prak tiku mtr i axial

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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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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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() ( )

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

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