THE COOPER UNION
Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 11 - Triaxial Compression Test On Cohesionless Soils
To determine the shear strength of a cohesionless soil sample. 

 

1) "Soil Testing for Engineers" by T. W. Lambe; - Chapter XI. 

2) "Engineering Properties of Soils and Their Measurements" - 4th edition by Joseph E. Bowles, Experiment No. 15.

3) A.S.T.M. Standards, 1994; (4.08) Designation D4767-9.

 

1) Pressurization tank

2) Loading frame

3) Two porous stones

4) Triple beam balance (sensitive to 0.1 gm)

5) Triaxial Chamber

6) O-rings

7) Rubber membrane

8) Top cap

 

Sample Preparation

1) Remove the load ring and pull out the clutch.

2) Lubricate the base pedestal with petroleum jelly.

3) Place the porous stone on the base pedestal, and make certain that there is no air trapped between the stone and the pedestal.

4) Inspect the latex membrane for holes by filling it with water and checking for leaks, or holding it up to the light.  Utilize an acceptable membrane and carefully stretch it around and over the base pedestal.  Attach rubber bands to create a seal.

5) Take the three part metal split mold, and place it around the membrane; take care not to pinch the membrane when closing the sections together.  Finally, slide the metallic ring clamp over the mold for a tight seal.

6) Employ the vacuum pump to create a tight fit.  This, in effect, removes any air trapped between the membrane and the form.  Put the membrane around and over the sides of the mold, and stretch it out in order to eliminate wrinkles.

7) Adhere to propoer mixing instructions (Synthetic Industries guidelines).  A small amount of water is introduced to facilitate better mixing of the soil and fibers.  Having completed this procedure, weigh and record the wet mass of a container with a mixed sample of sand and fiber.  The combined weight of mixture used, along with the known volume of the cylinder will provide the relative density by formula for the chosen percentage of fiber.  The dry unit weight of soil used may be determined upon completion of the test.

8) Place the sand-fiber mixture into the membrane using a series of teaspoons and tamps to achieve the desired relative density.  Do NOT use a vibrator technique to densify the soil, because that may cause upward migration of the fibers.

9) Put the block on top of the sample, and pull the membrane up  and over the block.  Secure it with rubber bands, while taking care to avoid wrinkles.
 

Experimental Procedure

1) The preparation of the specimen occurred directly within the loading machine.  Ensure that the top cap is in place over the porous stone, and that no air is trapped between these two pieces.  Using a level bubble on top of the cap, verify that the soil specimen is level.  This is very important to the axial longitudinal load.

2) In order to remove the mold from the sample, a partial vacuum must be applied by lowering the water in the burette approximately two (2) feet below its initial level.  This lowering creates a perceived "negative" pore pressure inside the sample, though the actual pressure is only a positive value less than atmospheric. 

3) Having lowered the pressure, carefully remove the mold from around the sample.  Note that the sample is acted upon on the surfaces of the impervious rubber membrane by an external pressure equal to the difference between atmospheric pressure outside, and an internal pressure of whatever amount below atmospheric the burette level has caused.

4) Using a pair of calipers, obtain the diameter of the sample to the nearest 0.5mm by taking measurments at the top, middle, and bottom, and averaging the results.  Also, measure the length of the sample to the nearest 0.5mm, between the two porous stones in at least three different places.  From these values, compute the initial area and volume.  Record these values on Data Sheet A.

5) Place the plastic triaxial chamber cylinder on the base of the triaxial equipment.  The soil sample will be entirely enclosed in this cylinder.  Carefully place the loading head on the cylinder, and put into place all the tie rods.

6) Attach the longitudinal deformation gauge into the loading head of the triaxial cylinder and its mounting, which will measure the movement of the loading piston after it contacts the top cap of the soil sample when the test begins.  Now place the piston into the loading head, making sure that it is seated properly in the cap which is, of course, on top of the soil sample.  Note, the piston should be well-lubricated, so as to move freely and have a proper fit.  A sufficient length of piston should be protruding above the top of the loading head to allow for the maximum longitudinal deformation anticipated for this soil; e.g, if the maximum strain is 20% (i.e., ez = 20% or 0.20), and the sample is six inches long, then a minimum of 1.20 inches of piston rod must be protruding.

7) At this time the load ring, which will measure the axial loading to the sample, should be in place in the machine.  Raise the loading table to a position where the piston of Step 6 is just in contact with the load ring.  Be sure to record the load ring number on Data Sheet A.

8) Tighten the nuts on all the tie rods, assuring a tight fit.  Make sure to perform this tightening with great care because too much shaking might disturb the soil sample.  Once the nuts are tight, readjust the piston rod in contact with the load ring.

9) The confining pressure may now be applied to the soil sample.  This will be done by pressurizing a tank of water or glycerine.  The fluid will be forced into the chamber surrounding the sample, and the pressure on the fluid in the tank is tranmitted to the fluid in the triaxial chamber, which is in turn transmitted to the soil sample. For example, if 30 p.s.i. of pressure is applied to the water in the tank, the water forced into the triaxial chamber is at 30 p.s.i., and therefore the soil is subjected to a confining pressure of 30 p.s.i.  During this entire process of filling the triaxial chamber with the pressurized fluid, the pitcock at the top of the loading head remains open.   When the triaxial chamber is completely filled with fluid, the pitcock is closed to prevent the fluid from pouring out.

