1. Direct Shear Test
CEP 701 PG Lab

2. Mohr-Coulomb Failure Criterion (in terms of total stresses)  c 
failure envelope
Cohesion
Friction angle f
f is the maximum shear stress the soil can take without failure, under normal stress of .

3. Mohr-Coulomb Failure Criterion (in terms of effective stresses) ’ c’ ’
failure envelope
Effective cohesion
Effective
friction angle f
u = pore water pressure
f is the maximum shear stress the soil can take without failure, under normal effective stress of ’.

4. Mohr-Coulomb Failure Criterion
Shear strength consists of two components: cohesive and frictional.
’f f ’  ' c’
’f tan ’
frictional component c’
cohesive component

8. Normal stresses and shear stresses on any plane can be obtained with the following equations

9. Principal stresses or

11. Mohr Circle of stress s’ t q s’1 s’3
Soil element q s’ t
Resolving forces in s and t directions,

12. Mohr Circle of stress t s’

13. Mohr Circle of stress t s’ q
(s’, t)
PD = Pole w.r.t. plane

14. Mohr Circles & Failure Envelope
Soil elements at different locations
Failure surface Y
~ stable X
~ failure ’

15. 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.

16. Direct shear test
This test is performed to determine the consolidated - drained shear strength of a sandy to silty soil.
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.
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.

17. 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.

18. 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.

19. Preparation of a sand specimen
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
Components of the shear box
Preparation of a sand specimen
Porous plates

20. Preparation of a sand specimen
Direct shear test
Preparation of a sand specimen
Specimen preparation completed
Pressure plate
Leveling the top surface of specimen

21. Direct shear test Steel ball P Test procedure Pressure plate
Step 1: Apply a vertical load to the specimen and wait for consolidation P
Pressure plate
Porous plates
Proving ring to measure shear force S

22. Direct shear test Steel ball P Test procedure Pressure plate
Step 1: Apply a vertical load to the specimen and wait for consolidation P
Test procedure
Pressure plate
Steel ball
Proving ring to measure shear force S
Porous plates
Step 2: Lower box is subjected to a horizontal displacement at a constant rate

23. PROCEDURE
Measure the thickness of soil specimen.
Apply the desired normal load.
Remove the shear pin.
Attach the dial gauge which measures the change of volume.
Record the initial reading of the dial gauge and calibration values.
Before proceeding to test check all adjustments to see that there is no connection between two parts except sand/soil.
Start the motor. Take the reading of the shear force and record the reading.
Take volume change readings till failure.
Add 5 kg normal stress 0.5 kg/cm2 and continue the experiment till failure
Record carefully all the readings. Set the dial gauges zero, before starting the experiment

24. Direct shear test Shear box Loading frame to apply vertical load
Dial gauge to measure vertical displacement
Shear box
Proving ring to measure shear force
Dial gauge to measure horizontal displacement
Loading frame to apply vertical load

25. Analysis of test results
Direct shear test
Analysis of test results
Note: Cross-sectional area of the sample changes with the horizontal displacement

26. Direct shear tests on sands
Stress-strain relationship
Shear stress, t
Shear displacement
Dense sand/ OC clay tf
Loose sand/ NC clay tf
Change in height of the sample
Expansion
Compression
Shear displacement
Dense sand/OC Clay
Loose sand/NC Clay

27. Direct shear tests on sands
How to determine strength parameters c and f
Shear stress, t
Shear displacement
tf3
Normal stress = s3
tf2
Normal stress = s2
tf1
Normal stress = s1
Shear stress at failure, tf
Normal stress, s f
Mohr – Coulomb failure envelope

28. Direct shear tests on sands
Some important facts on strength parameters c and f of sand
Direct shear tests are drained and pore water pressures are dissipated, hence u = 0
Sand is cohesionless hence c = 0
Therefore,
f’ = f and c’ = c = 0

29. 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)
Failure envelopes for clay from drained direct shear tests
Shear stress at failure, tf
Normal force, s f’
Normally consolidated clay (c’ = 0)
Overconsolidated clay (c’ ≠ 0)

30. 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)
Where,
ca = adhesion,
d = angle of internal friction

31. 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

32. THE END