Environmental Engineering Reference
In-Depth Information
P (
σ
)
Extensometer
Upper frame
Pulley
F
a
n
a
n
(
)
S
a
+
Porous stone
Base
Drainage
valve
Lower box fixed
to base
F = W
FIGURE 3.51
Laboratory direct shear box.
Load W
has been performed as a consolidated undrained (CU) test when load application rates are
high, although some drainage always occurs during shear. The shear force is applied
either by adding deadweights (stress-controlled) or by operating a motor acting through
gears (controlled strain) and gradually increasing the load until failure occurs.
Mohr's Envelope
Plotting the results for a sequence of tests under different normal stresses as points (
)
results in a Mohr's envelope. Because of the form of the test, only the normal and shear
stresses on a single plane are known. If it is assumed that the horizontal plane through the
shear box is identical with the theoretical failure plane (usually an inexact assumption), it
can be assumed that the stresses at failure are in the ratio of
τ
,
σ
τ
/
σ
tan
φ
to provide a point
on the envelope.
Residual or Ultimate Shear Strength
The direct shear test is often used for measuring the residual strength. The test is per-
formed by a back-and-forth sequential movement of the shear box after the failure of an
undisturbed specimen has occurred, or by testing a remolded specimen.
An alternate method, the
rotational shear test
, is considered to provide the lowest possi-
ble values for the residual strength. The specimen is ground down to produce all particles
smaller than 0.74 mm (#200 sieve) and placed between two circular plates which are
rotated and the shearing resistance measured.
The relationship between peak and ultimate strength for normally consolidated and
overconsolidated clays is given in
Figure 3.30,
and Mohr's envelope for the ultimate
drained strength
Simple Shear Test (ASTM D6528-00 for CU direct simple shear)
Purpose
The purpose is to measure drained or undrained strength parameters.
Apparatus
The soil specimen is contained either in a cylindrical rubber membrane reinforced by wire,
which permits shear deformation to be distributed fairly uniformly throughout the specimen
