Triaxial Shear Test-UU (IS-2720-Part-11)

Aim

To determine shear strength parameters of the given soil sample by conducting unconsolidated undrained (UU) triaxial shear test.

Theory And Application

Triaxial testing is a test used to measure the shear parameters of a given soil. The test is performed on a cylindrical soil/rock samples. This test is considered to be the most conveniently available conditions to suit the field situations.

Apparatus Required

• Triaxial testing machine complete with triaxial cell
• Water pressure unit with hand pump
• Provong ring
• Dial gauge
• Rubber membranes
• Membrane stretcher
• Sample trimming apparatus
• Bins for moisture content determinations
• Balance and box of weights
• Drying oven

Procedure

1. Trim the soil specimen (prepared from the sampling tube of an undisturbed sample tube using universal extractor frame or from a compacted soil specimen as per standard proctors method, at optimum moisture content or any other moisture content to suite the field situations). Using the trimming apparatus if necessary the trimmed specimen should be 76.2 mm long and 38.1 mm in diameter. The diameter and the length are measured at not less than 3 places and the average values are used for computation. Note the weight of the specimen (W1).
2. The specimen is then enclosed in a 38.1 mm diameter and about 100 mm long rubber membrane, using the membrane stretcher. Spreading back the ends of the membrane over the ends of the stretcher and applying suction between the stretcher and the rubber membranes does by inhalation. The membrane and stretcher are then easily slide over the specimen, the suction is released and membrane is unrolled from the ends of the stretcher.
3. Use non-porous stones on either side of the specimen as neither any pressure is to be measured nor any drainage of air or water is allowed.
4. Remove the porous cylinder from its base removing the bottom fly nuts.
5. The pedestal at the centre of the base of the cylinder on which the specimen is to be placed is cleaned and a 38.1 mm diameter rubber O-ring is rolled over to its bottom. The specimen along with the non-porous plate on either side is centrally placed over the pedestal and the bottom edge of the machine covering the specimen is sealed against the pedestal by rolling back the O-ring over the membrane.
6. The cap is placed over the top plate of the specimen and the top of the rubber membrane is sealed against the cap by carefully rolling over it another O-ring. This arrangement of rubber O-ring forms the effective seal between the specimen with the membrane and the water under pressure. The specimen is checked for its verticality and co-axiality with the cylinder chamber.
7. The chamber along with the loading plunger is carefully placed over its base without disturbing the soil specimen and taking care to see that the plunger rests on the cap of the specimen centrally. The loading frame is then adjusted so that it just touches the plunger top by naked eye. The chamber is then rotated if necessary such that the dial gauge, recording compression, rests centrally over the top of the screw which can be locked at any level and which is attached to the top of the cylinder chamber carrying the specimen. The cylinder is then attached to the base plate tightly by means of tightening the nuts.
8. The valve to drain out the chamber and the valve to drain out the air and water from the sample are closed and the air lock nut at the top of the cylinder is kept open to facilitate the exit of air as water enters the chamber through another valve which connects the chamber to the water storage cylinder subjected to a pressure by a hand pump or by any other means.
9. The water storage cylinder is filled with water completely and its top is then closed by means of a valve. Necessary pressure is built up in the cylinder by working the hand pump and the pressure communicated to the cylinder where the specimen is placed, by opening the connecting valve. The cylindrical chamber is allowed to be filled up completely which is indicated by the emergence of water through the air lock nut at the top of the chamber. Then the airlock nut is closed to develop necessary confining pressure by using the hand pump (or by any other means) and the same is maintained constant.
10. If necessary, bring the loading plunger down until it is in contact with the specimen top cap by means of hand operated loading device. This is indicated by a spurt in the reading of the proving ring dial gauge.
11. For this position, adjust the deformation dial gauge reading to zero.
12. Record the initial reading of the proving ring and compression dial gauge.
13. The vertical load is applied to the specimen by starting the motor at the loading frame. The change in the proving ring dial gauge gives the measure of the applied load. The deformation dial gauge gives the deformation in the soil specimen, which can be used to compute strain in the soil.
14. Take readings of proving ring dial gauge at 0.5, 1.0, 1.5, 2.0% (or any other smaller values) of strain and for every 1.0% strain thereafter up to failure or 20% strain whichever is earlier.
15. Throughout the test, make sure that the chamber, containing pressure is kept constant at the desirable value as indicated by the pressure gauge on the water cylinder. If necessary, the pressure can be made good for any possible losses by working the hand pump.
16. After specimen has failed or 20% strain is recorded, as the case may be (a) stop application of load (b) disconnect the chamber from water storage cylinder by closing the linger valve (c) open the air lock knob a little and (d) open the valve to drain out the water in the cylinder. After a few seconds open the airlock nut completely to facilitate quick draining out of water, by entry of air at top of the cylinder.
17. After the water is completely drained out, take out the cylinder from loading frame carefully, loosen the nuts and remove the Lucite cylinder from ts base, without disturbing the sample.
18. Note the space of the failed specimen, angle of shear plane if any and dimensions of the specimen.
19. Wipe the rubber membrane dry and find its weight W2 that should be same as W1.
20. Remove the membrane from the specimen and take a representative specimen preferably from the sheared zone.
21. Repeat the test with three specimens of the same soil sample subjected to three different lateral pressures (confining) of 0.5, 1.0 and 1.5 kg/cm² (5, 10 and 15 psi or 50, 100 and 150 kpa)

Calculations

1. Axial strain=ΔL/L=change in length/initial length. This is expressed as a % for convenience.
2. The stress intensity applied vertically is obtained by dividing the load, P by the cross-sectional area of the specimen. At any time when axial strain is e. area = A₀/(1-e) where A0 is the initial cross-sectional area of the sample. The intensity of stress = P/A. as the sample is enclosed in a rubber membrane and is sealed at either end, its volume is constant as no air or water can escape. So as the length decreases due to compression, area should increase which is assumed to be uniform. Therefore, A=A₀/(1-e)

Results

1. A graph is drawn between the deviator stress and strain. The deviator stress is the difference between the stresses in axial and radial direction i.e. (σ₁-σ₃) and is equal to the vertical stress P/A. σ₃ is the lateral confining pressure at any time, which is constant for a test. From the plot, determine the second result at half the ultimate stress, which can be taken as modulus of elasticity.
2. The mohr’s circle of stress to define the state of stress at failure is drawn for each sample. The circle has for its centre point (σ₁+σ₃)/2 and the radius equal to (σ₁-σ₃)/2. An envelope, which approximates to a straight line, is drawn touching the circle. The intercept made on Y-axis and the slope of the envelope gives the values of strength parameters of the soil C and φ respectively.