CITY
Significance:
The direct shear test is one of
the oldest strength tests for soils. In this laboratory, a direct shear device
will be used to determine the shear strength of a cohesionless soil (i.e. angle
of internal friction (Ø).
From the plot of the shear stress
versus the horizontal displacement, the maximum shear stress is obtained for a specific
vertical confining stress. After the experiment is run several times for
various vertical-confining stresses, a plot of the maximum shear stresses
versus the vertical (normal) confining stresses for each of the tests is
produced. From the plot, a straight-line approximation of the Mohr-Coulomb
failure envelope curve can be drawn, f may be determined, and, for cohesionless
soils (c = 0), the shear strength can be computed from the following equation:
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