Direct Shear Test - Soil Properties: Testing, Measurement, and Evaluation - page 379

Mass of oven-dried soil specimen (at end of test) plus can (g)
Mass of can (g)

[B] Shear Stress Data

Normal load on test specimen (lb)
Rate of shear (rate of horizontal displacement) (in./min)
Vertical dial readings, horizontal displacement dial readings, and
proving ring dial readings (in.)

Note —These data are obtained for each of three specimens tested at
different normal loads.

CALCULATIONS [A] Specimen Parameters

The initial area, volume, wet unit weight, water content, dry unit weight
of the specimen, and final water content are all calculated by methods
described in previous chapters.

[B] Shear Stress

Normal stress can be determined by dividing normal load (which in-
cludes the weight of the loading block and externally applied normal
force) by the initial area of the specimen. Horizontal shearing forces can
be computed by multiplying each dial reading by the proving ring cali-
bration. Each corresponding shear stress can be calculated by dividing
each horizontal shearing force by the initial area of the specimen. These
computations must be completed for each specimen tested at different
normal loads.

[C] Curves of Shear Stress and Specimen Thickness Change
versus Shear Displacement

For each test specimen, graphs of shear stress versus shear (horizontal)
displacement and of specimen thickness change (vertical dial readings)
versus shear (horizontal) displacement should be prepared. Both graphs
can be placed on the same graph sheet using shear (horizontal) dis-
placement as a common abscissa.
With these graphs completed, one can determine the maximum
shear stress for each specimen tested by evaluating the graph of shear
stress versus shear (horizontal) displacement. The maximum stresses
are taken to be either the peak shear stress on the graph or the shear
stress at a shear (horizontal) displacement of 10% of the original diam-
eter, whichever is obtained first during the test.

[D] Curve of Maximum Shear Stress versus Normal Stress

In order to evaluate the shear strength parameters (cohesion and angle
of internal friction), it is necessary to prepare a graph of maximum