TESTS TO MEASURE THE SHEAR STRENGTH OF SOIL
DIRECT SHEAR TEST
There is no mechanism to measure the pore water pressure hence this test is preferred under the drained condition ,however it can be conducted under undrained condition also.
MECHANISM TO CONDUCT DIRECT SHEAR TEST
There are two mechanism to conduct the direct shear test
1- STRAIN CONTROLLED (SCREW)
2- STRESS CONTROLLED (WITH THE HELP OF PULLEY AND WEIGHT)
TEST
Saturated sample of test is placed on the shear box.
Shear box has two parts , the lower part and the upper parts and both are separated to each other.
A normal force P1 is applied on the shear on the top of sample and when expulsion of pore water stops ,shearing is introduced on a predetermined horizontal plane.
At constant normal stress shear displacement is given and shear resistance is recorded on the proving ring dial guage.
Let N1 is proving ring dailguage reading and shear failure under normal stress sigma n2
Then shear stress at failure is given as
Shear stress = shear force / area
KN1/A
Where,
K = =proving ring constant
N = proving ring dial guage reading (1No-1um)
σn1 and 𝛕f1 is plotted on the graph and test is repeated at different values of normal stress and corresponding shear stress at failure is noted and plotted in the graph.
If the test is drained then
LIMITATION OF DIRECT SHEAR TEST
1- Failure plane is predetermined which may not be the weakest plane.
2- There is no mechanism to measure the pore pressure
3- There is no control over drainage condition
4-
LIMITATION OF DIRECT SHEAR TEST
1- Failure plane is predetermined which
may not be the weakest plane.
2- There is no mechanism to measure the
pore pressure
3- There is no control over drainage
condition
4- The stress distribution on the
failure plane is not uniform
5- Stress condition are known only at
failure hence it is difficult to drawn mohr circle from single observation
TRIAXIAL SHEAR TEST
Tri-axial
shear test is conducted in two step in which drainage value may be opened or
closed .
Acyndrical
soil specimen is preferred from a saturated soil mass which is enclosed inside
impermeable rubber membrane.
The length
of sample is usually two times its diameter. There is complete control over
drainage condition and there is mechanism to measure pore pressure.
STAGE -1
Confining
pressure stage/ cell pressure stage/consolidation stage
All around
confining pressure is applied using external water pressure.
If test is
unconsolidated then drainage values will be closed but if test is consolidated
then drainage values will be opened and expulsion of pore water is permitted.
Water
expulsion of pore water stops 1 stage is completed.
STAGE – 2
Keeping
confining pressure constant additional axial stress is applied called deviator
stress
Deviator
stress is increased gradually until the soil specimen fails shear.
If the test
is drainage then the drainage valves will be closed and if the test is drained
drainage valves will be open. deviator stress at the time of failure is termed
as confined compressive stress.
MERITS OF TRIAXIAL TEST
1- Failure plane is not predetermined
it is the weakest planes
2- Mohr circle can be drawn at failure
from single observation
3- Stress distribution of failure plan
is much more uniform
4- There is complete control over
drainage condition
5- There is mechanism to measure pore
pressure.
UNCONFINED COMPRESSIVE STRENGTH TEST
It is special case of Triaxial test in which confining pressure is 0 it
means there is no 1 stage ,no any rubber membrane is required.
Without rubber membrane dry soil and sand cannot be held in position
hence this test is conducted in saturated soil and clay but more suitable for
clay soil.
The saturated sample is subjected to axial loading and deviator stress
at failure is
Deviator stress at failure is termed as the unconfined compressive
strength.
UNCONFINED COMPRESSIVE STRENGTH
FORMULA
CLAY IN UNDRAIN CONDITION
THIS TEST IS SUITABLE FOR CLAY UNDER UNDRAINED CONDITION IN WHICH MOHRS
FAILURE ENVELOP IS HORIZONTAL.
VANE SHEAR TEST
In VANE SHEAR TEST test there is no mechanism to measure pore pressure and no drainage facility available.
It is suitable for soft saturated clay (marine clay) to find undrained
shear strength and it can also be used
to find sensitivity of clay.
It can be conducted in lab as well as in field. The only mechanism difference
for lab and field test is the size of vane .
The vane is punched into soil and torque is applied by rotating the vane
at 6 degree per minute.
The vane is calibrated to a spring having torsional stiffness of K . The
torque at shear failure is determined as :-
T = KQ
Where,
Q = angular rotation of vane at which complete shear failure occurs.
While punching the vane in the soil fallowing two main shearing occurs:-
1- ONE WAY SHEARING
2- TWO WAY SHEARING
TWO WAY SHEARING
If vane is punched in
such that top of vane is at certain depth below the ground level then shearing
occurs at side and at top and bottom both.
ONE WAY SHEARING
If the vane is pushed in the soil in a way that top of vane is at ground level , in this condition shearing will occurs only at sides and bottom .
DETERMINATION OF SENSITIVITY OF CLAY
Initially vane shear test
is performed on the undisturbed soil hence shear strength of undisturbed clay is determined.
Vane is further rotating
quickly several times hence ,soil becomes remoulded. The vane shear test is
again performed on remoulded soil to find remoulded shear strength
St = (vane shear
strength)undisturbed/(vane shear strength)remoulded.
PORE PRESSURE PARAMETERS
Impossible to
measure pore pressure by practical means
Parameter B and q
represent the response of change in pore pressure due to change in vertical
pressure under undrained.
The pore pressure change can be
classified into two stages
.
STAGE -1
confining pressure stage
The
parameter B represent the ratio of change in pore pressure to the change in
confining pressure.
FORMULA:-
For dry
condition b=0
For
saturated condition b=1
For
partially saturated condition 0<B<1
STAGE -2
Shear stage /deviator stress
Parameter A
is defined in terms of another parameter ‘A’ such that
‘A’ = A.B
The
parameter represent the ratio of change in pore pressure to the change in
deviator stress in shear stage under undrained condition.
FORMULA:-
The
parameter A depends upon the degree of saturation ,over consolidation
ratio, strain in soil and stratification of soil. Its value may be as low as
-0.5(for highly consolidated clay and dense sand)and may be as high as 3 for
loose sand.
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