This standard prescribes the method for determining the unconfined compressive strength of undisturbed or remoulded cohesive soil specimens. The test applies an axial load at a constant rate of strain to a cylindrical soil sample to rapidly estimate its undrained shear strength.
Specifies the method for determining the unconfined compressive strength of cohesive soil specimens.
Quick Reference Values
specimen height to diameter ratio2 to 2.5
minimum specimen diameter38 mm
maximum particle size1/8th of the specimen diameter
rate of axial strain0.5% to 2.0% per minute
maximum strain for failure20% axial strain
Key Formulas
Ac = A0 / (1 - ε) — Corrected cross-sectional area, where A0 is initial area and ε is axial strain
σ = P / Ac — Compressive stress, where P is the compressive load
cu = qu / 2 — Undrained shear strength, where qu is unconfined compressive strength
Practical Notes
This test is only valid for cohesive soils (clays) that will not expel bleed water during the loading phase and can stand unsupported.
Ensure the ends of the specimen are perfectly flat and perpendicular to its longitudinal axis to avoid stress concentrations and eccentric loading.
Always record the mode of failure (e.g., diagonal shear plane vs. bulging) along with the test results, as it indicates the soil's behavioral characteristics.