Geotechnical Civil & Structural Design guide Understand a concept

How much settlement is acceptable, and which kind governs

Distinguishing immediate from consolidation settlement, and why the damage limit is usually about differential movement rather than total.

The engineering question

A footing can be well within its bearing capacity and still damage the building. How much movement is acceptable, and which calculation predicts it?

Worked example

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

Results
Total settlement	409.952 mm
Immediate (elastic) settlement	12.012 mm
Consolidation settlement	397.94 mm
Stress increase at layer mid-depth	90 kPa
Assessment	Immediate 12 mm + consolidation 398 mm ≈ 410 mm for a 6 m compressible layer. Bearing-pressure stress at layer mid-depth 90 kPa.

Immediate settlement is an elastic response: q·B(1−ν²)·If/E.
Consolidation uses the compression index over the thickness of the compressible layer and the stress increase at its mid-depth.
Total movement is the sum. Reinforced concrete frames typically tolerate about 25 mm total but far less differentially.

Sanity check

If consolidation is reported as zero on a soft clay, the void ratio or the compression index has almost certainly been left at a default.

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 — serviceability limit states
Terzaghi, Peck & Mesri — Soil Mechanics in Engineering Practice