Primary Consolidation Settlement (NC Clay)
The slow consolidation settlement that governs a foundation on clay.
Example
You enter
- 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
- Stress increase from load (psf) 1000
You get
- Sc (in) 2.78
Checked against published worked examples from: FHWA.
Details, formula, and sources
ScopeSc = (Cc H/(1 + e0)) log10((sigma'0 + d_sigma)/sigma'0). A 10 ft NC clay (Cc 0.25, e0 0.90) under a 1,000 psf footing load settles 2.8 in - several times the immediate elastic dip, over years as water squeezes out. Because it grows with the LOG of the stress ratio, doubling the load increment only lifts it to 4.75 in - the first increment is the costly one. Single NC layer, no time rate. A design aid; the geotechnical engineer of record governs.
Formula or tableSc_ft = (Cc H / (1 + e0)) log10((sigma'0 + d_sigma) / sigma'0); Sc_in = 12 Sc_ft.
Source and editionThe Terzaghi primary consolidation settlement of a normally-consolidated clay Sc = (Cc H/(1 + e0)) log10((sigma'0 + d_sigma)/sigma'0), with the compression index Cc, 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 namescc, h_ft, e0, sig0_psf, dsig_psf, sc_in
- NC consolidation Sc = (Cc H/(1 + e0)) log10 of the effective-stress ratio at mid-layerTerzaghi / Das
- Compression index Cc, often ~0.009(LL - 10) for remolded clay; OC clay uses Cr below the preconsolidation stresssoil-mechanics references
- Scope single NC layer, one mid-layer stress, no time rate or secondary creepscope of this tile