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Compression Index Calculator

Calculate the compression index Cc from two points on an oedometer e–log σ' curve, or estimate it from the liquid limit using Skempton's correlation. Cc is the key soil parameter that drives consolidation-settlement predictions for clays.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Read two points off the straight virgin portion of the e–log σ' curve: void ratio e1 at stress σ1' and e2 at σ2'.
  2. Enter both void ratios and both effective stresses (kPa), with σ2' > σ1' and e1 > e2.
  3. Optionally enter the liquid limit LL (%) to also see Skempton's empirical estimate Cc ≈ 0.009·(LL − 10).

How it works

The compression index is the slope of the virgin compression line on a void-ratio versus base-10-log-of-effective-stress plot: Cc = (e1 − e2) / log10(σ2'/σ1') = Δe / Δ(log10 σ'). Because the denominator is a log ratio, only the RATIO σ2'/σ1' matters, not the absolute stresses.

For remoulded normally-consolidated clays, Skempton (1944) proposed the empirical correlation Cc ≈ 0.009·(LL − 10), where LL is the liquid limit in per cent. This is useful as a first estimate or a sanity check, but a measured oedometer value should always be preferred for design.

Worked example

Void ratio dropping from 0.90 to 0.75. If e1 = 0.90 at σ1' = 100 kPa and e2 = 0.75 at σ2' = 200 kPa, then Δe = 0.15 and log10(200/100) = log10(2) = 0.3010, so Cc = 0.15 / 0.3010 = 0.498. With a liquid limit LL = 45 %, Skempton gives Cc ≈ 0.009 × (45 − 10) = 0.315 for comparison.

Common mistakes

  • Using natural log instead of log10 in the denominator — Cc is defined on a base-10 log axis.
  • Picking points off the recompression (reload) part of the curve instead of the straight virgin compression line.
  • Entering e2 > e1; void ratio must DECREASE as effective stress increases, so e1 > e2.

Frequently asked questions

What is a typical value of Cc?

For soft normally-consolidated clays Cc is often in the range 0.2–0.6; stiff clays and silts are lower, and highly plastic or organic soils can be much higher. Always confirm with an oedometer test.

What is Skempton's correlation for Cc?

Cc ≈ 0.009·(LL − 10), where LL is the liquid limit in per cent. It was derived for remoulded normally-consolidated clays and is best treated as an order-of-magnitude estimate rather than a design value.

Does it matter which absolute stresses I use?

Only the ratio σ2'/σ1' matters because the formula uses log10(σ2'/σ1'). Two points one log-cycle apart (e.g. 100 and 1000 kPa) make Cc simply equal to Δe.

How is Cc different from the recompression index Cr?

Cc is the slope of the virgin (first-loading) compression line; Cr (or Cs, the swelling index) is the flatter slope of the unload–reload loop, typically 5–10 times smaller than Cc.

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