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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 1
Consolidated Drained Triaxial Shear Test for Sand
Objectives
To determine the effective shear strength parameters, c and f of
sand in the laboratory using the triaxial shear test (i.e., under
consolidated drained loading conditions).
Background
Triaxial shear test is a sophisticated, but reliable testing
method to determine the shear strength of all kinds of soils
simulating any loading and drainage condition.
It is basically a shear test in which a cylindrical soil
specimen, typically with length/diameter ratio of equal 2, confined
by a pressurized fluid, and is stressed additionally in one axis
until the sample is loaded to failure conditions.
Failure occurs along the weakest plane whose angle, when
measured from the horizontal, is
termed The pressure in the longitudinal or vertical axis is
called (major principal stress)
and is equal to (minor principal stress; confining pressure) at
the beginning of the test. The sample is stressed by additional
loading (i.e., deviator stress) in the vertical axis through a cap
on top of the sample. The incrementally increasing load divided by
the cross-sectional area of
the sample results in the deviator stress, . Therefore = +
3
3 } 1 = minor principal stress
(Cell pressure, confining pressure)
d = principal stress difference
(deviator stress)
d
1 = major principal stress
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 2
Triaxial Test Types
In general, with triaxial equipment, three types of tests can be
conducted:
Type of Test Parameters Determined
Unconsolidated-undrained (U-U) cu (u = 0)
Consolidated-drained (C-D) c,
Consolidated-undrained (C-U) c, c, , , and A
where cu = undrained cohesion of soil specimen
c = effective cohesion
= effective internal friction angle
A = the pore water pressure parameter
Note: = c + tan (c = cohesion, = effective normal stress) for
drained condition
u = 0 ; = cu for undrained condition
Today, we will be conducting consolidated drained triaxial test
in the laboratory on a sand specimen.
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 3
The Triaxial Cell
The above figure provides a schematic diagram of a triaxial cell
design. The apparatus utilizes a cylindrical sample that is placed
inside a cylindrical cell and sealed at both ends and pressurized.
One end has a pedestal on which the sample, topped with a solid
cap, is placed and sealed with a rubber membrane. Through the other
end a ram enters the cell to place a load on the sample causing it
to deform. The following figure illustrates details of the actual
triaxial testing machine used in the experiments.
Ram Clamp
Hold Down Bolt
Cap Drainage Tubes
Cell Valve
Bottom Drainage Valves
Ram or Piston Dial Gage Clamp
Cap Drainage Tube Clamp
Top Drainage Valves
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 4
Placement of a Sand Specimen in the Triaxial Cell
1 2
3 4
5 6
7
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 5
1. Obtain a triaxial base, install a transducer, saturate all
lines, and valves while checking the flow to ensure that no
blockage exists.
2. Apply a saturated porous stone and a rubber membrane over the
pedestal of the base and secure with two rubber O rings. The
pedestal is the clear plastic cylinder secured to the base.
3. Surround the membrane and pedestal with the three-part sample
former and secure with hose clamps.
4. A filter cloth is placed between the membrane and sample
former and the top of the membrane is pulled over the top of the
sample former.
5. Connect a vacuum line to the sample former. One of the
sections of the former has a hose connected to it to allow a vacuum
source to be connected to it to allow the air around the membrane
and inside the former to be withdrawn in order to create a firm
membrane. The filter was added to prevent the membrane from
blocking the vacuum hole and to ensure the removal of all the
air.
6. Connect a one meter length of tubing to one of the base
lines. This tubing will be used to create a negative pressure
within the sample.
7. Completely fill the membrane, sample valves and one meter
line with water and turn off valves.
8. Place a prescribed amount of soil (usually 265 g for a loose
sample and 270 g for a dense sample) within a volumetric flask and
add water to full. Swirl Flask to release most air. Fill flask with
water completely to the top.
9. Place a piece of paper over the top of the volumetric flask
and invert the flask. Place the neck of the flask just below the
top of the water within the membrane and withdraw the paper. Allow
all of the sand within the flask to fall into the membrane and the
quickly re-invert the flask without spilling much water.
10. Level off the sand at the top and place a cap on the sand.
Draw the membrane from the former to cover the cap and secure with
two rubber O rings.
11. Saturate the sample line by allowing water to slowly flow
through the lines and secure to the top of the sample cap. Turn cap
valve off.
12. Turn on the lower sample valve to which the meter length of
tube is attached. A small amount of water should exit and then the
flow should stop. Close the lower sample valve.
13. Remove the vacuum line from the sample former.
14. Remove the sample former (At this point the atmosphere is
providing a confining pressure to hold the sample together) and
measure the dimensions of the sample plus the apparatus or
membrane.
