Determining the Unconfined Compressive Strength of Cohesive Soil - Soil Properties: Testing, Measurement, and Evaluation - page 321

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CHAPTER TWENTY-ONE

Determining
the Unconfined
Compressive Strength
of Cohesive Soil

(Referenced Document: ASTM D 2166)

The unconfined compressive strength of cohesive soil, q u , is defined as the
load per unit area at which an unconfined prismatic or cylindrical speci-
men of soil will fail in a simple compression test. It is taken to be the max-
imum load attained per unit area or the load per unit area at 15% axial
strain, whichever is secured first during the performance of a test [1].
The unconfined compression test is perhaps the simplest, easiest,
and least expensive test for investigating the approximate shear
strength of cohesive soils in terms of total stress. (The reason the test is
usually limited to cohesive soils is that there is no lateral support, and
the soil sample must be able to stand alone. A cohesionless soil such as
sand cannot generally stand alone in this manner without lateral sup-
port.) The soil parameter cohesion c is taken to be one-half the uncon-
fined compressive strength (i.e., c = q u /2).

INTRODUCTION

Compression device (including deflection dial; see Figures 21-1
and 21-2)
Sample ejector, for ejecting the soil core from the sampling tube
Dial comparator or other suitable device, for measuring the speci-
men's physical dimensions
Timing device

APPARATUS
AND SUPPLIES

321