Determining the Unconfined Compressive Strength of Cohesive Soil - Soil Properties: Testing, Measurement, and Evaluation - page 332

The primary results of unconfined compressive strength tests are
the values of unconfined compressive strength q u and cohesion c . A test
report should also include, however, (1) an identification and visual de-
scription of the soil, (2) the type (undisturbed, compacted, or remolded)
and shape (cylindrical or prismatic) of the test specimen, (3) the dimen-
sions, height-to-diameter ratio, initial density, and water content of the
test specimen, and (4) average rate of strain to failure (%/min).
Normally, a graph of load per unit area versus unit strain is also pre-
sented, and sketches of the failed specimen might be included as well.
Because the ability of soil to support imposed loads is determined
by its shear strength, the shear strength of soil is important in founda-
tion design, lateral earth pressure calculations, slope stability analysis,
and many other considerations.
There are several methods of investigating the shear strength of
a soil in a laboratory, including the (1) unconfined compression test,
(2) triaxial compression test, and (3) direct shear test. As discussed pre-
viously, unconfined compression tests can generally be used only to in-
vestigate cohesive soils and provide approximate values of the strength
of these soils in terms of total stresses. Triaxial tests, described in
Chapter 22, and direct shear tests, discussed in Chapter 23, can be used
to investigate both cohesive and cohesionless soils.
Some typical values of the shear strength of cohesive soil (i.e.,
cohesion) are given in Table 21-1.

Table 21-1 Typical Values of Shear Strength of Cohesive Soil [3]

Shear Strength
(Half of Unconfined Compressive Strength)

(lb/ft 2 )

Consistency of Clay

Very soft 250

Soft 250-500

Medium 500-1,000

Stiff 1,000-2,000

Very stiff 2,000-4,000

Hard 4,000

ASTM 2007 Annual Book of ASTM Standards , West Conshohocken,
PA, 2007. Copyright, American Society for Testing and Materials, 100 Barr
Harbor Drive, West Conshohocken, PA 19428-2959. Reprinted with
permission.

[1]

REFERENCES

[2] Joseph E. Bowles, Engineering Properties of Soils and Their Measurement ,

2d ed., McGraw-Hill Book Company, New York, 1978.

[3] T. William Lambe, Soil Testing for Engineers , John Wiley & Sons, Inc.,

New York, 1951.