EngiMetric
Geotechnical Civil & Structural Design guide Understand a concept

What the ground under your foundation can actually carry

How to go from a soil description to an allowable bearing pressure, and how to tell whether bearing or settlement will actually govern your footing.

The engineering question

A geotechnical report gives you parameters rather than a number you can design to. How do those parameters become a pressure your footing can safely carry?

Worked example

The figures below were produced by running the Soil Bearing Capacity Calculator with the same inputs — during the build, from its authoritative engine.

Results
Allowable bearing pressure 304.381 kPa
Net ultimate pressure 902.3431 kPa
Nc (cohesion factor) 30.1396
Nq (surcharge factor) 18.4011
Nγ (weight factor) 22.4025
Shape Square
Assessment Terzaghi general shear: qu 913 kPa (allowable 304 kPa at FS 3). Verify with site-specific soil parameters and apply against a known critical failure plane.
  1. At φ = 30° the capacity factors are Nc ≈ 19.4, Nq ≈ 18.4 and Nγ ≈ 22.5.
  2. The cohesion term dominates when c is high: 10 × 19.4 × 1.3 ≈ 252 kPa, before the weight and surcharge terms are added.
  3. Divide the ultimate pressure by the safety factor to obtain the allowable value to carry into the structural design.

Sanity check

For a square footing, a wider footing must never reduce the allowable pressure — if it does, a shape factor has been applied incorrectly.

Tools for this problem

These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.

Engineering review required

This topic supports professional engineering work; it does not replace a sealed design. The result depends on site data, the governing standard and engineering judgement, and should be confirmed by a competent engineer before it is built.

References & standards

  • BS EN 1997-1 — Geotechnical design, bearing capacity and serviceability
  • Terzaghi (1943) — theoretical analysis of the bearing capacity of soils

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