The document provides information about stress distribution in soil due to self-weight and surface loads. It discusses Boussinesq's formula for calculating vertical stress in soil due to a concentrated surface load. The formula shows that vertical stress is directly proportional to the load, inversely proportional to depth squared, and depends on the ratio of radius to depth. A table of coefficient values used in the formula for different ratios of radius to depth is also provided.
Introduction to Machine Learning Unit-3 for II MECH
Stress Distribution
1. INTERNATIONAL UNIVERSITY
FOR SCIENCE & TECHNOLOGY
وا م ا و ا ا
CIVIL ENGINEERING AND
ENVIRONMENTAL DEPARTMENT
303322 - Soil Mechanics
Stress Distribution in Soil
Dr. Abdulmannan Orabi
Lecture
2
Lecture
7
2. Dr. Abdulmannan Orabi IUST 2
Das, B., M. (2014), “ Principles of geotechnical
Engineering ” Eighth Edition, CENGAGE
Learning, ISBN-13: 978-0-495-41130-7.
Knappett, J. A. and Craig R. F. (2012), “ Craig’s Soil
Mechanics” Eighth Edition, Spon Press, ISBN: 978-
0-415-56125-9.
References
3. Stress in soil due to self weight
Stress Distribution in Soil
Stress in soil due to surface load
3Dr. Abdulmannan Orabi IUST
4. Stress due to self weight
The vertical stress on element A can be determined
simply from the mass of the overlying material.
If represents the unit weight of the soil, the
vertical stress is
Variation of stresses with depth
A
Ground surface
zz ⋅= γσ
4Dr. Abdulmannan Orabi IUST
5. ∑=
⋅=⋅++⋅+⋅=
n
i
iinnz hhhh
1
2211 ...... γγγγσ
Stress due to self weight
Stresses in a Layered Deposit
The stresses in a deposit consisting of layers of
soil having different densities may be determined as
Vertical stress at depth z1
Vertical stress at depth z2
Vertical stress at depth z3
∗
∗ ∗
∗
∗ ∗
∗ ∗ ∗
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6. With uniform surcharge on infinite land surface
Stress due to self weight
Original
land surface
Conversion land surface
∗
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7. Stress due to self weight
∗
Vertical stresses due to self weight increase
with depth,
There are 3 types of geostatic stresses:
a. Total Stress, σtotal
b. Effective Stress, σ'
c. Pore Water Pressure, u
Vertical Stresses
7Dr. Abdulmannan Orabi IUST
8. Stress due to self weight
Consider a soil mass having a horizontal
surface and with the water table at surface
level. The total vertical stress at depth z is
equal to the weight of all material (solids +
water) per unit area above that depth ,i.e
Total vertical stress
!"!#$ %#! ∗
8Dr. Abdulmannan Orabi IUST
9. Stress due to self weight
The pore water pressure at any depth will be
hydrostatic since the void space between the solid
particles is continuous, therefore at depth z:
Pore water pressure
& ∗
If the pores of a soil mass are filled with water
and if a pressure induced into the pore water, tries
to separate the grains, this pressure is termed as
pore water pressure
9Dr. Abdulmannan Orabi IUST
10. Stress due to self weight
Effective vertical stress due to self weight of soil
The difference between the total stress ( !"!#$) and
the pore pressure (u) in a saturated soil has been
defined by Terzaghi as the effective stress ( ).'
'
!"!#$ −
The pressure transmitted through grain to grain at
the contact points through a soil mass is termed as
effective pressure.
10Dr. Abdulmannan Orabi IUST
11. Stress due to self weight
Stresses in Saturated Soil
If water is seeping, the effective stress at any
point in a soil mass will differ from that in
the static case.
It will increase or decrease, depending on the
direction of seepage.
The increasing in effective pressure due to the
flow of water through the pores of the soil is
known as seepage pressure.
11Dr. Abdulmannan Orabi IUST
12. A column of saturated soil mass with no seepage of
water in any direction.
