The document describes the engineering properties of soil through direct shear tests and triaxial shear tests. Direct shear tests are used to determine the shear strength of soils, especially cohesionless soils, by measuring the cohesion and angle of internal friction. Triaxial shear tests subject a soil sample to three principal stresses and can be conducted under unconsolidated-undrained, consolidated-undrained, or consolidated-drained conditions to evaluate shear resistance. Both tests provide important parameters for geotechnical designs involving soil strengths.

1. Engineering properties of soil

9. • Soils derive their strength
from contact between
particles capable of
transmitting normal as well
as shear forces.
• The contact between soil
particles is mainly due to
friction and the
corresponding stress
between the soil grains is
called the effective (or inter-
granular) stress s'.

10. Shear stress of soil
• Shear force is a force acting in a direction that's parallel to (over the top
of) a surface or cross section of a body.
• Thus, the shear strength of a soil is mainly governed by the effective stress.
Besides the effective stress between soil grains, the pore water contained
in the void spaces of the soil also exerts pressure which is known as pore
pressure, u.
• The sum of the effective stress and pore pressure acting an any given
surface within a compacted earth embankment is called the total stress s.
As the pore water cannot resist shear, all shear stresses are resisted by the
soil grains only. The effective stress is approximately equal to the average
intergranular force per unit area and cannot be measured directly.

12. Direct shear test
• Direct shear test or Box shear test is used to determine the shear
strength of the soil. It is more suitable for cohesionless soils.
• Shear strength of a soil is its maximum resistance to shearing stresses.
The shear strength is expressed as
of shearing resistance

13. Direct shear test
• Using direct shear test, one can find out the cohesion and angle of
internal friction of soil which are useful in many engineering designs
such as foundations, retaining walls, etc.
• This test can be performed in three different drainage conditions
namely unconsolidated-undrained, consolidated-undrained and
consolidated-drained conditions.
• In general, cohesionless soils are tested for direct shear in
consolidated drained condition.

14. Unconsolidated – Undrained test
• Drainage is not allowed at any point of the test, including before the
test when the usual stress is applied and during the test when the
shear stress is applied. As a result, no time is required for pore water
pressure to dissipate and the soil to consolidate, and no major
volume changes are anticipated.
• Because of the short drainage course, 5 to 10 minutes may be
sufficient for the entire test. Undrained samples, on the other hand
are often conducted only on low-permeability soils. It is also known
as the ‘Quick test’ since only a short amount of time is required for
testing before failure.

15. Unconsolidated – Undrained test
• It is quick test and may complete in 5-10 minutes. In this test water is
not allowed to leave the soil either during the consolidation stage
(confining stage) nor shear stage (deviator stage). Such tests are
suitable for low permeable soil such as clays with fast loading.
• The unconsolidated-undrained shear strength is applicable in
situations where load movement is so rapid that the time required for
the induced pore water pressure to dissipate or for consolidation to
take place during the loading period is insufficient.

16. Consolidated – Undrained test
• During the application of the normal stress, absolute drainage is
permitted, but no drainage is permitted during the application of the
shear stress. As a result, no volume changes occur during shear and
excess pore pressure grows.
• After the soil has settled to the desired degree under normal stress, 5
to 10 minutes may be sufficient for the test.The ‘consolidated quick
test’ is another name for this test. In geotechnical engineering, these
conditions are also general.

17. Consolidated – Undrained test
• The standard consolidated undrained test is compression test, in
which the soil specimen is first consolidated under all round
pressure in the triaxial cell before failure is brought about by
increasing the major principal stress.
• The soil is set to be consolidated by allowing the drainage through
the sample. The volume of the soil reduces without the air
replacement. This consolidation is done under the confining pressure.
In case of undrained conditions, the pore water is not allowed to
drain out of soil.

18. Consolidated – drained test
• In a 'consolidated drained' test the sample is consolidated and
sheared in compression slowly to allow pore pressures built up by
the shearing to dissipate. The rate of axial deformation is kept
constant, i.e., strain is controlled.
• Drainage is allowed fully before and during the test at all times. The
soil is consolidated under normal stress and shear is measured by
slowly applying shear stress to the soil, with drainage allowed at all
times. At no point does there appear to be any excess pore pressure,
and volume changes occur.

19. Consolidated – drained test
• In the case of cohesive soils, it can take 4 to 6 weeks to complete a
single test of this kind, while in the case of cohesionless soils, less
time is needed because the latter drain off quickly. Except for
research purposes, this test is rarely performed on cohesive soils. The
‘Slow Test’ or ‘consolidated slow test’ is another name for it.
• The type of soil and the reality of the situation will determine which of
these tests should be used. UU-tests are useful for determining the
short-term stability of foundations, excavations, and earth dams etc.
Depending on the drainage conditions in the area, either CU-tests or
CD-tests are suitable for long-term stability issues.

