Matest catalogue 2018

SECTION S | SOIL
543
TRIAXIAL TESTS
STANDARDS: BS 1377:8 | ASTM D2850, ASTM D4767, ASTM D7181 | NF P94-070, NF P94-074 | CEN-ISO | TS 17892-8.9
Introduction
A knowledge of stress-strain behaviour and the shear strength parameters of soils is required when soil is interacting with structures or
when soil is used as a construction material in many engineering purposes such as:
1. Excavations
2. Shallow foundations
3. Piles and deep foundations
4. Earth retaining structures, diaphragm walls, anchors
5. Slope stability
6. Ground improvement
7. Design of embankments, earth dams
The most widely used testing apparatus for investigating the stressstrain
behaviour and the strength parameters of soils is the triaxial
apparatus.
Triaxial tests are typically performed with two stages: an isotropic
loading followed by shear loading which is carried out up to failure.
A cylindrical saturated soil sample, undisturbed or reconstituted,
is placed in a rubber membrane in order to isolate it from direct
contact with the surrounding water with which the testing cell is
filled, and which is pressurized. The sample sits in the cell between
a rigid base and a rigid top cap and is loaded by means of a ram, at
a constant speed. The water drainage in or out the sample can be
allowed by means of opening or closing a valve.
The two phases are carried out under different combinations of
drainage conditions and give rise to 3 different standard triaxial
tests. Each test is usually performed on three saturated specimens
at three different confining pressures.
“CU” consolidated undrained test
This test is used in order to estimate the drained and undrained
shear strength parameters (effective cohesion c’, effective friction
angle φ’, and undrained shear strength cu). It is usually performed
on fine grained soils. In this test, the first phase is carried out
with an open drainage valve in order to allow the consolidation and
volume change to occur. During the second phase, the drainage
valve is closed and the change of pore water pressure is measured;
there is no volume change and distortions occur up to failure. It
gives three values of undrained shear strength, which are the radii
of the Mohr’s circles, and the values of the effective cohesion and
of the effective friction angle, which are derived from the envelope
of the effective stress Mohr’s circles. The stress-strain behaviour
and the strength parameters are then used to model engineering
problems when undrained conditions are present, after consolidation
processes or in long term applications.
CELL PRESSURE
SAMPLE
DRAINAGE
PORE PRESSURE
“UU” unconsolidated undrained test
This test is used in order to estimate the undrained shear strength
parameters (undrained shear strength cu and undrained friction
angle φu = 0). It is usually performed on fine grained soils. In this
test, both phases are carried out with the drainage valve closed. No
volume change is allowed during the test and distortions during the
shear phase occur up to the failure. This test gives a unique value
of undrained shear strength, as the envelope of the Mohr circles
plotted in total stresses is horizontal. The angle φu has to be zero
or it is an error in the test, e.g. poor saturation.
The stress-strain behaviour and the strength parameters are then
used to model engineering problems when undrained conditions
are present, typically in short term design.
“CD” consolidated drained test
This test is used in order to estimate the effective shear strength
parameters (effective cohesion c’, effective friction angle φ’). It is
usually performed on coarse grained soils. In this test, both phases
are carried out with the drainage valve open. Volume change occurs
in both phases and during the second phase, distortions occur up
to failure. The CD test gives the values of the effective cohesion and
of the effective friction angle, which are derived from the envelope
of effective stress Mohr’s circles. The stress-strain behaviour and
the strength parameters are then used to model engineering problems
when drained conditions are present, typically in long term
design.