Unconfined Compression Test
Plot a graph of s1 or the deviator stress as ordinate vs. or vertical strain in % as abcissa on Cartesian graph paper. Define qu at failure; this is s1 (peak).
Triaxial Test
8
TRIAXIAL TEST
Triaxial Test
8 8 . . 1 1 I I N N T T R R O O D D U U C C T T I I O O N N
From an inspection of the triaxial apparatus, it is concluded that any soil pore-fluid state, from a negative or vacuum state to a fully saturate state with an excess pore-fluid pressure, can be obtained with this equipment. Drained or undrained conditions can be investigated. For a drained test, as the load is applied to the soil specimen, one can allow the pore fluid to escape by opening the appropriate valve. An undrained test can be performed by closing the soil system to the atmosphere so that no pore fluid can escape during the test.
8. 8 .1 1. .1 1 D D
EEFIFINNIITTIIOONNSSU U
NCNCOONNSSOOLLIIDDAATTEEDD- -
UUNNDDRRAAIINNEE TTEESSTTWhich is also called the quick test (abbreviation commonly used are UU and Q test). This test is performed with the drain valve closed for all phases of the test. Axial loading is commenced immediately after the chamber pressure σ3 is stabilized.
C C
ONONSSOLOLIIDDAATTEEDD- -
UNUNDDRRAAIINNEEDD TTEESSTTAlso termed consolidated-quick test or R test (abbreviated CU or R). In this test, drainage or consolidation is allowed to take place during the application of the confining pressure σ3. Loading does not commence until the sample ceases to drain (or consolidate). The axial load is then applied to the specimen, with no attempt made to control the formation of excess pore pressure. For this test, the drain valve is closed during axial loading, and excess pore pressures can be measured.
C C
ONONSSOLOLIIDDAATTEEDD- -
DRDRAAIINNEED D TTEESTSTAlso called slow test (abbreviated CD or S). In this test, the drain valve is opened and is left open for the duration of the test, with complete sample drainage prior to application of the vertical load. The load is applied at such a slow strain rate that particle readjustments in the specimen do not induce any excess pore pressure.
Since there is no excess pore pressure total stresses will equal effective stresses. Also the volume change of the sample during shear can be measured.
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Determination of shear parameters of soils by triaxial test.
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Scope of this test is to determine the shear parameters, strength-deformation characteristics
Triaxial Test
From an inspection of the triaxial apparatus, it is concluded that any soil pore-fluid state, from a negative or vacuum state to a fully saturate state with an excess pore-fluid pressure, can be obtained with this equipment. Drained or undrained conditions can be investigated. For a drained test, as the load is applied to the soil specimen, one can allow the pore fluid to escape by opening the appropriate valve. An undrained test can be performed by closing the soil system to the atmosphere.
8 8 . . 4 4 S S T T A A N N D D A A R R D D D D E E S S I I G G N N A A T T I I O O N N
ASTM: D2850-70 AASHTO: 234-70
Figure 8.1 Schematic diagram of triaxial test
Triaxial Test
8 8 . . 5 5 M M A A T T E E R R I I A A L L S S & & E E Q Q U U I I P P M M E E N N T T
1. Triaxial cell.
2. Strain controlled compression machine (Figure 7.1) 3. Specimen trimmer
4. Wire saw
5. Vacuum source 6. Oven
7. Evaporating membrane 8. Calipers
9. Rubber membrane 10. Membrane stretcher
Figure 8.2Triaxial test device
Triaxial Test
8 8 . . 6 6 P P R R E E P P A A R R A A T T I I O O N N O O F F S S A A M M P P L L E E S S A A N N D D T T E E S S T T S S P P E E C C I I M M E E N N
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RREPEPAARRAATTIIOONN OOFFU U
NDNDIISSTTUURRBBEED D SSAAMMPPLLEE1. Obtain a sample and using the wire saw and miter box, trim the ends parallel to each other.
2. Place the sample in the soil lathe and trim it to a circular cross-section.
3. Reposition the sample in the miter box and cut it to a length of approximately 7 cm by trimming both ends.
4. Measure the average length and diameter of the sample using the veneer caliper.
Weigh the sample.
