Shear Strength of Soils
Summary
TLDRSaeed Hashemi's presentation delves into the critical geotechnical concept of soil's shear strength, essential for analyzing the stability of soil masses in structural engineering. It defines shear strength as the soil's resistance to failure and sliding, introduces the Coulomb failure criterion, and discusses how cohesion and friction angle affect soil strength. The presentation also covers the Mohr Circle for stress representation, the significance of effective stresses over total stresses, and the method to determine the failure envelope through testing. Lastly, it touches on the calculation of the failure plane's orientation, providing a comprehensive understanding of soil mechanics.
Takeaways
- 📚 Shear strength of soil is a fundamental concept in geotechnical engineering, defining the internal resistance of soil to resist failure and sliding.
- 🔍 Shear failure occurs when the shear stress equals the shear strength of the soil, leading to potential structural instability if not accounted for in construction.
- 🏗️ The safety of structures like buildings and bridges is closely tied to the shear strength of the soil they are built upon.
- 📏 Coulomb's failure criterion expresses the shear strength of soil as a linear function of the normal stress at failure, crucial for predicting soil behavior.
- 📐 The Mohr-Coulomb failure criterion introduces two parameters: cohesion (C) and the angle of internal friction (Φ), which are key to understanding shear strength.
- 🌐 Different soil types exhibit varying cohesion and friction angles, affecting their shear strength and resistance to failure.
- 📉 The relationship between shear stress and normal stress can be non-linear for some soils, indicating complexity in soil behavior.
- 🔄 The concept of effective stress is essential for considering underground pore pressure when analyzing soil stability.
- 🔍 Mohr's Circle is a graphical representation used to determine the state of stress in soil and to predict failure conditions.
- 🧪 Laboratory tests on soil specimens are necessary to plot Mohr's Circles and define the failure envelope, enhancing the accuracy of soil strength assessments.
- 📝 The orientation of the failure plane in soil can be calculated using the friction angle, providing insight into the direction of potential slips or collapses.
Q & A
What is the definition of shear strength of soil according to the presentation?
-Shear strength of soil is defined as the internal resistance per unit area that the soil mass can offer to resist failure and sliding along any plane inside it.
Why is understanding shear strength important in geotechnical engineering?
-Understanding shear strength is crucial in geotechnical engineering because it helps analyze the stability of soil masses, which is essential for the safety of structures like buildings and bridges.
What is the Coulomb failure criterion mentioned in the presentation?
-The Coulomb failure criterion is a linear function that expresses the shear strength of a soil at a point on a particular plane as a function of the normal stress at failure on the same point.
What are the two components of shear strength mentioned in the script?
-The two components of shear strength are cohesive and frictional, which are represented by the soil's cohesion (C) and the angle of internal friction (Phi).
How does the angle of internal friction relate to the stability of soil?
-The angle of internal friction (Phi) is related to the stability of soil because it represents the angle of repose at which the soil will naturally stand. A steeper angle indicates a firmer soil, which is more resistant to shear failure.
What is the significance of the Mohr Circle in the context of soil mechanics?
-The Mohr Circle is significant in soil mechanics as it graphically represents the state of stress in a soil element and is used to determine the maximum shear stress and the orientation of the failure plane.
What is the role of effective stress in the analysis of soil failure?
-Effective stress plays a critical role in the analysis of soil failure because it considers the influence of underground pore pressure. It is used to calculate the actual stress on the soil grains that can lead to failure.
How is the failure envelope determined using Mohr Circles?
-The failure envelope is determined by conducting multiple tests on identical soil specimens and plotting the Mohr Circles for each test condition. The envelope is then drawn to represent the boundary of all possible failure conditions.
What does the orientation of the failure plane indicate in soil mechanics?
-The orientation of the failure plane indicates the direction in which the soil is most likely to fail under a combination of shear and normal stresses, and it can be calculated as a function of the friction angle.
Why is it important to consider the effective stresses rather than total stresses when dealing with underground pore pressure?
-Considering effective stresses is important when dealing with underground pore pressure because it accounts for the reduction in stress on the soil grains due to the presence of water or air in the soil pores, leading to a more accurate prediction of soil behavior.
How does the presentation suggest increasing the number of tests can improve the accuracy of the failure envelope?