The total stress at the
elevation of point A can be
obtained from the saturated
unit weight of the soil and
the unit weight of water
above it. Thus,
Stress due to self weight
Stresses in Saturated Soil without Seepage
0
A
Solid particle
Pore water
)*
)&
+
+
12Dr. Abdulmannan Orabi IUST
13. 0
A Solid particle
Pore water
)*
)&
+
+
+
+
Forces acting at the points of contact of soil
particles at the level of point A
Stress due to self weight
Stresses in Saturated
Soil without Seepage
& ) ,)* − )- %#!
where
+ . +
/+ 0 1
%#! + .+ 2
)* 2 + 3 4 0
1 + 2 + . +4
13Dr. Abdulmannan Orabi IUST
14. Stress due to self weight
Stresses in Saturated Soil without Seepage
)
)
5
6
7
8
Valve (closed)
Stress at point A,
• Total stress:
• Pore water pressure:
• Effective stress:
* & )
* & )
*
'
* − * 0
Stress at point B,
• Total stress:
• Pore water pressure
• Effective stress:
: & ) ) ∗ %#!
: ,) ) - &
:
'
: − :
:
'
) %;<
14Dr. Abdulmannan Orabi IUST
15. Stress due to self weight
Stresses in Saturated Soil without Seepage
Stress at point C,
• Total stress:
= & ) ∗ %#!
> ,) - &
>
'
> − >
>
'
%;<
• Pore water pressure:
• Effective stress:
Total stress
Pore water
Pressure, u Effective stress
DepthDepth Depth
15Dr. Abdulmannan Orabi IUST
16. )
)
5
6
7
8
Valve (open)
?
(
@
AB
-
Stress due to self weight
Stresses in Saturated Soil with Upward Seepage
Stress at point A,
• Total stress:
• Pore water pressure:
• Effective stress:
* & )
* & )
*
'
* − * 0
16Dr. Abdulmannan Orabi IUST
17. Stresses in Saturated Soil with Upward Seepage
Stress due to self weight
Stress at point B,
• Total stress:
• Pore water pressure
• Effective stress:
: & ) ) ∗ %#!
: ,) ) - &
:
'
: − :
:
'
) %;< − &
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18. Stresses in Saturated Soil with Upward Seepage
Stress due to self weight
Stress at point C,
• Total stress:
• Pore water pressure:
• Effective stress:
= & ) ∗ %#!
: ,)
)
- &
>
'
> − >
>
'
%;< −
)
&
>
'
%;< − &
Note that h/H2 is the hydraulic gradient i
caused by the flow, and therefore
18Dr. Abdulmannan Orabi IUST
19. Total stress
Pore water
Pressure, u Effective stress
DepthDepth Depth
Stress due to self weight
Stresses in Saturated Soil with Upward Seepage
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20. Stress due to self weight
Stresses in Saturated Soil with Upward Seepage
At any depth z, is the pressure of the
submerged soil acting downward and is the
seepage pressure acting upward.
The effective pressure reduces to zero when these two
pressures balance.
This situation generally is referred to as boiling.
>
'
%;< − >C & 0
>C
%;<
&
. >C 3. 3+ D2.+ 3 .+2
For most soils, the value of >C varies from 0.9 to 1.1
%;<
&
>
'
20Dr. Abdulmannan Orabi IUST
21. )
)
5
6
7
8
Valve (open)
?
(
@
AB
-
Stress due to self weight
Stresses in Saturated Soil with Downward Seepage
Stress at point A,
• Total stress:
• Pore water pressure:
• Effective stress:
* & )
* & )
*
'
* − * 0
21Dr. Abdulmannan Orabi IUST
22. Stress at point B,
• Total stress:
• Pore water pressure
• Effective stress:
: & ) ) ∗ %#!
: ,) ) − - &
:
'
: − :
:
'
) %;< &
Stress due to self weight
Stresses in Saturated Soil with Downward Seepage
22Dr. Abdulmannan Orabi IUST
23. Stress due to self weight
Stress at point C,
• Total stress:
• Pore water pressure:
• Effective stress:
= & ) ∗ %#!
: ,) −
)
- &
>
' > − >
>
'
%;<
)
& >
'
%;< &
Stresses in Saturated Soil with Downward Seepage
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24. Pore water
Pressure, uTotal stress Effective stress
DepthDepth Depth
Stress due to self weight
Stresses in Saturated Soil with Downward Seepage
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25. Worked Examples
Example 1
A soil profile is shown in figure below. Calculate total
stress, pore water pressure, and effective stress at A,
B, C, and D.
