THE COOPER UNION
Albert Nerken School of Engineering
Soil Mechanics Laboratory
Experiment No. 8 - Direct Shear Test On Cohesionless Soils
To determine the shear strength of a cohesionless soil sample.

 

1) 'Soil Testing for Engineers' by T. W. Lambe; Chapter X.

2) "Engineering Properties of Soils and Their Measurements", 4th ed. by Joseph E. Bowles; Experiment No. 17.

3) A.S.T.M. Standards, 1997; (4.08) Designation D3080-90.

 

1) Direct shear machine

2) Shear box

3) Thickness gauge

4) Shear and Normal displacement gauges

5) Triple beam balance (sensitive to 0.1 gm)

6) Miscellaneous apparatus:

  1. Electric vibrator (vibro-tool)
  2. Metal funnel

 

1) Determine the dimensions of the shear box: the diameter if it is circular, or the length and width if it is square/rectangular, in inches.

2) Calculate the cross-sectional area of the soil sample in in2, and record this on Data Sheet (A).

3) Place the thickness gauge in the bottom of the shear box.  The thickness of the thickness gauge is 1.0000 in.  With the thickness dial indicator, obtain a reading on the thickness gauge, R1, and record this on Data Sheet (A).

[When the soil is placed in the shear box, it must be placed above the crack between the two pieces of the shear box.  In all probability, the thickness of the soil sample will be greater than one inch, and the tolerance of the thickness dial indicator is only one inch, thus the need for the thickness gauge.  By knowing the reading on the thickness dial indicator on the thickness gauge, R1, and by knowing the reading on the thickness dial indicator on the soil sample in the shear box, R2, the thickness of the soil sample in excess of the thickness gauge can be determined, (R2 - R1).  By adding the thickness of the thickness gauge and (R2 - R1), the thickness of the soil sample can be determined.]

4) Weigh a jar containing the dry, cohesionless soil to be tested, in grams, before the sample is prepared.  Record this weight on Data Sheet (B).

5) Place the soil in the shear box by using one of the following techniques.  In either technique, make sure that all the pins are in place to ensure that the two pieces do not shift during the placement of the soil sample:

  1. Loose Sample: pour the soil into the shear box through a metal funnel.  Ensure that the material falls only a small height.
  2. Dense Sample: place "n" teaspoons of soil at a time into the shear box.  After every "n" teaspoons, vibrate the sample evenly with a vibro-tool for 1-2 minutes.

When the soil is in place, level it off.

6) Determine the R2 reading on the thickness dial indicator mentioned in Step 3.  Record R2 on Data Sheet (A).

7) Determine the thickness of the soil sample, in inches.  Record this thickness on Data Sheet (A).

8) Determine the volume of the soil sample, V, in in3, and record it on Data Sheet (A).

9) Weigh the jar containing the dry, cohesionless soil, after the sample is prepared, in grams.  Record this weight on Data Sheet (A).

10) Determine the weight of the dry soil sample used, in grams.  This is Ws, the dry weight of the soil.  Record it on Data Sheet (A).

11) Place the shear box containing the sample into the direct shear machine.  Be sure not to disturb the sample when placing it into the machine.   If the sample is loose, it might densify and the test results will not match the calculated density.

12) Remove the pins from the shear box, otherwise the shearing strength of the pins themselves will be tested.

13) Place the normal load on the soil sample.  Be sure to include the weights of the loading block and the ball bearing as part of the normal load.

14) Place the shear displacement and the normal displacement gauges on the sample.  Ensure that they are both zeroed.  The calibration of the shear displacement gauge is 0.001 in/div, and the calibration of the normal displacement gauge is 0.0001 in/div.

15) Start the shear machine at a speed of 0.0495 in/min.  Bring the load ring almost into contact with the shear box, and ensure that it is zeroed.   One bottom of the box is stationary, while the top moves.

16) The test begins when the load ring dial starts to move.   Since the shear displacement and normal displacement gauges might start to move before the load ring dial, note the reading on both displacement gauges when the load ring dial moves.  This is the new zero.  Readings will be taken on all three gauges, but the controlling gauge is the shear displacement gauge: take all readings at every 0.005 in of displacement on this gauge.  To obtain the shearing load, enter the load ring calibration curve with the load ring reading.

17) Continue to take readings until at least the peak, or even the ultimate, stress is achieved.

18) Reverse the machine.

 

1) Calculate the dry unit weight, relative density, void ratio, and porosity.

2) Calculate the shear and normal stresses.

3) Since we have only one test at normal load and relative density, it will be assumed that the theory is correct, so obtain tanf from the equation: s = s tanf, where s is the normal stress, which is constant throughout the test, s is the shearing stress at failure (either peak or ulitmate), and f is the angle of internal friction of the soil.  Determine f at both peak and ultimate.

  1. tanf(peak) = s(peak) / s
  2. tanf(ultimate) = s(ultimate) / s

 

1) In order to obtain s(peak) and s(ultimate), plot on Cartesian graph paper the shearing stress, t, as the ordinate, and the shear displacement as the abcissa.  High relative density => peak and ultimate; low relative density => only ultimate.

2) In additon, directly below this graph, plot the thickness change graph on Cartesian graph paper with the normal shear displacement as the ordinate and the shear displacement as the abcissa.  High relative density => exhibits a slight decrease in thickness, then an increase in thickness; low density => exhibits a continual decrease in thickness.

 

 

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