-The presentation suggests that increasing the number of tests allows for more Mohr Circles to be plotted, which leads to a more precise and detailed failure envelope, enhancing the understanding of soil behavior under various stress conditions.
Outlines
🏗️ Shear Strength of Soils and Failure Criteria
Saeed Hashemi introduces the concept of shear strength in soils, a fundamental aspect of geotechnical engineering. Shear strength is the internal resistance of soil to resist failure and sliding along any plane within it. It is crucial for analyzing the stability of soil masses and ensuring the safety of structures like buildings and bridges. The presentation explains that soils fail in shear under various stress combinations. The Coulomb failure criterion is discussed, which expresses the shear strength as a linear function of the normal stress at failure. The two components of shear strength—cohesion and friction—are highlighted, with examples of different soil types and their respective values. The importance of understanding shear strength for geotechnical engineering is emphasized.
📊 Mohr's Circle and Failure Envelope in Soil Mechanics
The second paragraph delves into the application of Mohr's Circle for understanding stress states in soil mechanics. It explains how different stress conditions can be represented by Mohr's Circles and how the failure envelope is determined by the intersection of these circles with the envelope. The concept of effective stress is introduced, which is necessary when considering underground pore pressure. The process of finding the failure envelope using Mohr's Circles involves testing identical soil specimens under varying stress conditions until failure occurs. The orientation of the failure plane is also discussed, showing how it can be calculated based on the friction angle. The presentation concludes with a practical example of how these principles are applied in geotechnical engineering to predict and prevent soil failure.
Mindmap
Keywords
💡Shear Strength
💡Shear Failure
💡Coulomb's Failure Criterion
💡Friction Angle
💡Cohesion
💡Effective Stress
💡Mohr's Circle
💡Failure Envelope
💡Deviatoric Stress
💡Orientation of Failure Plane
Highlights
Shear strength of soils is a fundamental concept in geotechnical engineering.
Shear strength is the internal resistance of soil to resist failure and sliding along any plane.
Soils generally fail in shear under different combinations of stresses.
Shear strength is critical for analyzing the stability of soil masses and the safety of structures.
Shear failure occurs when shear stress equals the soil's shear strength at a point within the soil mass.
Coulomb's failure criterion expresses shear strength as a linear function of normal stress at failure.
The Mohr-Coulomb failure criterion includes cohesion (C) and the angle of internal friction (Φ).
Friction angle can be defined as the angle of repose of the soil.
Shear strength has two components: cohesive and frictional.
Different soil types have varying values of cohesion and friction angle.
The relationship between shear stress and normal stress can be non-linear for some soil types.
Higher values of cohesion and friction angle result in higher shear strength of soil.
The Mohr's Circle represents any stress state and can be used to define the failure envelope.
Effective stresses must be considered when there is underground pore pressure.
The orientation of the failure plane can be calculated based on the friction angle.
Laboratory tests are necessary to determine the failure envelope using Mohr's Circles.
The number of tests influences the precision of the failure envelope.
The maximum shear stress and normal stress at failure can be calculated from effective principle stresses.
Transcripts
hello everybody my name is Saeed hashemi
today I'm going to give you a short
presentation about sheer strength of
soils which is one of the most important
basics of geotechnical engineering
here is the outline of my today's
presentation first I'm going to
introduce the sheer strength of soils
and Define the shear failure concept
then I'll explain the more coulomb
failure Criterion and finally I'll
explain the sheer parameters related to
these Criterion
okay what is the shear strength of the
soil Shear strength is defined as the
internal resistance per unit area that
the soil Mass can offer to resist
failure and sliding along any plane
inside it in fact various studies showed
that soils generally fail in Shear under
different combination of stresses as you
can see in this figure
if at the point on any plane within a
soil Mass which is under loading such as
these embankment or this street footing
the shear stress becomes equal to the
shear strength of the soil then failure
will occur at that point therefore we
need this knowledge to analyze the
stability of soil masses
so in case of constructing any structure
such as a building or a bridge the
safety of that structure closely depends
on the sheer strength of the soil
as you can see in this photo
under footing soil fail in this grain
elevator in Canada and then the
structure collapsed while it had no
problem in its building elements such as
beams or columns
we know that soil is a material
comprised of discrete mineral grains and
decayed organic matter along with
intergranular gases and liquids as shown
here
when the shear stress along this surface
reaches the shear strength of the soil
