Direct Shear Test Video Lecture | Soil Mechanics - Civil Engineering (CE)

hello viewers the shear strength of the
soil is the resistance to deformation of
soil particles upon the action of a
shear stress when soil is loaded
shearing stresses are induced in it when
shearing stresses reach a limiting value
shear deformation takes place leading to
the failure of the soil mass the failure
may be in the form of sinking of a
footing or movement of wedge of soil
behind a retaining wall forcing it to
move out or slide in an earth embankment
the shearing resistance can be
determined in the laboratory by four
methods direct shear test tri-axial
shear test unconfined compression test
and vane shear test depending upon
drainage conditions three types of sea a
test have been developed unconsidered
ated undrained condition consolidated
undrained condition and consolidated
drained condition in this program we
shall demonstrate the direct shear test
as pariahs code to seven to zero part
thirteen to conduct this test we need
the direct shear apparatus that consists
of shear box
it is horizontally divided into two
parts
having connecting pins spacing screws
base plate with cross grooves on its toe
face loading pad with a steel ball on
its top two pairs of grid plates one
plane and other perforated porous stones
one pair metal plates one pair container
for shear box loading system to
distribute the load from the yoke over
the specimen normal to the shear plane
set of weights for normal load
proving ring with dial gauge accurate
2.01 mm to measure the shear force knob
for fixing the proving ring switch
electric motor hand we
rollers
locking pin clutch and here four
different rate of loading four different
soil micrometer dial gauges two number
two major vertical displacement dial
gauge to measure horizontal displacement
during shear
sample trimmer sampler
they're mud
stop watch soil specimen and water
content determination operators in this
program we shall determine the shear
strength parameters of the soil under
unconservative undrained condition first
of all take a specimen of soil to be
tested from undisturbed sample
you
assemble the two halves of the shear box
using connecting pins
place the baseplate inside the box now
place metal plate instead of perforated
stone as this test is for undrained
condition place one grid plate without
perforations over the metal plate such
that the grids are perpendicular to the
direction of shear carefully place the
soil specimen inside the box so that it
rests on the grid plate
you
you
over the top of the soil sample now
place another grid plate without
perforations such that the grids are
perpendicular to the direction of sear
slightly press this grid plate evenly so
that the grids are buried into the
sample now place another metal plate
over this grid plate place the loading
pad on the top of the metal plate place
the adjusting pins in the box
now place the seer box inside the seer
container
being the proving ring assembly in
contact with you are provided with top
half of the shear box
slight movement of the pointer in the
dial gauge is an indication that contact
is established
place the dial gauge for measurement of
horizontal displacement
fix the dial gaze on the loading yoke to
measure the vertical displacement
you
set.all dial gauges to zero
place a steel bowl on the spherical
groove
provided on the loading pad
you
see the loading yoke on this bowl raise
the upper half of the shear box through
1mm by operating the adjustment screws
remove the pins connecting the upper and
lower half of the shear box now apply
the required normal load on the hanger
of the loading lever for the present
case it is 0.5 kg per centimeter square
set the clutch and the gear for applying
shear displacement of 1.25 mm per minute
to the sample switch on the machine and
the stopwatch simultaneously
record the readings on the to
displacement dial gauges and the proving
ring dial gauge at regular interval of
time
continue applying the shear force till
the specimen fails which is indicated by
a kickback of the pointer in the proving
ring dial gauge if such a failure does
not occur continue searing till the
specimen undergoes a shearing
displacement of 2l mm here you see the
sheared soil specimen which was under
test conduct at least three tests on
separate specimen having the same
density and moisture content but
applying different normal loads here you
see a standard pro forma for direct
shear test for recording the values of
various parameters during conduct of
test record all the observations in the
performer for a normal stress 0.5 kg per
centimeter square the CS test at failure
is 0.37 kg per centimeter square shear
stress is equal to shear force divided
by area of specimen at a normal stress
of 1 kg per centimeter square the CS
test at failure is 0.55 kg per
centimeter square and for normal stress
of 1.5 kg per centimeter square the CS
test at failure is 0.7 7 kg per
centimeter square for the present soil
draw a graph by plotting normal stress
as the abscissa and the shearing stress
as the ordinates corresponding to
failure States join the points
corresponding to the state of failure by
a line
this line gives the mejor envelope for
the soil measure the intercept which
this line makes with the shearing stress
XS this gives the Khoisan C measure the
angle which the line makes with the
horizontal axis
this gives the angle of shearing
resistance for present test C is equal
to 0.14 kg per centimeter Square and Phi
is equal to 23 degree the value of C and
Phi will vary with different state of
soil