5. Use the sample trimmings to determine water content of the clay.
8. 8 .6 6. .2 2 P P
RREPEPAARRAATTIIOONN OOFF RREEMMOOUULLDDEDEDS S
AAMMPPLELE 1. Remold the clay thoroughly.2. Reform a cylindrical specimen using the sample former, taking care not to entrap air in to the clay.
3. Extrude the sample from the mould and square its ends using the miter box.
4. Measure the sample as before.
8 8 . . 7 7 T T E E S S T T P P R R O O C C E E D D U U R R E E
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LALACCEEMMEENNTT OOFF SSPPECECIIMMEENN IINN TTHHEE TTRRIIAAXXIIAALL TTEESTSTIINNGG MMAACCHHIINNEE 1. Boil the two-porous stones to be used with the specimen.Figure 8.3Triaxial test apparatus
Triaxial Test
2. De-air the lines connecting the base of the Triaxial cell 3. Attach the bottom platen to the base of the cell
4. Place the bottom porous stone (moist) over the bottom platen.
5. Take a thin rubber membrane of appropriate size to fit the specimen tightly. Take a membrane stretcher, which is a brass tube with an inside diameter of about ¼ in (6mm) larger than the specimen diameter. The membrane stretcher can be connected to a vacuum source. Fit the membrane to the inside of the membrane stretcher, and lap the ends of the membrane over the stretcher. Then apply the vacuum. This will make the membrane form a smooth cover inside the stretcher.
6. Slip the soil specimen to the inside of the stretcher with the membrane (step5) the inside of the membrane may be moistened for ease of slipping the specimen in. Now release the vacuum, and unroll the membrane-form the ends of the stretcher.
7. Place the specimen (step 6) on the bottom porous stone (which is placed on the bottom platen of the Triaxial cell), and stretch the bottom end of the membrane around the porous stone and bottom platen. At this time, place the top porous stone (moist) and the top platen on the specimen, and stretch the top of the membrane over it. For airtight seals, it is always a good idea to apply some silicone grease around the top and bottom plates before the membrane is stretched over them.
8. Using some rubber bands, tightly fasten the membrane around the top and bottom platens.
9. Connect the drainage line leading from the top platen to the base of the triaxial cell.
10. Place the Lucite cylinder from the top platen to the base triaxial cell on the base plate to complete the assembly.
Note
1. In the Triaxial cell, the specimen can be saturated by connecting the drainage line leading to the bottom of the specimen to a saturation reservoir. During this process, the drainage line leading from the top of the specimen is kept open to the atmosphere. The saturation of clay specimens takes a fairly long time.
2. For unconsolidated undrained test, if the specimen saturation is not required, nonporous plates can be used instead of porous stones at the top and bottom of the specimen.
8. 8 .7 7. .2 2 U U
NCNCOONNSSOOLLIIDDAATTEEDD- -
UUNNDDRARAIINNEEDD TTEESSTT1. Place the Triaxial cell (with the specimen inside it) on the platform of the compression machine.
2. Make proper adjustment so that the piston of the triaxial cell just rests on the top platen of the specimen.
3. Fill the chamber of the triaxial cell with water. Applying a hydrostatic pressure, 3, to the specimen through the chamber fluid. (Note: All drainage to and from the specimen should be closed now so that drainage from the specimen does not occur).
4. Check for proper contact between the piston and the top platen on the specimen. Zero the dial gauge of the proving ring and the gauge used for measurement of the vertical
Triaxial Test
5. Take proving ring dial readings for vertical compression intervals of 0.01inch (0.254mm) initially. This interval can be increased to 0.02 inch (0.508mm) or more later when the rate of increase of loads on the specimen decreases. The proving ring readings will increase to a peak value and then may decrease or remain approximately constant. Take about four to five readings after the peak point.
6. After completion of the test, reverse the compression machine and lower the triaxial cell, and then shut off the machine. Release the chamber pressure, and drain the water in the triaxial cell. Then remove the specimen and determine its moisture content.
