7/25/2019 Laboratory Manual - Triaxial CD Test
1/3
Aim of the experiment
To determine shear strength parameters i.e. angle of shearing
and cohesion of a given soilsample.
Theory
In triaxial test a cylindrical soil specimen is covered by a
rubber membrane, put under pressurein a confining chamber, and then
loaded on the main axis until the soil specimen fails. During
testing, several parameters are measured including the confining
pressure, the axial force, theaxial deformation, the generated pore
water pressure and the specimen volume change. The test
is repeated on similar specimen at different confining
pressures. The results are then used to
draw the Mohrs circles of each of the specimens at failure and
then to determine the shearstrength parameters: the shear intercept
and the friction angle.
There are three different types of triaxial tests:
unconsolidated-undrained (UU) tests,
consolidated- undrained (CU) tests, and consolidated-drained(CD)
tests. The difference between
all these tests is the drainage condition and they refer to
saturated soil specimens. Differentparameters are measured under
different specimen condition: a) during undrained and
unconsolidated conditions, the pore water pressure is measured;
b) during drained andconsolidated conditions, the volume change in
the specimen is measured.
Apparatus
1. Triaxial test cell with base, perspex cell and head.
Compression machine.2.
Lateral pressure assembly.
3.
Proving rings.
4. Dial gauge.
5. Rubber membranes.6.
Rubber O rings.
7. Split mould of 3.8 cm dia and 7.6 cm height.
8. Deaired water supply.9.
Porous stone.
10.
Filter paper.
11.Balance of 0.01 gm accuracy.12.
Drying oven.
13.
Stop watch.
14.Volume change burette.
15.Scale and vernier calipers
Procedure of CD Test for sandy soil specimen
First the weight of the specimen i.e., sand is obtained. Then a
split mold is used to form thespecimen during preparation. The
rubber membrane is already located to the lower palten using
the O rings. The split mold is placed around the membrane, and a
porous stone is placed on the
lower palten inside the membrane. Next, the mold is filled with
sand, which is introduced using afunnel to achieve a very loose
condition. A densely packed specimen can be achieved by
tamping moist sand. The porous stone and top cap are put in
place, and the membrane is sealed
to the top cap. Finally, the split mould is dismantled. After
that the cell is placed, bolted, andfilled with water. Pressure is
applied to the water and the vacuum inside the specimen is
released.
7/25/2019 Laboratory Manual - Triaxial CD Test
2/3
The test must be conducted following the stress path that
closely simulates the stress history of
the specimen in the field. The most common stress path consists
of applying the confiningpressure (by means of the control panel)
followed by the deviator load.
The dial indicators used for measuring the force and deformation
are zeroed. Valves connecting
the top and bottom of the specimen are kept open for drained
tests or closed for undrained testsconditions. In the simplest
case, only the dial indicator for the force and dial indicator for
the
deformations are read at predetermined increments. For more
elaborate testing, volumetricchanges must also be monitored as
indicated on the control panel, using the pipettes as well as
changes in pore pressure if they are allowed. The test ends when
the sample get failed or when
the strain exceeds 20%. The cell is unloaded and dismantled, and
the specimen is removed.
Calculations and results:
A) Specimen Data :(Specimen No.________)1) Type of test
performed :Consolidated drained (CD)
2) Type of specimen :3) Diameter of specimen, Do(mm) :4) Initial
area of specimen, Ao (mm
2) :
5) Initial height of specimen, Ho (mm):
6) Volume of specimen, Vo (mm3) :
7) Mass of specimen (gm) :
8) Wet unit weight of specimen :
9) Water content of the specimen :
10) Dry unit weight of specimen :
B) Triaxial Compression Data:
1) Consolidation pressure on test specimen, 3
(N/mm2) :
2) Rate of axial strain :
3) Initial height of specimen, H0(mm) :
4) Proving ring calibration :