failure happens in case of a Shear
failure the soil grains which is here
slide over each other along the failure
surface
and crushing of cranes won't occur if
the soil is not Consolidated actually
these two photos are examples of sheer
failure due to Landslide phenomenon
originally the sheer strength of a soil
at a point on a particular plane was
expressed by coulomb as a linear
function of the normal stress at failure
on the plane at the same point and this
function as you can see here Tau f is
the maximum shear stress the soil can
take without failure under normal stress
Sigma thus failure will occur at any
point in the soil where the critical
combination of shear and normal stresses
develops also as you can see in this
graph
the state of stress in two Dimensions
can be shown on a plot of shear stress
versus versus normal stress Sigma in
this graph derived C
is the cohesion of the soil and Phi here
which is this angle
it will be the angle of internal
friction or friction angle actually a
friction angle can be defined as the
angle of repose which the soil still is
naturally so a firm soil will have a
steeper angle of reposed than a loose
one
as I mentioned before Shear strength has
two components cohesive and frictional
in this slide you can see the different
values of cohesion and friction angle
for different soil types for example in
case of pure gravel there is no cohesion
between the grains and C is zero and the
shearing resistance is developed only by
inter particle forces by Phi or in case
of plastic clay which is shown here the
only parameter that resists against the
shear is cohesion and Phi is zero
because there is no friction between the
grains it should be noted that the
relationship between Tau
and sigma
can be non-linear for some soil types
and finally you should notice that the
higher the values C and Phi higher the
shear strength of a soil
okay more circles
any stress State can be represented by
coulomb equation as I mentioned before
and also a more more Circle which is
here or here
can be defined by the total or effective
principle stresses Sigma 1 dash and
sigma 3 Dash
we can draw in more circles for
different test conditions for example in
if the combination of stresses touches
the failure envelope the element will
fail as shown here and for element X
but in case of element y the more Circle
as you can see here will not touch the
envelope and element remains safe
in this element element y with applying
the Delta Sigma stress after the
hydrostatic estrus condition more Circle
becomes larger and with this stress
condition that we have in here the soil
elements does not fail why because the
more circle is located within the
failure envelope but please note that
more circles cannot extend to the area
above the envelope because when it
touches the envelope here this is the
failure envelope the element fails and
it is not possible to pass this line
in this slide you can see that with
increasing the Delta Sigma or deviatoric
stress more Circle becomes larger
until touches the failure envelope and
the sample fails at exactly this point
it should be mentioned that the shear
stress in a soil element can be resisted
only by the skeleton of the soil grains
so we need to consider the effective
stresses if we have underground pore
pressure so in this case we have to
calculate the effective stresses and
then we can draw the more Circle as you
can see in here instead of the more
Circle that we have we can have with the
total stresses and also the center of
the Mohr Circle and also the maximum
shear stress can be calculated by these
two simple equations if you wanna if we
have the sigma V and sigma H according
to our experimental test then we can
derive the sigma Dash Sigma V Dash and
sigma H dash and then we can draw our
more Circle easily
uh for finding the failure envelope by
more circles we need to test several
identical specimens in the first stage
we need to apply the hydrostatic stress
condition to the specimen and in the
second stage we will increase the the
butyric stress or vertical stress to the
specimens until the specimen failed so
with each test we will have one more
Circle so we need at least at least
three more circles to be able to draw
our failure envelope and in case of
total stress and in case of having
underground pore pressure then we have
to calculate the effective stresses
from the total stresses and then we can
draw our
more circles but in terms of Sigma Dash
and then we can draw our failure
envelope easily but keep in mind that
with increasing the number of tests we
can have more precise failure envelope
and finally the last slide is about the
orientation of failure playing as you
can see here the orientation of failure
play in a soil element which failed due
to the combination of shear stress and
normal stress as well can be calculated
as a function of a friction angle it's
equal to 45 degrees plus Phi over 2.
actually this angle Theta can be also
shown on more Circle which is two times
Theta
and the stresses at failure points Tau F
and sigma F Dash can be calculated as a
function of effective principle stresses
Sigma 1 dash and sigma 3 Dash so we can
calculate these two stresses based on
the applied stresses that we had
recorded in the experimental test in the
laboratory
I hope you enjoyed the presentation and
thank you so much for your attention
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