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1. Compute the unit strain from the deformation readings as
2. Δax = ΔL/L0 and fill in the appropriate column of the data sheet. Also compute the adjusted instantaneous area A = Ac/ (1-Δax) and place this in the appropriate column of the data sheet. The c subscript above refers to the fact that these should be the dimensions not at the start of the test, but after consolidation of the specimen.
3. Compute the axial load using the load readings. If a load (proving) ring is used, the load P is P = DR x load-ring constant where DR is the load-dial reading in units of deflection. Put these data in the appropriate column of the data sheet.
4. Compute the principal stress difference
5. (σ1 - σ3) = P/A and fill in the appropriate column of the data sheet.
6. Knowing that 7. σ1 = σ3 + (σ1 - σ3)
8. Compute the principal stress ratio: σ1/σ3
8 8 . . 9 9 R R E E S S U U L L T T S S
1. Draw a graph of the axial strain (%) vs. deviatory stress. From this graph, obtain the value of at failure ( = f)
2. The minor principal stress on the specimen at failure is 3 (i.e. the chamber confining pressure). Calculate the major principal stress at failure as
1 3
3. Draw a Mohr’s circle with 1 and 3 as the major and minor principal stresses. The radius of the Mohr’s circle is equal to Su.
Triaxial Test
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Triaxial test is a soils laboratory test to determine shear strength parameters. The shear strength of soil is needed to design foundation, slopes, tunnels, dams, and other geotechnical
Chart 8.2Showing results of triaxial test Chart 8.1 Showing stress vs strain curve 0
2 4 6 8 10 12 14 16
0 2 4 6 8 10 12 14
Stress change lb/in2
Axial strain %
Triaxial Test
The sample, which is cylindrical, is tested inside a Perspex cylinder filled with water under pressure. The sample under test is enclosed in a thin rubber membrane to seal it from the surrounding water. The pressure in the cell is raised to the desired value, and the sample is then brought to failure by applying an additional vertical stress.
One of the major advantages of the triaxial apparatus is the control provided over drainage from the sample. When no drainage is required (i.e. in undrained tests), solid end caps are used. When drainage is required, the end caps are provided with porous plates and drainage channels. It is also possible to monitor pore-water pressures during a test.
8 8 . . 1 1 1 1 P P R R E E C C A A U U T T I I O O N N S S
1. Limit the magnitude of the excess pore pressure by testing at a very slow strain rate, as it is impossible to have a state of no excess pore pressure
2. A drain test can take up to a week. For this duration, membrane leaks, soil set up and equipment corrosion can be significant.
Triaxial Test
8 8 . . 1 1 2 2 S S A A M M P P L L E E P P R R O O B B L L E E M M
SOIL TESTING LABORATORY
UNCONSOLIDATED UNDRAINED TRIAXIAL TEST (Preliminary data)
Sample No. 15 Project No. SR 2828 Boring No. B-21 Location Newell, N.C Depth of sample 3 ft
Description of Sample Reddish brown silty clay Tested by John Doe
Date 1/26/89
Moist unit weight of specimen (beginning of test) 122.7 lb/ft3
Moisture content (end of test) 16.9 (%)
Dry unit weight of specimen 105.0 lb/ft3
Initial average length of specimen, Lo 5.82 cm Initial average diameter of specimen, Do 2.50 cm
Initial area, Ao 4.91 cm2
Gs 2.78
Final degree of saturation 71.9 %
Cell confining pressure, σ3 10.0 psi
Proving ring calibration factor 6000 lb/in
Triaxial Test
SOIL TESTING LABORATORY
UNCONSOLIDATED-UNDRAINED TRIAXIAL TEST (Axial stress – strain calculation)
Specimen
1. Draw a graph of the axial strain (%) vs. deviatory stress. From this graph, obtain the value of at failure ( = f)
2. The minor principal stress on the specimen at failure is 3 (i.e. the chamber confining pressure). Calculate the major principal stress at failure as
Triaxial Test