Determining the Unconfined Compressive Strength of Cohesive Soil - Soil Properties: Testing, Measurement, and Evaluation - page 324

sample of trimmings for this purpose, placing them immediately
in a covered container. The water content determination shall be
performed in accordance with Method D 2216.

(3) Remolded Specimens —Specimens may be prepared either from
a failed undisturbed specimen or from a disturbed sample, provid-
ing it is representative of the failed intact specimen. In the case of
failed intact specimens, wrap the material in a thin rubber mem-
brane and work the material thoroughly with the fingers to assure
complete remolding. Avoid entrapping air in the specimen. Exercise
care to obtain a uniform density, to remold to the same void ratio as
the undisturbed specimen, and to preserve the natural water con-
tent of the soil. Form the disturbed material into a mold of circular
cross section having dimensions meeting the requirements of (1).
After removal from the mold, determine the mass and dimensions
of the test specimens.

(4) Reconstituted Specimens —Specimens shall be prepared to the
predetermined water content and density prescribed by the indi-
vidual assigning the test (Note 2). After a specimen is formed, trim
the ends perpendicular to the longitudinal axis, remove from the
mold, and determine the mass and dimensions of the test specimen.

Note 2 —Experience indicates that it is difficult to compact,
handle, and obtain valid results with specimens that have a
degree of saturation that is greater than 90%.

The unconfined compression test is quite similar to the test for com-
pressive strength of concrete, where crushing concrete cylinders is car-
ried out solely by measured increases in end loadings. A cohesive soil
specimen, prepared as previously described, is placed in a compression
device (Figures 21-1 and 21-2) and subjected to an axial load, applied
to produce axial strain at a rate of 1 ⁄ 2 to 2%/min. The resulting stress and
strain are then measured. The unconfined compressive strength is
taken as the maximum load attained per unit area or the load per unit
area at 15% axial strain, whichever is secured first during the test.
The actual step-by-step procedure is as follows (ASTM 2166-06 [1]):

PROCEDURE

(1) Place the specimen in the loading device so that it is centered
on the bottom platen. Adjust the loading device carefully so that the
upper platen just makes contact with the specimen. Zero the de-
formation indicator or record the initial reading of the electronic
deformation device. Apply the load so as to produce an axial strain
at a rate of 1 ⁄ 2 to 2%/min. Record load, deformation, and time values
at sufficient intervals to define the shape of the stress-strain curve
(usually 10 to 15 points are sufficient). The rate of strain should be
chosen so that the time to failure does not exceed about 15 min
(Note 3). Continue loading until the load values decrease with
increasing strain, or until 15% strain is reached. Indicate the rate