Direct Shear Test - Soil Properties: Testing, Measurement, and Evaluation - page 377

Obtain data readings of time, vertical and horizontal displace-
ment, and shear force at desired interval of displacement. Data
readings should be taken at displacement intervals equal to 2% of
the specimen diameter or width to accurately define a shear stress-
displacement curve.

Note 3 —Additional readings may be helpful in identifying the
value of peak shear stress of overconsolidated or brittle material.

Note 4 —It may be necessary to stop the test and re-gap the
shear box halves to maintain clearance between the shear box
halves.

(4) For a consolidated drained test, apply the shearing force and
shear the specimen slowly to ensure complete dissipation of excess
pore pressure. The following guide for total elapsed time to failure
may be useful in determining rate of loading

Time to failure ( t f ) 50 t 50

where:
t50 time required for the specimen to achieve 50% consolidation

under the normal force.

Note 5 —If the material exhibits a tendency to swell, the soil
must be inundated with water and must be permitted to
achieve equilibrium under an increment of normal stress large
enough to counteract the swell tendency before the minimum
time to failure can be determined. The time-consolidation
curve for subsequent normal stress increments is then valid
for use in determining t f .

Note 6 —Some soils, such as dense sands and overconsolidated
clays, may not exhibit well-defined time-settlement curves.
Consequently, the calculation of t f may produce an inappropri-
ate estimate of the time required to fail the specimen under
drained conditions. For overconsolidated clays which are
tested under normal stresses less than the soil's preconsolida-
tion pressure, it is suggested that a time to failure be esti-
mated using a value of t 50 equivalent to one obtained from
normal consolidation time-settlement behavior. For clean
dense sands which drain quickly, a value of 10 min may be
used for t f . For dense sands with more than 5% fines, a value
of 60 min may be used for t f . If an alternative value of t f is se-
lected, the rationale for the selection shall be explained with
the test results.

Determine the appropriate displacement from the following equation:

d r d f t f

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