Foundation Settlement Calculator

Immediate settlement happens right away as soil elastically compresses under the new load, using the formula Si = qB(1 − ν²)Is/E — this is largely instantaneous and driven by the soil's stiffness (E). Consolidation settlement is the slower process, sometimes taking months or years, where pore water is squeezed out of fine-grained soil over time, calculated separately with Sc = (Cc/(1+e0)) × H × log10((σ0+Δσ)/σ0). The calculator adds both together for total settlement, but they occur on very different timescales.

The shape factor Is in the elastic settlement formula accounts for how the foundation's geometry distributes stress into the soil below it, and 0.82 is a standard approximation for a square or roughly square footing. A long strip footing or a very elongated rectangular footing distributes stress differently and would use a different Is value, so this calculator's immediate-settlement estimate is most accurate for compact, near-square footing shapes rather than long continuous footings.

Rather than asking for it directly, the calculator estimates σ0 internally as an assumed soil unit weight of 120 pcf multiplied by half the compressible layer's thickness, representing the existing overburden pressure at the layer's midpoint before your foundation load is added. Because this unit weight is assumed rather than entered, a site with meaningfully lighter or heavier soil (e.g., loose fill versus dense clay) will see its consolidation settlement result shift even though you can't adjust that assumption as an input.

It uses Terzaghi's standard time factor (T90 = 0.848) combined with an assumed coefficient of consolidation, Cv = 0.01 ft²/day, which is representative of soft clay draining from both the top and bottom of the layer (double drainage). Stiffer clays consolidate faster, and a layer that can only drain from one side (single drainage, such as sitting on an impermeable layer) would take roughly four times as long for the same thickness — so this figure is illustrative of typical soft-clay behavior rather than a site-specific prediction.