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The clay's compression index, from a laboratory oedometer (consolidation) test.
Higher plasticity clays generally have higher Cc values.
The thickness of the compressible clay layer.
For a layer drained on only one side, some methods use half-thickness for the drainage path length — but the compressible thickness itself is the full layer thickness for this settlement formula.
The clay's initial (in-situ) void ratio.
The in-situ value from the oedometer, before any load is applied. Settlement here is computed as a CHANGE in void ratio referred back to this one, so it sets the denominator of the whole calculation. A sample that dried on the way from the borehole, or swelled after the confining stress was released, no longer carries the in-situ ratio — which is why sample quality shows up in settlement predictions more than in strength ones.
The effective overburden stress at the mid-height of the clay layer before loading.
At MID-HEIGHT, because stress increases with depth and the mid-height value is what represents the layer as a whole; taking it at the top understates the starting stress for the entire thickness. Effective means total stress less pore water pressure, so the water table's position moves this directly — and a water table assumed deeper than it really is overstates the starting stress, which understates the settlement that follows.
The additional stress applied by the new structure or fill, at the same depth.
From a stress distribution analysis (e.g. Boussinesq method) under the new load.
Primary consolidation settlement
5.81 in
This gives ultimate PRIMARY consolidation settlement only, assuming the clay is normally consolidated — overconsolidated clays need a modified formula with the preconsolidation pressure, and secondary (creep) settlement is not included here.
- Equivalent in meters
- 0.15 m
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Primary Consolidation Settlement Calculator: 5.81 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Terzaghi 1D primary consolidation settlement: Sc = (Cc x H / (1 + e0)) x log10((σ'0 + Δσ) / σ'0), where Cc is the compression index, H is the clay layer thickness, e0 is the initial void ratio, σ'0 is the initial effective stress, and Δσ is the added stress
Inputs used
- Compression Index (Cc)
- 0.3
- Compressible Layer Thickness (H)
- 16.5 ft
- Initial Void Ratio (e0)
- 0.8
- Initial Effective Stress (σ'0)
- 2088.54 psf
- Added Stress (Δσ)
- 1044.27 psf
Intermediate steps
- Equivalent in meters
- 0.15 m
Confidence note: This gives ultimate PRIMARY consolidation settlement only, assuming the clay is normally consolidated — overconsolidated clays need a modified formula with the preconsolidation pressure, and secondary (creep) settlement is not included here.
What this calculation does not cover
- Primary consolidation only. Immediate (elastic) settlement that happens as the load goes on, and secondary compression (creep) that carries on after the excess pore pressure has dissipated, are both outside this figure — and for organic and highly plastic clays the creep component can be a large share of the total. What a structure actually settles is the sum of all three.
- The formula is the normally consolidated one. There is no preconsolidation pressure input, so an overconsolidated deposit — a weathered near-surface crust, ground that has been previously loaded, desiccated or drawn down — is treated as if it had never seen a higher stress, and the result overstates its settlement. A load that starts below the preconsolidation pressure and finishes above it needs the two-part recompression-plus-virgin calculation instead, which this page does not perform.
- This is the end-of-primary magnitude, not a timescale. Coefficient of consolidation, drainage path length and degree of consolidation play no part in the arithmetic, so the answer says nothing about how much settlement will have occurred by handover, how long a surcharge has to stay in place, or whether vertical drains would change the programme.
- One layer, one stress, one point. The added stress you enter is applied uniformly through the full layer thickness, whereas the real stress under a footing or an embankment decays with depth. Enter the mid-depth value the help asks for and a layer that is thick relative to the width of the load is materially mis-estimated — under-predicted, substantially so in the worst cases — so split it into sub-layers, each with its own added stress and its own initial effective stress, and add the results, as you must anyway for a profile with several compressible strata. Compression is assumed one-dimensional, with no lateral yield or squeezing out from under the load.
- Compression index is capped at 1.0 and initial void ratio at 2.5, and a laboratory value above either is replaced with the limit rather than refused — highly plastic, organic and peaty deposits therefore come out low. This is also a settlement magnitude, not a foundation design: it applies no tolerable-settlement criterion, returns nothing about differential settlement between points, and does not substitute for a site investigation with oedometer testing interpreted by a geotechnical engineer.
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This result is a specification — 5.81 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Terzaghi 1D primary consolidation settlement: Sc = (Cc x H / (1 + e0)) x log10((σ'0 + Δσ) / σ'0), where Cc is the compression index, H is the clay layer thickness, e0 is the initial void ratio, σ'0 is the initial effective stress, and Δσ is the added stress
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