Allowable Load for a Settlement Limit (NC Clay)
The largest stress increase that keeps a clay's settlement within an allowable limit.
Example
You enter
- Allowable settlement (in) 2
- Compression index Cc 0.25
- Clay layer thickness H (ft) 10
- Initial void ratio e0 0.9
- Initial effective stress at mid-layer (psf) 2000
You get
- Max allowable stress increase 677 psf
- Stress ratio 1.3386
Checked against published worked examples from: FHWA.
Details, formula, and sources
ScopeThe maximum load-induced stress increase that keeps a clay's settlement within an allowable limit - d_sigma = sigma'0 (10^(Sc(1 + e0)/(Cc H)) - 1). To hold a 10 ft NC clay (Cc 0.25, e0 0.90, sigma'0 2,000 psf) to a 2 in settlement, the added stress can be at most 677 psf; tighten the limit to 1 in and only 314 psf is allowed - because settlement grows with the log of the stress ratio, a tighter limit allows disproportionately less load. Single NC layer, no time rate. A design aid; the geotechnical engineer of record governs.
Formula or tabled_sigma = sigma'0 (10^(Sc_ft (1 + e0) / (Cc H)) - 1), Sc_ft = Sc_in / 12; final_stress = sigma'0 + d_sigma.
Source and editionThe Terzaghi primary consolidation relation solved for the allowable load increment, the inverse of the settlement tile, as compiled in the Das and NAVFAC references, by name.
Free accessThe primary-consolidation relation is a public soil-mechanics result; NAVFAC DM-7 is public-domain and free online.
What governsEstimate. AHJ and licensed professional govern.
API field namessc_allow_in, cc, h_ft, e0, sig0_psf, dsig_psf, stress_ratio
- Inverse relation d_sigma = sigma'0 (10^(Sc(1 + e0)/(Cc H)) - 1) at mid-layerTerzaghi / Das
- Log stress ratio settlement grows with the log of the stress ratio, so a tighter limit allows disproportionately less loadsoil-mechanics references
- Scope single NC layer, one mid-layer stress, no time rate or secondary creepscope of this tile