Direct Shear Test - Soil Properties: Testing, Measurement, and Evaluation - page 376

displacement indicator and apply the appropriate normal load.
(The normal load includes both the weight of the loading block and
the externally applied normal force.)
Note 1 —The decision to dampen the porous stones before in-
sertion of the specimen and before application of the normal
force depends upon the problem under study. For undisturbed
samples from below the water table, the porous stones are
usually dampened. For swelling soils, wetting should probably
follow application of the normal force to prevent swell not
representative of field conditions.
(2) For a consolidated test, consolidate the test specimen under
the appropriate normal force. As soon as possible after applying
the initial normal force, fill the water reservoir to a point above
the top of the specimen. Maintain this water level during the con-
solidation and subsequent shear phases so that the specimen is at
all times effectively submerged. Allow the specimen to drain and
consolidate under the desired normal force or increments thereof
prior to shearing. During the consolidation process, record the
normal displacement readings before each increment of normal
force is applied and at appropriate times [see ASTM Method D
2435 (Chapter 20)]. Plot the normal displacement readings
against elapsed time. Allow each increment of normal force to re-
main until primary consolidation is complete. The final increment
should equal the previous normal force developed and should pro-
duce the specified normal stress.
Note 2 —The normal force used for each of the three or more
specimens will depend upon the information required. Appli-
cation of the normal force in one increment may be appropri-
ate for relatively firm soils. For relatively soft soils, however,
several increments may be necessary to prevent damage to the
specimen. The initial increment will depend upon the strength
and sensitivity of the soil. This force should not be so large as
to squeeze the soil out of the device.
(3) Separate the upper and lower halves of the shear box frame
by a gap of approximately 0.025 in. (0.64 mm) so the specimen
can be sheared. Position the shear-deformation (horizontal dis-
placement) indicator and set both the vertical and the horizontal
displacement indicators to zero. Fill the shear box with water for
saturated tests. Apply the shearing force and shear the specimen.
After reaching failure, stop the test apparatus. This displacement
may range from 10 to 20% of the specimen's original diameter or
length. For all tests except those under consolidated drained con-
ditions (in a controlled-displacement case), the rate of shear (i.e.,
the rate of horizontal displacement) should be on the order of
0.05 in./min.