D
C
B
A Ground surface
G.W.T
Sand
Clay
Sandγ = 16.3 kN/m^3
γ = 15.1 kN/m^3
γ = 19.8 kN/m^3
1.8 m
1.6 m
2.9 m
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26. Stress due to self weight
Total stress Effective stress Pore water pressure
DepthDepthDepth
γ1 X H1
γ1 X H1 + γ2 X H2
γ1 X H1 + γ2 X H2 + γ3 X H3
γ1 X H1 + γ2 X H2 + γsub X H3
γw X Hw
26Dr. Abdulmannan Orabi IUST
27. To analyze problems such as compressibility of
soils, bearing capacity of foundations, stability
of embankments, and lateral pressure on earth
retaining structures, we need to know the
nature of the distribution of stress along a
given cross section of the soil profile.
Stress due to surface load
Introduction
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28. When a load is applied to the soil surface, it
increases the vertical stresses within the soil
mass. The increased stresses are greatest
directly under the loaded area, but extend
indefinitely in all directions.
Introduction
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Stress due to surface load
29. •Allowable settlement, usually set by building
codes, may control the allowable bearing
capacity.
•The vertical stress increase with depth must
be determined to calculate the amount of
settlement that a foundation may undergo
Introduction
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Stress due to surface load
30. Introduction
Foundations and structures placed on the
surface of the earth will produce stresses in
the soil
These stresses will decrease with the
distance from the load
How these stresses decrease depends upon
the nature of the soil bearing the load
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Stress due to surface load
31. Individual column footings or wheel loads
may be replaced by equivalent point loads
provided that the stresses are to be
calculated at points sufficiently far from
the point of application of the point load.
Stress Due to a Concentrated Load
31Dr. Abdulmannan Orabi IUST
Stress due to surface load
32. Stresses in soil due to surface load
Vertical stress due to a concentrated load
• Boussinesq’s Formula
• Wastergaard Formula
Stress Due to a Concentrated Load
32Dr. Abdulmannan Orabi IUST
33. Stress Due to a Concentrated Load
Boussinesq’s Formula for Point Loads
Joseph Valentin Boussinesq (13 March 1842 – 19 February
1929) was a French mathematician and physicist who made
significant contributions to the theory of hydrodynamics, vibration,
light, and heat.
33Dr. Abdulmannan Orabi IUST
Stresses in soil due to surface load
34. In 1885, Boussinesq developed the
mathematical relationships for determining
the normal and shear stresses at any point
inside a homogenous, elastic and isotropic
mediums due to a concentrated point loads
located at the surface
Vertical Stress in Soil
Stress Due to a Concentrated Load
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35. The soil mass is elastic, isotropic (having
identical properties in all direction
throughout), homogeneous (identical elastic
properties) and semi-infinite depth.
The soil is weightless.
Stress Due to a Concentrated Load
Assumption:
35Dr. Abdulmannan Orabi IUST
Vertical Stress in Soil
36. The distribution of σz in the elastic medium
is apparently radially symmetrical.
The stress is infinite at the surface directly
beneath the point load and decreases with the
square of the depth.
Vertical Stress in Soil
Stress Due to a Concentrated Load
36Dr. Abdulmannan Orabi IUST
37. At any given non-zero radius, r, from the point
of load application, the vertical stress is zero
at the surface, increases to a maximum value at
a depth where , approximately, and
then decreases with depth.
E 39.25°
Vertical Stress in Soil
Stress Due to a Concentrated Load
37Dr. Abdulmannan Orabi IUST
38. Vertical Stress in Soil
According to Boussinesq’s analysis, the vertical stress
increase at point A caused by a point load of magnitude P
is given by
Stress Due to a Concentrated Load
D
∆
∆ M
∆ N
O
P
Q
.
P
D
1
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39. Vertical Stress in Soil
Stress Due to a Concentrated Load
According to Boussinesq’s analysis, the vertical stress
increase at point A caused by a point load of magnitude P
is given by
2 2 5/2
3 1
2 [1 ( / ) ]
z
P
z r z
σ
π
=
+
39Dr. Abdulmannan Orabi IUST
… … . 7 − 1
1
∆
.
Q
or
2z b
P
I
z
σ =
40. Equation shows that the vertical stress is
Directly proportional to the load
Inversely proportional to the depth squared, and
Proportional to some function of the ratio ( r/z).
