UNCONFINE COMPRESSION TEST
ASTM D2166, BS EN ISO 17892
1. Scope*
1.1 This test method covers the determination of the unconfined
compressive strength of cohesive soil in the intact,
remolded, or reconstituted condition, using strain-controlled
application of the axial load.
1.2 This test method provides an approximate value of the
strength of cohesive soils in terms of total stresses.
1.3 This test method is applicable only to cohesive materials
which will not expel or bleed water (water expelled from the
soil due to deformation or compaction) during the loading
portion of the test and which will retain intrinsic strength after
removal of confining pressures, such as clays or cemented
soils. Dry and crumbly soils, fissured or varved materials, silts,
peats, and sands cannot be tested with this method to obtain
valid unconfined compression strength values.
4. Summary of Test Method
4.1 In this test method, a cylindrical soil specimen is
unconfined laterally while loaded axially at an axial strain rate
between 0.5 to 2 %/min. Measurements are made of elapsed
time, axial deformation, and axial load. The unconfined compressive
stress, qu, is calculated as the compressive stress at
failure. The shear strength, su, is one half of the unconfined
compressive strength.
5. Significance and Use
5.1 The primary purpose of the unconfined compression test
is to quickly obtain a measure of compressive strength for
those soils that possess sufficient cohesion to permit testing in
the unconfined state.
5.2 Samples of soils having slickensided or fissured
structure, samples of some types of loess, very soft clays, dry
and crumbly soils and varved materials, or samples containing
significant portions of silt or sand, or both (all of which usually
exhibit cohesive properties), frequently display higher shear
strengths when tested in accordance with Test Method D2850.
Also, unsaturated soils will usually exhibit different shear
strengths when tested in accordance with Test Method D2850.
8. Procedure
8.1 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 deformation 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 7).
Continue loading until the load values decrease with increasing
strain, or until 15 % strain is reached. Indicate the rate of strain
in the report of the test data, as required in 10.3.6. Determine
the water content of the test specimen using the entire
specimen, unless representative trimmings are obtained for this
purpose, as in the case of intact specimens. Indicate on the test
report whether the water content sample was obtained before or
after the shear test, as required in 10.3.1.
NOTE 7—Softer materials that will exhibit larger deformation at failure
should be tested at a higher rate of strain. Conversely, stiff or brittle
materials that will exhibit small deformations at failure should be tested at
a lower rate of strain.
8.2 Make a sketch, or take a photo, of the test specimen at
failure showing the slope angle of the failure surface if the
angle is measurable.
8.3 A copy of a example data sheet is included in Appendix
X1. Any data sheet can be used, provided the form contains all
the required data.