Direct Shear Test - Soil Properties: Testing, Measurement, and Evaluation - page 380

Figure 23-5 Shear Diagram for Direct Shear Test

shear stress (ordinate) versus normal stress (abscissa) for each speci-
men tested (see Figure 23-5). The same scale must be used along both
abscissa and ordinate. A straight line is drawn through the plotted
points and extended to intersect the ordinate. The angle between the
straight line and a horizontal line (ø in the figure) gives the angle of in-
ternal friction, and the value of shear stress where the straight line in-
tersects the ordinate ( c in the figure) gives the cohesion.
It is adequate, in theory, to have only two points to define the
straight-line relationship shown in the figure. In practice, however, it is
better to have three (or more) such points through which the best
straight line can be drawn. That is why the test procedure calls for three
or more separate tests to be performed on three or more specimens from
the same soil sample at different normal stresses.

An unconsolidated undrained direct shear test was performed in the
laboratory according to the procedure described in this chapter. The fol-
lowing data were obtained:

NUMERICAL
EXAMPLE

[A] Specimen Data

Diameter of specimen, D 0 2.50 in.

Initial height of specimen, H 0 1.00 in.

Mass of specimen at beginning of test 161.52 g

Initial water content data:
Mass of wet soil sample plus can 248.43 g

Mass of oven-dried soil sample plus can 216.72 g

45.30 g
Final water content data:
Mass of wet soil specimen plus can at end of test

Mass of can

226.20 g