Triaxial Shear Test

1. objective

The standard consolidated undrained test is compression test, in which the soil specimen is first consolidated under all round pressure in the triaxial cell before failure is brought about by increasing the major principal stress.

It may be performed with or without measurement of pore pressure although for most applications the measurement of pore pressure is desirable. Triaxial Shear Test is the fastest Triaxial Shear Test to obtain shear strength parameters (c,Φ) of soil.

2. apparatus required

3. reference

IS 2720(Part 11):1993 Determination of the shear strength parameters of a specimen tested in unconsolidated undrained triaxial compression without the measurement of pore water pressure (first revision). Reaffirmed- Dec 2016.

4. Procedure

  • Preparation of Sample :
    1. Remove wax sealing from field sample tube.
    2. Place sample cutter tube (38 mm inner dia.) on field sample tube.
    3. Insert sample cutter tube in the soil with the help of hydraulic jack.
    4. Take out the sample cutter tube from field sample tube by pushing soil with hydraulic jack.
    5. Transfer soil sample from sample cutter tube to split mould of proper length (76 mm).
    6. Take out soil specimen from split mould.

  • Loading of Sample :
    1. Clean base of triaxial cell.
    2. Put porous stone over bottom pedestal and a filter paper of 38 mm dia. over this porous stone.
    3. Place soil specimen over filter paper and put another filter paper then porous stone on the top of the soil specimen.
    4. Place about 8 filter paper strips vertically around soil specimen extending from top porous stone to bottom porous stone to facilitate uniform and quick saturation.
    5. Put rubber membrane around the soil specimen with the help of stretcher.
    6. Place O ring around top and bottom pedestal in the grooves.
    7. Place the triaxial cell and tight the nut to the base plate.
  • Testing of Specimen :
    1. Saturate the soil sample from 24 to 48 hours, by opening drainage valve, which is connected with burette filled with water. Water level in burette is kept little more than the top of specimen.
    2. After saturation triaxial cell is filled with water and all around cell pressure (σ3) is applied by mercury controlled device. The pore water pressure is measured the sample is saturated until it satisfies B parameter of 1 (not less than 90% of σ3).
    3. Four soil specimen of a sample are tested at 0.5, 1.0, 1.5, and 2.0 kg / cm2 of lateral pressure (σ3). For consolidated Un-drained test (CU), the sample is to be placed for consolidation and B parameter has to be checked. The drainage reading during consolidation in the burette is to be recorded in time interval of 1, 4, 9, 16, 25, 36.....minutes up to 24 hrs.
    4. On account of consolidation the length and diameter of specimen changed.
    5. Changed length, cross sectional area and rate of strain on consolidated specimen have to be calculated.
    6. Apply calculated rate of strain on consolidated specimen and note down the deformation and corresponding load on specimen untill the failure of specimen.
    7. Four specimen has been tested at four confining pressure (0.5, 1, 1.5 and 2 Kg/cm2) as explained above.
    8. Now from above reading plot Mohr’s circle and get the shear parameter C and Ø.

    5. observation and recording

    The machine is set in motion (or if hand operated the hand wheel is turned at a constant rate) to give a rate of strain 2% per minute. The strain dial gauge reading is then taken and the corresponding proving ring reading is taken the corresponding proving ring chart. The load applied is known. The experiment is stopped at the strain dial gauge reading for 15% length of the sample or 15% strain.

    Compression Gauge Reading Load Gauge Reading Compression of Sample Strain Corrected Area Load Deviator Stress
    1 - σ 3 )
    Vertical Stress
    1)
    - (σ 1 / σ 3 )
    0
    50
    100
    150
    200
    250
    300
    350
    400
    450

    Table 1 : Recordings during Triaxial Shear Test

    6. calculation

    Sample No. Wet bulk density gm/cc Cell pressure kg/cm2 Compressive stress at failure Strain at failure Moisture content Shear strength (kg/cm2) Angle of shearing resistance
    1
    2
    3

    Table 2 : Triaxial Shear Test Result

    7. General Remarks

    It is assumed that the volume of the sample remains constant and that the area of the sample increases uniformly as the length decreases. The calculation of the stress is based on this new area at failure, by direct calculation, using the proving ring constant and the new area of the sample. By constructing a chart relating strain readings, from the proving ring, directly to the corresponding stress.

    The strain and corresponding stress is plotted with stress abscissa and curve is drawn. The maximum compressive stress at failure and the corresponding strain and cell pressure are found out.

    The stress results of the series of Triaxial Shear Tests at increasing cell pressure are plotted on a Mohr stress diagram. In this diagram a semicircle is plotted with normal stress as abscissa shear stress as ordinate.

    The condition of the failure of the sample is generally approximated to by a straight line drawn as a tangent to the circles, the equation of which is ԏ = c + σ tan(Φ). The value of cohesion, c is read of the shear stress axis, where it is cut by the tangent to the mohr circles, and the angle of shearing resistance (Φ) is angle between the tangent and a line parallel to the shear stress.

    For normally consolidated soils, c= 0; however,
    for over-consolidated soils, c> O.

    A typical range of values of A at failure for clayey soils is given below:

    Type of Soil A at Failure
    Clays with High sensitivity 0.75 to 1.5
    Normally Consolidated Clays 0.5 to 1.0
    Over Consolidated Clays -0.5 to 0.0
    Compacted Sandy Clay 0.5 to 0.75

    Table 3 : Triaxial Shear Test Result

    8. video

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