Florida International University Department of Civil and Environmental Engineering CEG 4011 L Geotechnical Engineer ing I Laboratory Dr. Luis A. Prieto-Portar PhD, PE, SE. Lab Report #01 The Direct Shear Test (ASTM D-3080) Perfo rme d on x x Mar ch 20 10 Team Members: Member Attendance Writing Assignment Completed
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9. Apply horizontal load, S , to the top half of the shear box. The rate
of shear displacement should be between 0.1 to 0.02 in/min.
Record the readings of the vertical dial gauge and the proving ringgauge, which measures the horizontal load, S for every tenth small
division displacement in the horizontal dial gauge.
Continue until the following happens at the proving ring dialgauge:
• Reaches a maximum and then falls
• Reaches a maximum and then remains constant.
10. Repeat the test (Steps 1 to 9) two more times. For each test, the dry unit weight of compaction of the sand specimen should be the same as that of the first sample.
A direct shear test is used to find the shear strength parameters of a soil. Stress failure is caused by
slippage of soil particles, which may lead to sliding of one body of soil relative to the surrounding
mass. The shear stress and displacement is not uniformly distributed within the soil, therefore, as
the soil is initially displaced the shear stress increases at a fast rate and then as more displacement
occurs, the rate decreases. This can be seen in the plot of shear stress versus horizontal
displacement where the slope of the graph is steep initially and then decreases as displacement
increases.
There are advantages and disadvantages to using a direct shear test. Some of the advantages are
that it is cheap, fast and simple, especially for the testing of sand and failure occurs along a singlesurface, which approximates observed slips or shear type failures in natural soils. However, the
main disadvantage is that the failure plane is forced and may not be the most critical plane which
failure can occur. Other disadvantages are that non-uniform stress conditions exist in the specimen,
and the principal stresses rotate during shear, and the rotation cannot be controlled. While
conducting this experiment, it was determined that some factors might have induced errors in the
data that was being recorded. Before weighing the sand sample in the porcelain dish, a part of it
spilled. This might have led to some overestimation in the values for the weight of the sand placed
in the shear box. Another factor that might have induced error was the way in which the gauges
were being read. They were read simultaneously and at a very fast pace. This might have led to
some inaccuracies while recording the readings.
Also, the tools used for the experiment were not perfect. The horizontal reading gauge could not be
placed in a perfectly horizontal position, which definitely caused some underestimation of the
horizontal shear displacement recorded. Finally, the sand placed in the shear box didn’t have a
perfectly flat surface. This probably led to some inconsistencies while recording the values for the
vertical shear displacement. From the experiment the maximum shear stress was found to be 3.5
psi, 5.6 psi and 8.6 psi for the first, second and third trials respectively.
Prieto-Portar, Luis. “08. The Direct Shear Test” Florida International University. 1 Apr. 2008<http://web.eng.fiu.edu/~prieto/geo1/Laboratories/08-Direct-Shear-Test/Index.htm>.
"Shear Strength in Soils". <http://esig4.uwyo.edu/classes/fa2007/ce3600/8_shear/shear.htm>.
Sivakugan, N. "Shear Strength of Soils." <www.geoengineer.org/files/Strength-Sivakugan.ppt>.