PPT for shear strength of soil which gives you an overview of the topic and mostly the content is inspired by prof. Muni Budhu

1. Shear Strength of Soils
By:- Pankaj Drolia

2. Shear Strength
Shear strength of a soil is the maximum internal resistance to applied shearing forces

3. Throughout this session we will be considering two groups of soil:-
● Type I soils—loose sands and normally consolidated and lightly overconsolidated
clays
● Type II soils—dense sands and overconsolidated clays

5. ● The peak shear stress of Type II soils is suppressed and the volume expansion
decreases when the normal effective stress is large.
● Just before peak shear stress is attained in Type II soils, shear bands develop. Shear
bands are loose pockets or bands of soil masses that have reached the critical state
shear stress. Denser soil masses adjacent to shear bands gradually become looser as
shearing continues.
● All soils reach a critical state, irrespective of their initial state, at which continuous
shearing occurs without changes in shear stress and volume for a given normal
effective stress.
● The critical state shear stress and the critical void ratio depend on the normal
effective stress.
● Higher normal effective stresses result in higher critical state shear stresses and
lower critical void ratios. The critical void ratio is not a fundamental soil property.
● At large strains, the particles of some overconsolidated clays become oriented
parallel to the direction of shear bands, and the final shear stress attained is lower
than the critical state shear stress.
● Higher overconsolidation ratios of homogeneous soils result in higher peak shear
stresses and greater volume expansion

6. ● Volume changes that occur under drained condition are suppressed under undrained
condition. The result of this suppression is that a soil with a compression tendency under
drained condition will respond with positive excess porewater pressures during undrained
condition, and a soil with an expansion tendency during drained condition will respond
with negative excess porewater pressures during undrained condition.
● Cohesion, defined as the shearing resistance from intermolecular forces, is generally small
for consideration in geotechnical application.
● Soil tension resulting from surface tension of water on soil particles in unsaturated soils
creates an apparent shear resistance that disappears when the soil is saturated. You need to
be cautious in utilizing this additional shearing resistance in certain geotechnical
applications such as shallow excavations.
● Cementation—the chemical bonding of soil particles—is present to some degree in all
natural soils. It imparts shear strength to the soil at zero normal effective stress. The shear
strain at which this shear strength is mobilized is very small. You should be cautious in
using this shear strength in designing geotechnical systems because in most of these
systems the shear strain mobilized is larger than that required to mobilize the shear
strength due to cementation.

7. Models for interpreting the shear strength of
soil
1. Coulomb’s failure criterion
2. Taylor’s failure criterion
3. Mohr-Coulomb failure criterion
4. Tresca failure criterion

8. Coulomb’s Failure criterion
Coulomb’s law requires the existence or the development of a critical sliding plane, also called slip plane.

9. 1. Shear failure of soils may be modeled using Coulomb’s frictional law,
, where τf is the shear stress when slip is initiated, σ’ is the normal effective stress on the
slip plane, ф’ is the friction angle, and α is the dilation angle.
2. The effect of dilation is to increase the shear strength of the soil and cause the
Coulomb’s failure envelope to be curved.
3. Large normal effective stresses tend to suppress dilation.
4. At the critical state, the dilation angle is zero.
5. For cemented soils, Coulomb’s frictional law is where Ccm is called
the cementation strength and ξо is the apparent friction angle.

11. Taylor’s failure criterion
Taylor (1948) used an energy method to derive a simple soil model. He assumed that the
shear strength of soil is due to sliding friction from shearing and the interlocking of soil
particles.
Unlike Coulomb failure criterion, Taylor failure criterion does not require the assumption
of any physical mechanism of failure, such as a plane of sliding. It can be applied at every
stage of loading for soils that are homogeneous and deform under plane strain conditions
similar to simple shear.

12. The critical state shear strength is:
The peak shear strength is:

13. Mohr-Coulomb failure criterion
Coupling Mohr’s circle with Coulomb’s frictional law allows us to define shear failure
based on the stress state of the soil.

14. ● Failure occurs, according to the Mohr–Coulomb failure criterion, when the soil
reaches the maximum principal effective stress obliquity, that is max.
● The failure plane or slip plane is inclined at an angle to the
plane on which the major principal effective stress acts.
● The maximum shear stress,τ(max) = , is not the failure shear
stress.

15. Tresca failure criterion
● The shear strength of a fine-grained soil under undrained condition is called the
undrained shear strength
● We use the Tresca failure criterion—shear stress at failure is one-half the principal
stress difference—to interpret the undrained shear strength
● The undrained shear strength, su, is the radius of the Mohr total stress circle.
● The undrained shear strength depends on the initial void ratio. It is not a
fundamental soil shear strength parameter.

19. Points to be noted
● The friction angle at the critical state, , is a fundamental soil parameter.
● The friction angle at peak shear stress for dilating soils, , is not a fundamental
soil parameter but depends on the capacity of the soil to dilate.

20. LABORATORY TESTS TO
DETERMINE
SHEAR STRENGTH PARAMETERS

21. A Simple Test to Determine Friction Angle of
Clean Coarse-Grained Soils
The critical state friction angle, , for a clean coarse-grained soil can be found by
pouring the soil into a loose heap on a horizontal surface and measuring the slope angle
of the heap relative to the horizontal. This angle is sometimes called the angle of repose,
but it closely approximates

22. Shear Box or Direct Shear Test
● This test is useful when a soil mass is likely to fail along a thin zone under plane
strain conditions.
● The shear box consists of a horizontally split, open metal box.
● Soil is placed in the box, and one-half of the box is moved relative to the other half.
Failure is thereby constrained along a thin zone of soil on the horizontal plane.

