharge term qNq, and the third is the self-weight term half gamma B prime N gamma. Each one picks up shape and depth multipliers that adjust the strip footing formula for the real geometry.
  • At phi equals zero the failure surface is a circular arc and Nc is either 5.7 (Terzaghi) or 5.14 (the exact Prandtl solution). The difference matters for soft clays where the whole capacity comes from that one term.
  • Terzaghi's original method has no depth factors. The general method (Hansen and Vesic) adds depth factors that reward embedding the footing, which is why a deeper footing can carry more even when the overburden is the same.
  • Water below the footing replaces the effective unit weight in the third term. Water above it also lightens the overburden in the second term. Both reduce the capacity, and the second effect is usually the larger one.
  • Eccentricity shrinks the effective area. The Meyerhof rule replaces B with B minus 2e, so a 3 m footing loaded 0.3 m off centre behaves like a concentric 2.4 m one.