10) While Step 9 is being performed, it is imperative that the piston remain in contact with the load ring at all times, otherwise the pressure in the triaxial chamber will push the piston out.   When the chamber is completely filled with water or glycerine, the laod ring should be adjusted to read zero.  It will, in the process of filling the chamber with water or glycerine, attain some small reading.  This can be attributed to the following: the underside of the pistom rod will be acted upon by a force equal to the chamber fluid pressure multiplied by the cross-sectional area of the piston rod.  The sample is now ready for testing.  Note: A reading on the burette must be taken both before and after the triaxial chamber is filled with pressurized fluid to determine the initial volume change.  Record this on Data Sheet A.

11) The burette may now be slowly raised to a much higher elevation and filled with water up to the 15 ml. mark.  This burette can be raised as soon as the chamber pressure is applied.  [Note: This procedure assumes a burette with numbers increasing down the side.   Adjust accordingly for burettes numbered oppositely.]  The 50 ml. mark, which is near the bottom of the burette, should be at about the elevation of the center of gravity of the soil sample.  Make sure that there are no air bubbles in the burette at this time because erroneous volume change results will  be recorded.  The burette is now capable of being raised because a partial vacuum is no longer needed to keep the soil sample erect.  This is now being done by the confining pressure.  [Aside: For a burette with the numbers increasing downward, the following is true: When a sample is decreasing in volume, the water level in the burette will rise.  This is due to the fact that water is passing from the sample into the burette.  For example, the burette reading might go from 13 ml. to 12 ml., indicating a volume change of 1 ml.   This volume change is given a negative sign.  Conversely, if a sample increases in volume, the level of water in the burette will decrease.  This indicated that water is passing from the burette in to the sample.  For example, the burette reading might go from 25 ml. to 27 ml., indicating a volume change of 2 ml.  This is a positive volume change.]

12) Set the loading machine to a strain rate of 0.02 inches per minute.  The axial load is taken from the load ring readings.  The axial deformation is to be calculated, based upon the readings obtained from the longitudinal deformation gauge.  A reading on the strain gauge should be taken every 0.02 inches.  Record these values on the Data Sheet.  Also determine the volume change by reading the burette and record this as well.  Note that this is a constant rate of strain test.

13) Continue to load the soil specimen until one of two things occurs: either failure of the specimen is obtained, or the test is well beyond the peak stress.  In loose sands, failure is denoted by a bulging of the sample; the load ring readings remain constant and the volume changes are relatively small.  In dense sands, failure is denoted by definite failure or fracture planes, and the load ring readings fall off after a peak.  This is a brittle failure.

14) After failure has occurred, lower the burette back to its position in Step 2, and back off the chamber pressure to zero.  Open the pitcock at the top of the loading head, and at this time the fluid in the triaxial chamber will drain back into the reservoir tank.   When all the fluid is drained, remove the tie rods, loading head, and plastic triaxial chamber cylinder.  The tested specimen is now exposed and under a partial vacuum from the lowered burette.

15) Remove the membrane and the soil from the base pedastal, making sure that all the sand is removed from it.  Rinse all of the sand into a large evaporating dish from the o-rings and porous stones, and thoroughly rinse the membrane to loose any remaining particles.   Drain off as much water a possible from the evaporating dish.  Record the number and tare of the evaporating dish, then place the sample and dish into the oven until the next lab session.  At that time, remove the dried specimen and weigh the dish and soil to the nearest 0.1 g on a triple beam balance.  Record all weights on the Data Sheet.

 

1) Determine the specific gravity, and the maximum and minimum dry densities.  Also determine the dry unit density and relative density of the sample.

2) Compute the strains, ez = DL/L0 X 100%, and the axial load, P, by multiplying the load ring reading by the calibration value.

3) Compute the instantaneous area of the sample, Ai, the deviator stres, p = P/Ai, in p.s.i., and the total axial stress s1 = p + s3, the chamber pressure.

4) Compute the ratio, s1/s3, the unit volume strain, d = DV/V0 x 100%, and the initial void ratio, ei.

 

1) On cartesian paper, plot the stress-strain curve for the sample by plotting the total axial stress, s1, as the ordinate and the unit axial strain, ez, as the abcissa.

2) On cartesian paper, plot the unit volume strain, d, as ordinate versus the unit axial strain, ez, as abcissa

3) From these the peak and ultimate total axial stress drawn from the plot in Step 1, plot Mohr's Circle of Stress for the sample, by plotting the total axial stress s1 on the ordinate, and the chamber pressure, s3, on the abcissa, which is the axis for the normal stress.

4) From the plot in Step 4, plot two items: the angle of internal friction, F, as the ordinate versus the relative density, Dd, as abcissa, and the tangent of F as ordinate and Dd as abcissa.  This is done for both peak and ultimate F angles.

5) Next, plot the curve of ei as ordinate versus d as abcissa and s3 as variable.  This is to obtain a void raitio at zero percent unit volume strain for various chamber pressures.   These void ratios are known as the critical void ratio.  The same type of curve may be drawn with relative density and obtaining critical relative densities.

6) Finally, draw two plots from the data obtained in Step 5: one of critical void ratio as ordinate versus chamber pressure as abcissa, and another with the critical relative density as ordinate versus the same abcissa.

 

 

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