23. ● Vertical forces are applied through a metal platen resting on the top serrated
plate.
● Horizontal forces are applied through a motor for displacement control or
by weights through a pulley system for load control.
● The horizontal displacement, Δx, the vertical displacement, Δz, the vertical
loads, Pz, and the horizontal loads, Px, are measured
● Usually, three or more tests are carried out on a soil sample using three
different constant vertical forces.
● Failure is determined when the soil cannot resist any further increment of
horizontal force
● From the recorded data, you can find the following strength parameters:
(and su, if fine-grained soils are tested quickly).

24. TRIAXIAL TEST
● The most versatile of all the shear testing methods
● Drainage conditions can be controlled, porewater pressure measurements can be
made.
● Volume changes can be measured.
● The failure plane is not fixed. The specimen can fail on any weak plane or can simply
bulge.
● The stress distribution on the failure plane is uniform.

26. General Procedure
● In the triaxial test, a cylindrical sample of soil, usually with a length-to-diameter
ratio of 2
● The sample is laterally confined by a membrane, and radial stresses are applied by
pressuring water in a chamber.
● The axial stresses are applied by loading a plunger.
● If the axial stress is greater than the radial stress, the soil is compressed vertically
and the test is called triaxial compression.
● If the radial stress is greater than the axial stress, the soil is compressed laterally
and the test is called triaxial extension.

28. Consolidated-Drained (CD) Test
A consolidated drained compression test is performed in two stages.
1. The first stage is consolidating the soil to a desired effective stress level by
pressurizing the water in the cell and allowing the soil sample to drain until the
excess porewater pressure dissipates.
2. In the second stage, the pressure in the cell (cell pressure or confining pressure) is
kept constant, and additional axial loads or displacements are added very slowly
until the soil sample fails.

30. The CD test is a drained test, a single test can take several days if the hydraulic
conductivity of the soil is low (e.g., fine-grained soils).
The Mohr–Coulomb failure criterion is used to interpret the results of a CD test
The results of CD tests are used to determine the long-term stability of slopes,
foundations, retaining walls, excavations, and other earthworks.

31. Consolidated-Undrained (CU) Test
● The purpose of a CU test is to determine the undrained and drained shear strength
parameters
● The CU test is conducted in a similar manner to the CD test except that after
isotropic consolidation the axial load is increased under undrained condition and
the excess porewater pressure is measured
● The effective stress path is nonlinear because when the soil yields, the excess
porewater pressures increase nonlinearly, causing the ESP to bend.

33. ● Analysis of both short-term and long-term stability of slopes, foundations,
retaining walls, excavations, and other earthworks.
● Mohr Coulomb and Tresca failure criterion are used.

34. Unconsolidated - Undrained (UU) Test
The UU test consists of applying a cell pressure to the soil sample without drainage of
porewater followed by increments of axial stress. The cell pressure is kept constant and
the test is completed very quickly because in neither of the two stages—consolidation and
shearing—is the excess porewater pressure allowed to drain.

36. ● Used for fine grained soil.
● Tresca failure criterion is used.
● Analysis of short-term stability of slopes, foundations, retaining walls,
excavations, and other.

37. Unconfined Compression (UC) Test
● Purpose is to determine the undrained shear strength of saturated clays quickly.
● No radial stress is applied to the sample.
● The axial (plunger) load, Pz, is increased rapidly until the soil sample fails, that is, it
cannot support any additional load.
● The loading is applied quickly so that the porewater it cannot drain from the soil;
the sample is sheared at constant volume

38. The results from UC tests are used to:
● Estimate the short-term bearing capacity of fine-grained soils for foundations.
● Estimate the short-term stability of slopes.
● Compare the shear strengths of soils from a site to establish soil strength variability
quickly and cost-effectively (the UC test is cheaper to perform than other triaxial
tests).
● Determine the stress–strain characteristics under fast (undrained) loading
conditions.

39. Field Tests

40. Vane Shear Test (VST)
● Undrained shear strength of soft clays can be determined by VST
● Apparatus- Consists of a vertical steel rod having four stainless
steel blades fixed at its bottom end.

41. The undrained shear strength from a vane shear test is calculated from
where T is the maximum torque, h is the height, and d is the diameter of the vane.

42. Other field tests
● Standard Penetration Test (SPT) : Results from SPT have been correlated to several
soil parameters.Most of these correlation are weak.

43. ● Cone Penetrometer test (CPT):The cone resistance qc is normally correlated with
the undrained shear strength. Several adjustments are made to qc.
● Where Nk is cone factor and PI is plasticity Index

44. Points to be noted
● Various field tests are used to determine soil strength parameters
● You should be cautious in using these correlations of field test results, especially
SPT, with soil strength parameters in design

45. Sources
● SOIL MECHANICS AND FOUNDATION
BY Dr. MUNI BUDHU,
Professor, Department of Civil Engineering & Engineering Mechanics,University of
Arizona
● Google

46. Thank You !