Vertical Stress in Soil
Stress Due to a Concentrated Load
where
2 5/2
3 1
2 [1 ( / ) ]
bI
r zπ
=
+
40Dr. Abdulmannan Orabi IUST
… … … … . 7 − 2
41. It should be noted that the expression for z is
independent of elastic modulus (E) and
Poisson’s ratio (µ), i.e. stress increase with depth
is a function of geometry only.
Vertical Stress in Soil
Stress Due to a Concentrated Load
41Dr. Abdulmannan Orabi IUST
45. Equation may be used to draw three types of pressure
distribution diagram. They are:
The vertical stress distribution on a horizontal
plane at depth of z below the ground surface
The vertical stress distribution on a vertical plane
at a distance of r from the load point, and
The stress isobar.
Vertical Stress in Soil
Pressure Distribution Diagram
45Dr. Abdulmannan Orabi IUST
46. The vertical stress distribution on a horizontal
plane at depth of z below the ground surface
U
5
5
Vertical Stress in Soil
Distribution on a horizontal plane
46Dr. Abdulmannan Orabi IUST
47. The vertical stress
distribution on a vertical
plane at a distance of r
from the point load
.
Vertical Stress in Soil
Distribution on a vertical plane O
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48. U
Vertical Stress in Soil
Stress isobars
An isobar is a line which
connects all points of equal
stress below the ground
surface. In other words, an
isobar is a stress contour.
48Dr. Abdulmannan Orabi IUST
49. What is the vertical stress at point A of figure below
for the two loads, P1 and P2 ?
P1 = 350 kNP2 = 470 kN
Z=2.5m
2.3 m1.1 m
A
Worked Examples
Example 2
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50. A four concentrated forces are located at corners of
a rectangular area with dimensions 8 m by 6 m as
shown in figure in the next slide. Compute the
vertical stress at points A and B, which are located
on the lines A – A’ , B – B’ at depth of 4 m below
the ground surface.
Worked Examples
Example 3
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51. 700 kN700 kN
700 kN700 kN
4 m
4 m
8 m
B
A’
A
B’
Worked Examples
Example 3
Dr. Abdulmannan Orabi IUST 51
52. Vertical Stress in Soil
Westergaard Formula
Westergaard proposed a formula for the
computation of vertical stress by a point load,
P at the surface as
O +
2V +
.
/
In which µ is Poisson’s ratio
+ 1 − 2X /,2 − 2X-
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… . 7 − 3
53. Vertical Stress in Soil
Stress below a Line Load
The vertical stress increase due to line load , ,
inside the soil mass can be determined by using the
principles of the theory of elasticity, or
2
V P
This equation can be rewritten as
/
2
V 1
P
1P
P
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… . 7 − 4
54. Vertical Stress in Soil
Vertical Stress caused by a horizontal line load
The vertical stress increase ( ) at point A in
the soil mass caused by a horizontal line load
can be given as :
2 P
V P
1
/
P
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… . 7 − 5
55. Vertical Stress in Soil
Vertical Stress caused by a strip load
The fundamental equation for the vertical stress
increase at a point in a soil mass as the result of
a line load can be used to determine the vertical
stress at a point caused by a flexible strip load of
width B.
The term strip loading will be used to indicate a
loading that has a finite width along the x axis
but an infinite length along the y axis.
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56. Vertical Stress in Soil
Vertical Stress caused by a strip load
α
β
6
B
Vertical stress at point A can be determined by equation:
[ sin cos( 2 )]o
z
q
σ α α α β
π
= + +
P
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… . 7 − 6
58. _
6
4
"
+
_
1 2 2[( )( ) ( )]o
z
q a b b
a a
σ α α α
π
+
= + −
Vertical Stress Due to Embankment Loading
The vertical stress increase in the soil mass due to
an embankment of height H may be expressed as
Vertical Stress in Soil
" )where:
`4+ a`
) `4+ a`
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… . 7 − 7
59. ^ 7 6
2 `
120 aO+
3 `
2 `
Refer to figure below. The magnitude of the load is
120 kPa. Calculate the vertical stress at points,
A , B, and C.
