When to use: Estimate the settlement magnitude (not just the ultimate/allowable bearing pressure) under a shallow spread footing. For granular soil this uses an SPT-N–correlated elastic settlement method; for cohesive soil it uses classic 1-D consolidation theory. This is distinct from the Bearing Capacity (Terzaghi ultimate) and Foundation Bearing (contact pressure/kern) tools, which don't estimate settlement magnitude.
Disclaimer: This is a preliminary, educational estimate only. Actual settlement depends on subsurface variability, soil layering, groundwater, and construction sequencing. A licensed geotechnical engineer's site-specific analysis governs actual foundation design.
Estimate immediate (elastic) settlement for shallow footings on granular soil, or consolidation settlement for footings on cohesive soil, in inches — with a general rule-of-thumb flag for whether the magnitude typically warrants a closer look. This tool estimates settlement magnitude specifically, complementing the Bearing Capacity (Terzaghi ultimate bearing) and Foundation Bearing (contact pressure/kern) calculators, neither of which estimate how much a footing will actually settle.
For sand and other granular soils, settlement is estimated with the standard elastic settlement equation S = q·B·(1−μ²)·If/Es (Bowles, Foundation Analysis and Design), using a soil modulus Es correlated from the SPT N-value via the Kulhawy & Mayne (1990) relation Es ≈ α·pa·N (α≈5 for sand, pa = atmospheric pressure ≈ 2.12 ksf). A simplified embedment correction — Meyerhof's (1965) depth factor Fd = 1+0.33(Df/B), capped at 1.33 — reduces settlement for deeper footings, consistent with observed behavior that embedment increases confinement.
For clay, settlement is dominated by time-dependent consolidation rather than immediate elastic compression, and is estimated with Terzaghi's classic one-dimensional consolidation equation: Sc = (Cc·H)/(1+e0) · log10((σ'0+Δσ)/σ'0), where Cc is the compression index, e0 the initial void ratio, σ'0 the initial effective overburden stress at the layer's mid-height, Δσ' the stress increase caused by the new footing load at that depth, and H the compressible layer thickness. Δσ' is entered directly here; it can be approximated with a simplified 2:1 stress-distribution method if a site-specific value isn't available.
The tool flags total settlement as typically acceptable below about 1 inch, tolerable for many structures between 1 and 2 inches, and warranting further review above 2 inches. These are general engineering rules of thumb, not code limits — actual tolerable total and differential settlement is governed by the specific structure type, foundation system, and applicable building code, and must be confirmed by a geotechnical engineer.
Granular (sandy) soils drain quickly, so settlement under load is essentially immediate/elastic and is commonly estimated with SPT-correlated elastic methods. Cohesive (clay) soils have low permeability, so load-induced pore pressure dissipates slowly, producing time-dependent consolidation settlement governed by the soil's compressibility (Cc, e0) rather than an elastic modulus.
The Bearing Capacity (Terzaghi) calculator checks whether the soil will fail in shear under the applied pressure — a strength/stability check. This tool instead estimates how much the footing will settle under service loads, even if bearing capacity has ample safety factor — settlement can govern foundation size even when shear failure is not a concern.
In practice, geotechnical engineers compute the stress increase at a layer's mid-depth using methods like the 2:1 approximation, Boussinesq point-load theory, or influence charts, based on the footing's plan dimensions, depth, and the layer's depth below the footing. This tool accepts Δσ' as a direct input to keep the scope focused on the consolidation settlement calculation itself.
Use the average, representative (uncorrected) blow count from the influence zone beneath the footing — commonly taken as a depth of about B to 2B below the footing base. A geotechnical report will provide corrected N-values (for overburden and hammer energy) that should be used for final design; this uncorrected-N estimate is intended for preliminary screening only.
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