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
Details, formula, and sources
The 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.
d_sigma = sigma'0 (10^(Sc_ft (1 + e0) / (Cc H)) - 1), Sc_ft = Sc_in / 12; final_stress = sigma'0 + d_sigma.
The 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.
The primary-consolidation relation is a public soil-mechanics result; NAVFAC DM-7 is public-domain and free online.
Estimate. AHJ and licensed professional govern.
Field names used by the API: sc_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