20. Apparatus Required for Direct Shear Test

21. Apparatus Required for Direct Shear Test

22. Apparatus Required for Direct Shear Test
•Shear box
•Shear box container
•Base plate with cross groves on its top
•Porous stones (2 Nos)
•Plain Grid plates (2Nos)
•Perforated grid plates (2Nos)
•Loading pad with steel ball
•Digital weighing machine
•Loading frame with loading yoke
•Proving ring
•Dial gauges (2 Nos)
•Weights
•Tampering Rod
•Spatula
•Rammer
•Sampler

23. Test procedure for direct shear test

24. Test procedure for direct shear test

25. Test procedure for direct shear test

26. Test procedure for direct shear test

27. Test procedure for direct shear test

28. Test procedure for direct shear test

29. Observations and Calculations for Direct Shear Test

30. Observations and Calculations for Direct Shear Test

31. Observations and Calculations for Direct Shear Test
Shear strength of the given soil sample is = ______________ kN/m2.

32. Triaxial Shear Test
• Triaxial shear strength test on soil measures the mechanical
properties of the soil.
• In this test, soil sample is subjected to stress, such that the
stress resulted in one direction will be different in
perpendicular direction.
• The material properties of the soil like shear resistance,
cohesion and the dilatancy stress is determined from this
test. The test is most widely used and is suitable for all types
of soils.

33. Triaxial Shear Test
• The tri-axial shear test is a commonly adopted soil test as the
test is suitable for all types of soil.
• This test was first developed by Casagrande to overcome the
drawbacks of the direct shear test.
• The triaxial shear test is more versatile and the drainage
conditions can also be well controlled.
• Triaxial shear test on soil includes the process of subjecting the
soil sample to stress such that the stress resulted in one
direction will be different in perpendicular direction.

34. Triaxial Shear Test

35. Triaxial Shear Test Apparatus
• Triaxial testing machine complete with triaxial cell : This unit
have the provision to insert a cylindrical soil specimen that is
sealed by means of rubber membrane to restrict the entry of
lateral fluid. Radial fluid pressure and the vertical stress is
applied by means of a piston arrangement. The unit also have
the provision to prevent the drainage of specimen. The fluid
pressure in the cell can be measured by means of a pressure
gauge.
• Equipment for loading
• Equipment to measure load and Deformation : Proving is used
to measure the load application by the piston. A dial guage is
used to measure the deformation of the specimen

36. Triaxial Shear Test Principle
• A cylindrical specimen, generally
having a length to diameter ratio of 2,
is used in the test and is stressed
under conditions of axial symmetry in
the manner shown in figure below.

37. Triaxial Shear Test Principle

38. Triaxial Shear Test Principle
• The normal size of the sample will be 76mm x 38mm &
100mm x 50mm.
• The sample is subjected to three principal stresses. Among
this three stresses, two are stress due to water pressure
that is within the confining cell. The two values are equal.
• The application of a load on the top of the cell by means of a
ram as shown above, is the applied third stress. This stress is
different from other two stresses.

39. Triaxial Shear Test Principle
• The sample tested has its ends sealed by top cap and
bottom pedestal by rubber O-rings.
• If these involves pore pressure measurement, porous stones
can be placed at the bottom and the top of the specimen.
• Pressure transducers are used to measure the pressure that
is developed inside the specimen.

40. Triaxial Shear Test Principle
• The triaxial shear testing can be conducted by two steps:
• Step 1: The soil sample is prepared and set in the triaxial
cell. Then the confining pressure is applied.
• Step 2: Here, deviator stress is applied, which is an
additional axial stress.
• This induces shear stresses within the sample.
• The axial stress applied is increased till the soil sample fails.
• The applied stresses, axial strain and the pore water
pressure is measured for both the above steps.

41. Triaxial Shear Test Procedure
The specimen can be prepared either remoulded or
undisturbed. Undisturbed soil can be tested on soils that have
sufficient cohesion. In order to make remoulded soils, cohesive
soil is collected and compacted properly. Care is taken while
preparing the cohesion less soils. The triaxial shear test can be
conducted in different variations. The most commonly
employed types are:
• Unconsolidated Undrained Test (UU)
• Consolidated Undrained Test (CU)
• Consolidated Drained Test (CD)

42. 1. Unconsolidated Undrained Test (UU)
As the name tells, the soil sample is subjected to cell pressure
with no provision of drainage. Here the cell pressure is
maintained to a constant value and the applied deviator stress
is increased till the sample fails. This is called as quick test.

43. 2. Consolidated Undrained Test (CU)
Here, during the application of cell pressure on the sample,
drainage is permitted. And the deviator stress is applied
keeping the cell pressure constant and no provision of further
drainage.

44. 3. Consolidated Drained Test (CD)
• This test is also called as drained or slow test. Here the
deviator stress is increased by allowing the drainage to
happen as it was and the cell pressure is also kept constant.
• Here the rate of loading is applied slowly so that excess pore
pressure is not developed within the sample.
• The prepared specimen is enveloped in the membrane and
positioned in the triaxial cell.
• To this, the desired lateral pressure is applied. Till the
specimen fails, the lateral pressure is applied. The vertical
deformation and the load readings are recorded.
• The main objective of the test is to determine the values of
cohesion and angle of internal friction.