Direct Shear Test - Soil Properties: Testing, Measurement, and Evaluation - page 375

and place it in the shear box. Compact the soil until the desired unit
weight is obtained. Continue placing the compacting soil until the
entire specimen is compacted.

Note 3 —The required thickness of the compacted lift may be
determined by directly measuring the thickness of the lift or
from the marks on the tamping rod which correspond to the
thickness of the lift being placed.

(2) Material required for the specimen shall be batched by thor-
oughly mixing soil with sufficient water to produce the desired
water content. Allow the specimen to stand prior to compaction in
accordance with the following guide:

Classification ASTM D 2487 Minimum Standing Time, h

SW, SP No Requirement

SM 3

SC, ML, CL 18

MH, CH 36

(3) Compacted specimens may also be prepared by compacting soil
using the procedures and equipment used to determine moisture-
density relationships of soils (ASTM Test Methods D 698 or D 1557)
and trimming the direct shear test specimen from the larger test
specimen as though it were an undisturbed specimen.

To carry out a direct shear test, a soil specimen is prepared and placed in
a direct shear box (see Figures 23-1 and 23-2), which may be round or
square. A normal load of specific (and constant) magnitude is applied.
The box is “split” into two parts horizontally (see Figures 23-1 and 23-2),
and if the lower half is held stationary while the upper half is pushed
with increasing force, the soil will ultimately experience shear failure
along horizontal surface A . This procedure is carried out in the direct
shear apparatus (see Figures 23-3 and 23-4), and the particular normal
load and shear stress that produced shear failure are recorded. The soil
specimen is then removed from the shear box and discarded, and another
specimen of the same soil is placed in the shear box. A normal load either
higher or lower than that used in the first test is applied to the second
specimen, and a shear force is again applied with sufficient magnitude to
cause shear failure. The normal load and shear stress that produced
shear failure are recorded for the second test. The entire procedure may
be repeated for another specimen and another different normal load.
The actual step-by-step procedure is as follows:

PROCEDURE

(1) Measure the diameter (or side), height, and mass of the speci-
men. Assemble the shear box with the frames aligned and locked
in position. Carefully insert the test specimen. Place the loading
block in place and connect the loading device. Position the vertical