cvg3109_lab_consolidated drained tests for sand_2014

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



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.



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



Placement of a Sand Specimen in the Triaxial Cell

1 2

3 4

5 6

7



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.



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.



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.



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'



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



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)

cvg3109_lab_consolidated drained tests for sand_2014

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