Worked Examples
Example 4
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60. 1- Under the center: The increase in the vertical
stress ( ) at depth z ( point A)under the center
of a circular area of diameter D = 2R carrying
a uniform pressure q is given by
Vertical Stress in Soil
Vertical Stress due to a uniformly loaded circular area
1 −
1
Q/ 1 /
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… . 7 − 8
61. Vertical Stress in Soil
Vertical Stress due to a uniformly loaded circular area
6
6'
Q
6'
6
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62. Vertical Stress in Soil
2- At any point: The increase in the vertical
stress ( ) at any point located at a depth z at
any distance r from the center of the loaded
area can be given
Vertical Stress due to a uniformly loaded circular area
where and are functions of z/R and r/R.
1' '
1' '
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… . 7 − 9
63. Vertical Stress in Soil
Vertical Stress due to a uniformly loaded circular area
7
7'
.
Q
7
7'.
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64. Vertical Stress in Soil
Variation of with z/R and r/R.1'
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65. Vertical Stress in Soil
Variation of with z/R and r/R.1'
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66. Variation of with z/R and r/R.'
Vertical Stress in Soil
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67. Vertical Stress in Soil
Variation of with z/R and r/R.'
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68. Vertical Stress in Soil
Vertical Stress Caused by a Rectangular loaded area
The increase in the vertical stress ( ) at depth z under a
corner of a rectangular area of dimensions B = m z and
L = n z carrying a uniform pressure q is given by:
z o zq Iσ =
c 3 +3 . 2 0 2 .+
d
+ 2
where :
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… . 7 − 10
69. Vertical Stress in Soil
Vertical Stress Caused by a Rectangular loaded area
c
1
4V
2 ` ` 1
` ` 1
` 2
` 1
+ e
2 ` ` 1
` − ` 1
The influence factor
can be expressed as
`
d
+ 2
where :
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… . 7 − 11
70. The increase in the stress at any point below a
rectangular loaded area can be found by dividing
the area into four rectangles. The point A’ is the
corner common to all four rectangles.
Vertical Stress in Soil
Vertical Stress Caused by a Rectangular loaded area
1 2
34
6'
* f
g c g
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76. Approximate Method
B
B + z
2
1
z
"
O
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2V:1H method
A simple but approximate method is sometimes used for
calculating the stress change at various depths as a
result of the application of a pressure at the ground
surface.
The transmission of stress is
assumed to follow outward
fanning lines at a slope of 1
horizontal to 2 vertical.
77. Approximate Method
For uniform footing (B x L) we can estimate the
change in vertical stress with depth using the Boston
Rule. Assumes stress at depth is constant below
foundation influence area
B
B + z
2
1
z
" d
,d - , -
"
O
"
O
d
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… . 7 − 12
2V:1H method
78. Approximate Method
B + z
L
B
z
Stress on this plane "
j
d ∗
Stress on this plane at depth z,
" d
,d - , -
Rectangular footing
B
B + z
2
1
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2V:1H method
79. Newmark Method
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• Stresses due to foundation loads of arbitrary
shape applied at the ground surface
• Newmark’s chart provides a graphical
method for calculating the stress increase due
to a uniformly loaded region, of arbitrary
shape resting on a deep homogeneous
isotropic elastic region.
80. Newmark Method
• The Newmark’s Influence Chart method
consists of concentric circles drawn to scale,
each square contributes a fraction of the
stress.
• In most charts each square contributes
1/200 (or 0.005) units of stress. (influence
value, I)
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81. Newmark Method
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The use of the chart is
based on a factor
termed the influence
value, determined from
the number of units
into which the chart is
subdivided.
Influence value 0.005
A
B
1 unit
82. Newmark Method
A B Influence
value = 0.005
Total number of block on chart = 200 and influence
value = 1/200
83. The influence chart may be used to compute
the pressure on an element of soil beneath a
footing, or from pattern of footings, and for
any depth z below the footing. It is only
necessary to draw the footing pattern to a
scale of z = length AB of the chart. (If z=
6m and AB = 30mm, the scale is 1/200).
Newmark Method
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84. The footing plan will be placed on the influence
chart with the point for which the stress is desired at
the center of the circles.
Newmark Method
The units (segments or partial segments) enclosed
by the footing are counted, and the increase in
stress at the depth z is computed as
" c j
Where I is the influence factor of the chart.
" +00 2 0. . +. + 2+ 3 +3 0. .
j `4 . 3 2 , 0+. + +. `+ 2-
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… . 7 − 13