About Consolidation Settlement Calculator
The consolidation settlement calculator computes the primary one-dimensional consolidation settlement of a saturated clay layer using Terzaghi consolidation theory. It automatically selects the correct equation based on whether the soil is normally consolidated or overconsolidated and whether the applied stress increment crosses the preconsolidation pressure.
Enter the compressible layer thickness H, the initial void ratio e0, the compression index Cc, the recompression (swelling) index Cr, the initial vertical effective stress, the increase in effective stress, and the preconsolidation pressure. The tool returns the total settlement, the recompression and virgin compression contributions, the overconsolidation ratio, and an e-log p′ curve of the loading path.
How It Works
- Enter layer thickness H (mm), initial void ratio e0, indices Cc and Cr, initial effective stress sigma0′ (kPa), stress increase dSigma (kPa), and preconsolidation pressure sigmaP′ (kPa).
- If sigmaP′ <= sigma0′ the clay is normally consolidated and the full increment compresses on the virgin line using Cc.
- If the soil is overconsolidated and the final stress stays at or below sigmaP′, the whole increment uses the recompression index Cr.
- If the increment crosses sigmaP′, the settlement is split: Cr from sigma0′ up to sigmaP′, then Cc from sigmaP′ up to the final stress, and the two parts are summed.
Worked Example
A 3000 mm clay layer has e0 = 0.8, Cc = 0.3, Cr = 0.05, initial effective stress sigma0′ = 100 kPa, and a stress increase dSigma = 100 kPa. For a normally consolidated clay (sigmaP′ = 100 kPa) the settlement is Sc = (0.3 / 1.8) * 3000 * log10(200 / 100) = 0.16667 * 3000 * 0.30103 = 150.5 mm. If instead the clay is overconsolidated with sigmaP′ = 150 kPa, the increment crosses sigmaP′: the recompression part is (0.05 / 1.8) * 3000 * log10(150 / 100) = 14.67 mm and the virgin part is (0.3 / 1.8) * 3000 * log10(200 / 150) = 62.47 mm, giving a total of 77.1 mm.
Formulas
- Normally consolidated (sigmaP′ <= sigma0′)
Sc = (Cc / (1 + e0)) * H * log10((sigma0′ + dSigma) / sigma0′)- Overconsolidated, recompression only (sigma0′ + dSigma <= sigmaP′)
Sc = (Cr / (1 + e0)) * H * log10((sigma0′ + dSigma) / sigma0′)- Overconsolidated, increment crosses sigmaP′
Sc = (Cr / (1 + e0)) * H * log10(sigmaP′ / sigma0′) + (Cc / (1 + e0)) * H * log10((sigma0′ + dSigma) / sigmaP′)- Overconsolidation ratio
OCR = sigmaP′ / sigma0′
Standards & References
- Terzaghi one-dimensional consolidation theory (1925)
- ASTM D2435 Standard Test Methods for One-Dimensional Consolidation Properties of Soils
- Das, Principles of Geotechnical Engineering
Frequently Asked Questions
What is the difference between the compression index Cc and the recompression index Cr?
Cc is the slope of the virgin compression line on the e-log p′ plot and governs settlement once the stress exceeds the preconsolidation pressure. Cr (also called the swelling or recompression index) is the much smaller slope on the reload line and governs settlement while the soil is still overconsolidated. Cr is typically about one fifth to one tenth of Cc.
How does the calculator decide between the normally consolidated and overconsolidated equations?
It compares the preconsolidation pressure sigmaP′ with the initial effective stress sigma0′ and the final stress. If sigmaP′ <= sigma0′ the soil is normally consolidated and uses Cc. If the soil is overconsolidated, it then checks whether the final stress stays below sigmaP′ (use Cr only) or crosses it (split into a Cr part and a Cc part).
What is the overconsolidation ratio (OCR)?
OCR is the preconsolidation pressure divided by the current effective overburden stress, sigmaP′ / sigma0′. An OCR of 1 means the soil is normally consolidated; an OCR greater than 1 means it has previously carried a higher stress and will settle far less for small load increases.
Does this calculator include secondary compression or time effects?
No. It computes only the primary consolidation settlement, which is the volume change as excess pore water pressure dissipates. Secondary compression (creep) and the time rate of consolidation governed by the coefficient of consolidation cv are separate calculations not covered here.