Transcript Document

Direct Shear Test
CEP 701 PG Lab
Mohr-Coulomb Failure Criterion
(in terms of total stresses)

 f  c   tan

Friction angle
Cohesion
f
c


f is the maximum shear stress the soil can take without
failure, under normal stress of .
Mohr-Coulomb Failure Criterion
(in terms of effective stresses)

 f  c' ' tan '
 '  u
’
Effective
cohesion
f
c’
’
u = pore water
pressure
Effective
friction angle
’
f is the maximum shear stress the soil can take without
failure, under normal effective stress of ’.
Mohr-Coulomb Failure Criterion
Shear strength consists of two
components: cohesive and frictional.

 f  c' ' f tan '
f
’
c’
’f tan ’
frictional
component
c’
’f
'
Normal stresses and shear stresses on any plane can be obtained
with the following equations
Principal stresses
or
Mohr Circle of stress
’1
’
’3
’3

q
Soil element
’1
Resolving forces in  and  directions,
 1'   3'

Sin2q
2
'
'
'
'






 '  1 3  1 3 Cos2q
2
2
 '  
   
2

2
'
1
' 2
3
    

  
 2 
 

' 2
3
'
1
Mohr Circle of stress

 '  
   
2

2
'
1
' 2
3
    

  
 2 
 

' 2
3
'
1
 1'   3'
2
 3'
 1'   3'
2
1'
’
Mohr Circle of stress

 '  
   
2

2
'
1
' 2
3
    

  
 2 
 

' 2
3
'
1
(’, )
 1'   3'
q
 3'
2
 1'   3'
2
PD = Pole w.r.t. plane
1'
’
Mohr Circles & Failure Envelope

Failure surface
Y
X
 f  c' ' tan '
Y
X
’
Soil elements at different locations
Y ~ stable
X ~ failure
Direct shear test
NEED AND SCOPE
In many engineering problems such as
•
design of foundation,
•
retaining walls,
•
slab bridges,
•
pipes,
•
sheet piling,
The value of the angle of internal friction and cohesion of the
soil involved are required for the design.
Direct shear test is used to predict these parameters quickly.
Direct shear test
1. This test is performed to determine the
consolidated - drained shear strength of a sandy to
silty soil.
2. The shear strength is one of the most important
engineering properties of a soil, because it is
required whenever a structure is dependent on the
soil’s shearing resistance.
3. The shear strength is needed for engineering
situations such as determining the stability of
slopes or cuts, finding the bearing capacity for
foundations, and calculating the pressure exerted
by a soil on a retaining wall.
Apparatus
1.
Direct shear box apparatus
2.
Loading frame (motor attached).
3.
Dial gauge.
4.
Proving ring.
5.
Tamper.
6.
Straight edge.
7.
Balance to weigh upto 200 mg.
8.
Aluminum container.
9.
Spatula.
PROCEDURE
•
Check the inner dimension of the soil container.
•
Put the parts of the soil container together.
•
Calculate the volume of the container. Weigh the
container.
•
Place the soil in smooth layers (approximately 10 mm
thick). If a dense sample is desired tamp the soil.
•
Weigh the soil container, the difference of these two is
the weight of the soil. Calculate the density of the soil.
•
Make the surface of the soil plane.
•
Put the upper grating on stone and loading block on top
of soil.
Direct shear test
Direct shear test is most suitable for consolidated drained tests
specially on granular soils (e.g.: sand) or stiff clays
Preparation of a sand specimen
Porous
plates
Components of the shear box
Preparation of a sand specimen
Direct shear test
Preparation of a sand specimen
Leveling the top surface
of specimen
Pressure plate
Specimen preparation
completed
Direct shear test
Test procedure
P
Steel ball
Pressure plate
Porous
plates
S
Proving ring
to measure
shear force
Step 1: Apply a vertical load to the specimen and wait for consolidation
Direct shear test
Test procedure
P
Steel ball
Pressure plate
Porous
plates
S
Proving ring
to measure
shear force
Step 1: Apply a vertical load to the specimen and wait for consolidation
Step 2: Lower box is subjected to a horizontal displacement at a constant rate
PROCEDURE
8. Measure the thickness of soil specimen.
9. Apply the desired normal load.
10. Remove the shear pin.
11. Attach the dial gauge which measures the change of volume.
12. Record the initial reading of the dial gauge and calibration values.
13. Before proceeding to test check all adjustments to see that there is
no connection between two parts except sand/soil.
14. Start the motor. Take the reading of the shear force and record the
reading.
15. Take volume change readings till failure.
16. Add 5 kg normal stress 0.5 kg/cm2 and continue the experiment till
failure
17. Record carefully all the readings. Set the dial gauges zero, before
starting the experiment
Direct shear test
Shear box
Dial gauge to
measure vertical
displacement
Proving ring
to measure
shear force
Loading frame to
apply vertical load
Dial
gauge
to
measure horizontal
displacement
Direct shear test
Analysis of test results
Normalforce(P)
  Normalstress 
Area of crosssectionof thesample
Shear resistancedevelopedat thesliding surface (S)
  Shear stress 
Area of crosssectionof thesample
Note: Cross-sectional area of the sample changes with the horizontal
displacement
Direct shear tests on sands
Shear stress, 
Stress-strain relationship
Dense sand/
OC clay
f
f
Loose sand/
NC clay
Expansion
Compression
Change in height
of the sample
Shear displacement
Dense sand/OC Clay
Shear displacement
Loose sand/NC Clay
Direct shear tests on sands
Shear stress, 
How to determine strength parameters c and 
Normal stress = 3
Normal stress = 2
f3
f2
f1
Normal stress = 1
Shear stress at failure, f
Shear displacement
Mohr – Coulomb failure envelope

Normal stress, 
Direct shear tests on sands
Some important facts on strength parameters c and  of sand
Sand is cohesionless
hence c = 0
Direct shear tests are
drained and pore water
pressures
are
dissipated, hence u = 0
Therefore,
’ =  and c’ = c = 0
Direct shear tests on clays
In case of clay, horizontal displacement should be applied at a very
slow rate to allow dissipation of pore water pressure (therefore, one
test would take several days to finish)
Shear stress at failure, f
Failure envelopes for clay from drained direct shear tests
Overconsolidated clay (c’ ≠ 0)
Normally consolidated clay (c’ = 0)
’
Normal force, 
Interface tests on direct shear apparatus
In many foundation design problems and retaining wall problems, it
is required to determine the angle of internal friction between soil
and the structural material (concrete, steel or wood)
P
Soil
S
Foundation material
 f  ca   ' tan
Where,
ca = adhesion,
 = angle of internal friction
Advantages of direct shear apparatus
 Due to the smaller thickness of the sample, rapid drainage can
be achieved
 Can be used to determine interface strength parameters
 Clay samples can be oriented along the plane of weakness or
an identified failure plane
Disadvantages of direct shear apparatus
 Failure occurs along a predetermined failure plane
 Area of the sliding surface changes as the test progresses
 Non-uniform distribution of shear stress along the failure surface