What Is Unconfined Compressive Strength?
A simple, widely used strength test for cohesive soils
What the test measures
The unconfined compressive strength (UCS) test (commonly per ASTM D2166) applies an increasing axial load to a cylindrical soil sample, with no lateral confining pressure, until the sample fails. The peak load divided by the sample's cross-sectional area gives the unconfined compressive strength — essentially a measure of the soil's undrained shear strength for cohesive (clay-like) soils.
How the test is run
- Trim an undisturbed (or remolded) specimen to a right cylinder with a height of 2 to 2.5 times its diameter; ASTM D2166 calls for a diameter of at least about 30 mm (1.3 in), with the largest particle smaller than a tenth of the diameter (a sixth for specimens 72 mm or more across).
- Load it axially at a strain rate of about 0.5% to 2% per minute, chosen so the specimen fails within about 15 minutes — fast enough that the test is undrained.
- Record load and deformation; failure is the peak load, or the load at 15% strain if no peak occurs first.
- Sketch the failure mode — a clean shear plane or bulging — and measure the moisture content (see What Is Soil Moisture Content?).
Calculating qu and undrained shear strength
As the specimen shortens, it bulges, so the load is divided by a corrected area rather than the original: axial strain ε = ΔL ÷ L₀; corrected area Ac = A₀ ÷ (1 − ε); qu = P ÷ Ac. For a saturated clay tested undrained, the undrained shear strength is half of qu: su = qu ÷ 2.
Example: a 38 mm diameter specimen (A₀ = 1,134 mm²) fails at 150 N at 5% strain. Ac = 1,134 ÷ 0.95 = 1,194 mm², so qu = 150 ÷ 1,194 = 0.126 N/mm², or 125.6 kPa, and su ≈ 62.8 kPa. The Unconfined Compressive Strength Calculator applies the same area correction.
Why 'unconfined' matters
The test deliberately applies no confining pressure around the sample, unlike a triaxial test which does. This makes UCS testing simpler and faster to run, but it's only meaningful for cohesive soils that can hold together and stand as a sample without lateral support — it isn't applicable to cohesionless soils like clean sand, which would simply collapse without confinement.
Fissured, cemented or partly saturated clays can also give misleading results, because the specimen may fail along a pre-existing crack or carry load through suction. Where the design depends on the strength, a triaxial test is the more reliable choice.
ASTM D2166 itself describes the result as an approximate strength in terms of total stresses. Its scope excludes dry and crumbly soils, fissured or varved materials, silts, peats and sands, because none of them gives a valid unconfined result.
Consistency classes
qu gives a quick, widely recognized description of clay consistency. The classes below follow the long-established Terzaghi and Peck scale.
| Consistency | qu (kPa) | qu (tsf) |
|---|---|---|
| Very soft | Below 25 | Below 0.25 |
| Soft | 25–50 | 0.25–0.5 |
| Medium | 50–100 | 0.5–1.0 |
| Stiff | 100–200 | 1.0–2.0 |
| Very stiff | 200–400 | 2.0–4.0 |
| Hard | Above 400 | Above 4.0 |
Common uses for the result
UCS results are used to estimate a cohesive soil's shear strength for foundation bearing capacity checks, slope stability analysis, and general soil consistency classification (soft, medium, stiff, hard) — a quick, widely used indicator in geotechnical investigations of clay-rich sites.
For a strip footing on saturated clay under undrained (φ = 0) conditions, classical bearing capacity theory gives an ultimate bearing pressure of roughly 5.14 × su plus the overburden pressure at the footing base. Engineers then apply a factor of safety and check settlement, so this is a starting point for design, not an allowable pressure.
Common mistakes
- Dividing by the original area rather than the corrected area.
- Testing a disturbed or dried sample and reporting it as undisturbed strength.
- Applying su = qu ÷ 2 to a sandy, fissured or partly saturated soil.
- Testing too slowly, allowing drainage in a test meant to be undrained.
FAQ
- What is the difference between UCS and undrained shear strength?
- Unconfined compressive strength qu is the axial stress at failure. For a saturated clay tested undrained, the undrained shear strength su is half of it: su = qu ÷ 2.
- Can you do a UCS test on sand?
- No. A clean sand has no cohesion to hold a specimen together without lateral support, so it collapses. Sands are tested with confined methods such as the triaxial or direct shear test.
- What is a typical UCS for a stiff clay?
- A stiff clay has a qu of roughly 100 to 200 kPa (1 to 2 tsf) on the Terzaghi and Peck consistency scale.
- Why is the area corrected during the UCS test?
- Because the specimen shortens and bulges as it's loaded. Dividing the load by the enlarged area, A₀ ÷ (1 − ε), gives the true stress rather than overstating it.
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More From the Blog
Sources
ASTM D2166
Standard Test Method for Unconfined Compressive Strength of Cohesive Soil — specimen size, strain rate, the 15% strain limit and scope.
NICET performance checklist, ASTM D2166/D2166M-16
Specimen size, strain-rate and stopping requirements.
NICET performance checklist, ASTM D2166/D2166M-16 — referenceTerzaghi & Peck, Soil Mechanics in Engineering Practice
Consistency of clays in terms of unconfined compressive strength.