15. Assemble the cell, cell bolts, cell top and ram. Introduce
the ram through the cell top and into the sample cap. When the ram
in contact with the bottom of the hole in the sample cap, lock the
ram in place.
16. Fill the cell with water and place the apparatus in the
testing frame. Saturate and connect the cell and sample lines to
the appropriate valves.
17. Adjust and apply a cell pressure of 210 kPa.
18. Adjust back pressure to 200 kPa and open valves to
sample.
19. Adjust and turn on the volume measuring devise.
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 6
20. Adjust the load ring over the ram leaving a gap of about 2
to 3mm.
21. Add a dial gauge to the ram. Compress the gauge slightly,
lock and then read.
22. Release the ram by turning the ram Lock.
23. By hand, rotate the loading mechanism, taking readings of
load. The readings should stabilize at some value and then as the
Ram touches the sample the readings will begin to rise abruptly. As
the readings begin to rise from the plateau reading, stop the
loading process and return the reading just to the plateau
value.
24. Take and record the reading on the deformation dial
gauge.
25. Prior to loading the specimen, determine the B parameter.
See next section for more details
26. Take all initial readings and the turn on the motor and
engage the auto loading system of the testing apparatus to begin
the test.
27. Take readings of Load, Deformation and Volume at regular
time intervals as per lab supervisor instructions and record the
readings on the sheet provided.
28. Continue the test for at least 10% strain and stop the test
when the load becomes stable or begins to decrease but do not
exceed 15% strain.
29. Turn off all valves, disconnect lines, disassemble cell,
remove sample, and cleanup.
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 7
Determination of the Pore Pressure Parameter, B
When measuring changes in the internal pore water pressure of a
sample it is important that the sample be fully saturated. Since
air is compressive gas, it would expand or contract with changing
pressures and thus alter the pore water pressure response. To
evaluate the pore water pressure response of a sample, a parameter
known as B value is determined. The B value is a measure of
response in the pore water pressure to an externally applied stress
to the sample.
1. Consolidate the sample at the desired cell and back
pressure.
2. Read the sample transducer pressure.
3. Turn off all valves to the sample to prevent drainage.
4. Note the cell pressure.
5. Elevate the cell pressure (usually 50 to100 kPa)
6. Read the pore pressure.
7. Calculate B as follows:
3
uB
Where: u = change in pore water pressure
3 = change in cell pressure
8. Re-establish the former cell pressure of 110 kPa. Caution: If
pressure is allowed to drop below 110 kPa, the sample may
collapse.
9. Note: For saturated soft soils is approximately 1. For
saturated stiff soils may be less than 1. A triaxial test specimen
with a B value of 0.95 or greater is usually considered
saturate.
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 8
Typical results for triaxial test
General Comments
1. For saturated sandy soils, the effective cohesion is
generally 0.
2. The specimens have to be sheared at relatively low shear rate
such that no excess pore-water pressure is generated during
shearing stage.
0
4
8
12
0 4 8 12 16
Axial Strain , (%)
(lb
/in
.2)
315 lb/in.2
1 = 15 + 11.6 lb/in.2
f 11.6 lb/in.2
p' = ( '1+ '3)/2 (kPa)0 100 200 300 400 500
q =
(1-
3)/
2 (
kP
a)
0
20
40
60
80
100
120
w = 16.4%
w = 21.5%
w = 27.0%
) ave = 11.6
mave
= 4.0 kPa
c'ave
= 4.07 kPa
'ave
= 11.8
19
She
ar s
tre
ss,
'
Normal stress, c1a3 b3 b1a1 c3
a1
a3 u
b1
b3 u
c1
c3 u
(at failure a1 < b1 < c1)
u = 0 : =
c'
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh 9
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh
10
Table 1. Consolidated-Drained Triaxial Test Preliminary Data
Description of soil: ______________________________________
Specimen No. _________
Sample Identification:
_____________________________________________________________
Location:
_____________________________________________________________
Tested by: ________________________________ Date:
_______________________________
Item Quantity
1. Initial average length of specimen, L0
2. Initial average diameter of specimen, D0
3. Initial area, 204
A D
4. Specific gravity of soil solids, GS
5. Pore Pressure Parameter,
3
uB
6. Cell confining pressure, 3
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CVG 3109: Soil Mechanics Laboratory
Geotechnical Laboratory, University of Ottawa, Won Taek Oh
11
Table 2. Consolidated-Drained Triaxial Test Axial Stress-Strain
Calculation
Specimen deformation,
L
Vertical strain,
0
L
L
Piston load,
P
Corrected area,
1
oc
AA
Deviatory stress,
P
A
(1) (2) (3) (4) (5)