Unconfined Compressive Strength Test

Unconfined Compressive Strength Test
(UCS — IS 2720 Part 10)

Laboratory determination of unconfined compressive strength (qu) and undrained shear strength (Su = qu/2) of cohesive soils per IS 2720 Part 10 — for embankment stability, pile design and foundation bearing capacity.

Overview

What is the Unconfined Compressive Strength Test?

The Unconfined Compressive Strength (UCS) test measures the compressive strength of a cylindrical cohesive soil specimen with no lateral confining pressure. The specimen fails in shear under axial load, giving qu (kPa). The undrained shear strength Su = qu/2 — the most basic soil strength parameter for soft clay and stiff clay foundations, pile skin friction and embankment stability.

Conducted as per IS 2720 Part 10, the UCS test is applicable to cohesive soils (clays and silts) that maintain their shape without confinement. The test is rapid and inexpensive — a primary screening test before more complex triaxial testing. Global Lab conducts UCS on both undisturbed (tube samples) and remoulded specimens at field density, providing sensitivity ratio (St = qu(undisturbed)/qu(remoulded)) for soft clay assessment.

Undrained Shear Strength Su

Su = qu/2 — most direct measure of cohesive soil shear strength without drainage.

Undisturbed Tube Samples

UCS on undisturbed samples from Shelby tubes preserves in-situ soil structure.

Sensitivity Ratio (St)

St = qu undisturbed / qu remoulded — identifies sensitive and quick clays.

Stress-Strain Curve

Complete axial stress vs axial strain curve plotted to failure.

NABL Certified Reports

All test and calibration results are backed by traceable measurements and standardized procedures.

Consistency Classification

qu classifies soil as very soft, soft, medium, stiff, very stiff or hard per IS 1498.

Test in Action

Unconfined Compressive Strength Test — Site Images

Cylindrical Clay Specimen in Unconfined Compression Apparatus

Axial Loading at 1.25% Strain/Minute to Failure

qu and Su from Stress-Strain Curve — IS 1498 Consistency Class

Test Procedure

How the Unconfined Compressive Strength Test is Conducted

1

Specimen Preparation

Cylindrical specimen (38mm dia × 76mm or 50mm × 100mm) trimmed from undisturbed tube sample or remoulded at field water content and density.
2

Compression Test Setup

Specimen placed in strain-controlled compression apparatus. Deformation dial gauge/LVDT attached.
3

Axial Loading

Specimen compressed at 1.25% axial strain/minute until failure (> 15% strain if no clear failure).
4

qu Determination

Peak axial stress = qu. Mohr circle with zero confining pressure plotted. Su = qu/2.
5

IS 1498 Classification & Report

qu classifies consistency (very soft <25kPa, soft 25–50, medium 50–100, stiff 100–200kPa+). NABL report within 3 days.

Standards & Specifications

Applicable Test Standards

Standard	Title	Scope	Type
IS 2720 Part 10	Methods of Test for Soils — Determination of Unconfined Compressive Strength	UCS Test	Indian Standard
IS 1498	Classification and Identification of Soils — Consistency Classification	Soil Classification	Indian Standard
IS 2911	Code of Practice for Design and Construction of Pile Foundations	Pile Design	Indian Standard
ASTM D2166	Standard Test Method for Unconfined Compressive Strength of Cohesive Soil	UCS Standard	ASTM

Applications

Where is the Unconfined Compressive Strength Test Used?

Embankment stability analysis on soft clay foundations — undrained Su for φ=0 analysis

Soft clay bearing capacity for mat and raft foundation design

Pile skin friction estimation in cohesive soils per IS 2911

Quick clay and sensitive clay identification for landslide hazard assessment

Soil consistency classification in geotechnical investigation reports

Cohesive soil stabilisation effectiveness — pre and post lime/cement treatment

Why Choose Global Lab

Our Advantages

NABL Accredited ReportsLegally valid IS 2720 Part 10 UCS certificates for geotechnical design.

Undisturbed Sample TestingShelby tube and block sample UCS preserves natural soil structure for reliable Su.

Sensitivity Ratio ReportedSt = undisturbed/remoulded qu — identifies sensitive clays posing construction risk.

Fast 3-Day ReportUCS test and NABL report within 3 working days.

Pan-India CoverageGeotechnical lab at all 7 NABL-accredited locations.

Combined Strength PackageUCS + Direct shear + Atterberg for complete cohesive soil characterisation.

Unconfined Compressive Strength Test(UCS — IS 2720 Part 10)