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Soil Mechanic LaboratoryCivil Engineering DepartmentUniversity of Indonesia
Laboratory Manual - Triaxial
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9.1. INTRODUCTION
9.1.1 OBJECTIVE OF THE EXPERIMENT:
To determine the friction angle () and cohesion values () of the soil.
9.1.2 MATERIAL AND EQUIPMENT:
Triaxial Test machine unit
Vacuum source
Rubber membrane
Membrane stretcher
Tissue paper
Extruder Spatula
Can
Oven
Scale with accuracy 0.01 gr
Soil specimen
Calipers
Wire saws
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9.1.3 BASIC THEORY
Consolidation is the phenomenon of the volume shrinkage that is slowly
occurred on the fully saturated soil with low permeability due partial pore water
dissipation. The process continues until excess pore water pressure due to the increase
in total stress has completely disappeared.
The increase of the pore water pressure causing pressure gradient in pore water
which causes the transient flow of the pore water move towards to the limit of the free
flow in the soil layer.
The flow or drainage will be continued until the pore water pressure is equal to
a value that is affected by the steady ground water level positions. The final value is
called steady-state pore water pressure. In general, the values of the static pore water
pressure and the steady-state will be the same, but maybe the excess pore water
pressure. The overly decreasing of the pore water pressure into the steady-state is
called the dissipation and if all the things happen, the soil is in a drained condition.
Prior to the excess pore water pressure dissipation, the soil is in an undrained condition.
There are three kinds of Triaxial Test:
1. Unconsolidated Undrained Test
In this test, water doesn’t be allowed to flow from the soil sample. The pore
water pressure also does not be measured in this test. Hence, there is only an
UNDRAINED Shear Strength which can be determined.
The formula which is used in this test is:
()
The using on the field includes the final condition and the foundation of the
embankment, pile foundation and spread footing (shallow foundation) in thenormally consolidated soil. In this condition, the critical design condition after
surcharging (in the end of construction) the pore water pressure is huge, the
consolidation hasn’t started yet. After the beginning of the consolidation, void ratio
and the water volume is decreasing, the pressure is increasing, so the embankment
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Laboratory Manual - Triaxial
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or the foundation has more safety. In the other words, the effective pressure is
occurred.
Usage in practice:
Figure 9.1 The embankment were built quickly on the clay layer
Figure 9.2 The reservoir/Dam were built quickly without any change of the amount of water in
the soil core.
Figure 9.3 The foundation quickly placed on top of the soil.
2. Consolidated Undrained Test
In this test, the soil sample is given a normal pressure and the water is
allowed to flow from the sample. The normal pressure works until the consolidation
is over, which the change of soil sample’s content no longer happens. Then the
water path from the sample is closed and the sample is given an undrained shear
strength. The normal pressure is still working; the pore water pressure is measured
during the shear strength is given.
The formula used in this test is:
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Laboratory Manual - Triaxial
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() ( )
Usage in practice:
Figure 9.4. Embankment were elevated (2), here the consolidation has occurred in the bottom
layer (1).
Figure 9.5. Due to decline in the water level suddenly from (1) to (2) on the soil core, there is
still a pore water pressure. And there is no drainage of water out of the core.
Figure 9.6. The rapid construction of the slopes embankment.
3. Drained Test
In this test, the soil sample is given a normal pressure and the water is
allowed to flow until the consolidation is over. Then, the shear strength is given or
the shift is done drained-ly. To keep the pore water pressure persistently in zero
value, the test velocity has to be slow (this case depends on the permeability
coefficient).
The formula used in this test is:
()
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Usage in practice:
Figure 9.7. The embankment were built slowly (layer by layer) on top of the clay layer.
Figure 9.8. Reservoir / Dam with permanent water seepage.
Figure 9.9. Excavation or clay slopes, where the layer has been consolidated.
In this module, we will do the Unconsolidated Undrained Triaxial Test. The
formulas used are the following:
= The given vertical stress
= The horizontal stress
= The calibration from the proving ring
= The initial area of the soil sample
= The elongation of the initial sample
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= The initial length of the soil sample
= The reading of the maximum proving ring
From the Mohr’s Diagram, the relation between the soil friction angle, the
pressure and the shear strength can be determined:
Figure 9.10. Mohr diagram to find the value of cohesion (c) and friction angle ().
The three kind of soil sample failure can be known from the Triaxial test, as the
following:
General Shear Failure
The addition of the load on the foundation is followed by the settlement of the
foundation itself. When the loading reaches qu, the failure is suddenly occurred and it is
followed by the enlargement of the failure on the surface to the sub-surface.
Figure 9.11. The graph of the relationship q vs settlement, the peak is seen clearly
Local Shear Failure
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In other condition when the foundation can stand the load after reaching q u, even
though the surface settlement is suddenly occurred. From the graphic of the relation
between q and the settlement, the peak is not clearly seen.
Figure 9.12. The graph of the relationship q vs settlement, the peak is not seen clearly
Punching Shear Failure
The foundation that is supported by the quite loose soil after reaching qu, the
graphic of the relation between q and the settlement can be determined close to the linear.
Figure 9.13. The graph of the relationship q vs settlement, close to be linear
9.2. THE EXPERIMENT
9.2.1
PREPARATIONa. Remove the undisturbed soil samples from the tube and put it into a cylindrical mold
test (using mechanical extruder) and cut with wire saws.
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Figure 9.14. Molding process of the undisturbed test samples.
b. Flatten both ends of the soil samples in the test cylinder using a spatula. Then remove
the test sample from the test cylinder with manual extruder.
Figure 9.15. Extruding process of the test sample from the test cylinder (left) and test samples
that have been finished (right).
c. Measure the dimensions of the soil samples (L = 2-3 D).
d. Weigh the initial weight of the soil samples.
Figure 9.16. The process of weighing the test sample after it is molded.
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9.2.2 Procedure
a. Set the rubber membrane on the sample by using a mounting or installer tool:
Set the rubber membrane on the walls of the tool.
Suck the air which is exists between the membrane and the wall of the tool with a
suction pump.
Put the soil sample into the installer tool.
Remove the soil samples from the tool so that the sample wrapped in a
membrane.
Figure 9.17. The test sample that have been installed with rubber membrane.
b. Put the soil and the porous stone into the Triaxial cell, and close it tightly.
Figure 9.18. The process of installation of the test sample to the triaxial apparatus.
c. Install the triaxial cell in units of Triaxial
d. Set the speed of the decline 1-2% of the sample’s height
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e. Fill the Triaxial cell with glycerine to the brim with giving a pressure on the tube. At
the time the glycerin almost filled tubes, air in the tube removed so that the glycerin
can fulfill the cell. The function of Glycerin is to keep the pressure σ3 can be evenly
distributed throughout the cell surface and the amount of can be read on the
manometer.
For this experiment given the value:
σ3 = 0.50 kg/cm2
σ3 = 0.75 kg/cm2
σ3 = 1.00 kg/cm2
the depth of soil sample = 2.00 m
Figure 9.19. The process of filling the triaxial cell with glycerin / water.
f. Do pressing on the soil samples from the top (vertical).
g. Do the dial reading of the Load Dial every drop increasing 0.02 inch or 0025 mm.
h. Once done, the test sample is inserted into the oven to obtain the water content.
9.3. DATA PROCESSING