Part 6 :
Earth Pressure & Earth Retaining
Structures
1
2
Earth Retaining Structures
3
At Rest Earth Pressure
The ratio of the horizontal principal effective stress to the
vertical principal effective stress is called the lateral earth
pressure coefficient at rest (Ko)
For coarse-grained soils, (Jaky, 1944)
1 sin
o
K
For fine-grained soils, (Massarsch,1979)
( ) ( )
%
0.44 0.42
100
o
o overconsolidated o normallyconsolidated
PI
K
K K OCR
4
Earth Pressure Concept
5
‘ ‘
‘ ‘
‘ ‘
2
‘
1 sin 1 sin
2
1 sin 1 sin
1 sin
tan 45
1 sin 2
2
a v
a
a v a a
c
K
K c K
Rankine Active Earth Pressure
(for smooth frictionless wall)
6
‘ ‘
‘ ‘
‘
2
‘
1 sin 1 sin
2
1 sin 1 sin
1 sin ‘
tan 45
1 sin 2
2
p v
p
p v p p
c
K
K c K
Rankine Passive Earth Pressure
(Smooth frictionless wall)
Note: 𝑘𝑝 =
1
𝑘𝑎
7
Slip Planes
Slip Planes within a soil mass near a retaining wall
8
Variation of active and passive lateral earth pressures
Variation of Stresses with Depth for cohesionless soils
9
Stresses due strip load
Variation of Stresses with Depth for water, surcharge, and strip load
10
Water pressure cancels out
Example 5-1 For the retaining wall shown, find:
a) Resultant active force and passive force
b) Overturning moment and resisting moment
𝑘𝑎 =
1 − 𝑠𝑖𝑛30
1 + 𝑠𝑖𝑛30
= 0.333, 𝑘𝑝=
1
𝑘𝑎
= 3
11
Active side
depth g’ ’ ka a(soil) a(surch.)
kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 16 0 0.333 0 3.33
4 16 64 0.333 21.3 3.33
6 10.2 84.4 0.333 28.1 3.33
Passive side
depth g’ ’ kp p
m kN/m
3
kN/m
2
kN/m
2
0 16 0 3 0
1 16 16 3 48
3 10.2 36.4 3 109.2
area No. Force Aram moment
kN m kN.m
1 42.6 3.3 142.1
2 49.4 1.0 47.2
surcharge 20.0 3.0 60.0
Fa = 112.0 Mo = 249.2
3 24.0 2.3 56.0
4 157.2 0.9 136.8
Fp = 181.2 Mr = 192.8
active side
passive
side
For the retaining wall shown, find:
a) Resultant active force and passive force
b) Overturning moment and resisting moment
Example 5-2
𝑘𝑎 =
1 − 𝑠𝑖𝑛30
1 + 𝑠𝑖𝑛30
= 0.333, 𝑘𝑝=
1
𝑘𝑎
= 3
Water pressure cancels out
12
For the retaining wall shown, find:
a) Resultant active force and passive force
b) Overturning moment and resisting moment
Example 5-3
Active side
depth g’ ’ ka a(soil) a(surch.)
kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 16 0 0.333 0 3.33
4 16 64 0.333 21.3 3.33
4 10.2 64 0.271 17.3 2.71
6 10.2 84.4 0.271 22.9 2.71
Passive side
depth g’ ’ kp p
m kN/m
3
kN/m
2
kN/m
2
0 16 0 3 0
1 16 16 3 48
1 10.2 16 3.69 59.0
3 10.2 36.4 3.69 134.3
area No. Force Aram moment
kN m kN.m
1 42.6 3.33 142.1
2 40.2 0.95 38.4
5 13.3 4.00 53.3
6 5.4 1.0 5.4
Fa = 101.6 Mo = 239.1
3 24.0 2.33 56.0
4 193.4 0.87 168.3
Fp = 217.4 Mr = 224.3
active side
passive
side
surcharge
Water pressure cancels out
13
Horizontal Stresses Due to Surface Strip Load
14
Active side
depth g’ ’ ka a(soil) a(surch.)
kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 16 0 0.333 0 3.33
4 16 64 0.333 21.3 3.33
6 10.2 84.4 0.333 28.1 3.33
Passive side
depth g’ ’ kp p
m kN/m
3
kN/m
2
kN/m
2
0 16 0 3 0
1 16 16 3 48
3 10.2 36.4 3 109.2
area No. Force Aram moment
kN m kN.m
1 42.6 3.33 142.1
2 49.4 0.95 47.2
strip 34.2 3.96 135.4
Fa = 126.2 Mo = 324.7
3 24.0 2.3 56.0
4 157.2 0.9 136.8
Fp = 181.2 Mr = 192.8
active side
passive
side
Example 5-4
For the retaining wall shown, find:
a) Resultant active force and passive force
b) Overturning moment and resisting moment
𝑘𝑎 =
1 − 𝑠𝑖𝑛30
1 + 𝑠𝑖𝑛30
= 0.333, 𝑘𝑝=
1
𝑘𝑎
= 3
Water pressure cancels out
15
Active Case With Cohesive Backfill (C > 𝟎)
Active pressure distribution to
be considered
16
Active side
depth g’ ’ ka c’ a
kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 17 0 0.406 15 -19.1
4 17 68 0.406 15 8.5
6 10.2 88.4 0.406 15 16.8
Passive side
depth g’ ’ kp c’ p
m kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 16 0 2.46 15 47.1
1 16 16 2.46 15 86.4
3 10.2 36.4 2.46 15 136.6
area No. Force Aram moment
kN m kN.m
1 5.2 2.4 12.6
2 27.3 0.9 24.3
Fa = 32.5 Mo = 36.9
3 66.8 2.5 163.6
4 223.0 0.9 206.3
Fp = 289.8 Mr = 369.9
active side
passive
side
Example 5-5
𝑧𝑐 =
2𝑐′
𝛾 𝑘𝑎
=
2(15)
17 0.406
𝑘𝑎 =
1 − 𝑠𝑖𝑛25
1 + 𝑠𝑖𝑛25
= 0.406, 𝑘𝑝=
1
𝑘𝑎
= 2.46
For the retaining wall shown, find:
a) Resultant active force and passive force
b) Overturning moment and resisting moment
17
Active pressure at depth z =
Rankine Pressure with Sloping Surface
Resultant active force =
2 2
2 2
cos cos cos
cos
cos cos cos
a
K
Rankine active pressure
18
Coulomb’s Earth Pressure
21
2
a a
P K Hg
The resultant active force behind dry sand
can be calculated as
Coulomb’s limit equilibrium approach, obtained expressions for Ka where wall friction (d ) is present,
the wall face is inclined at an angle b, and the backfill is sloping at an angle
19
Consider the retaining wall shown. Calculate the Coulomb’s
active force per unit length of the wall and the angle it makes
with the horizontal q. Assume d = (2
3
𝜙′)
𝑘𝑎 =
𝑠𝑖𝑛2(75 + 32)
𝑠𝑖𝑛2 75 sin(75 −
64
3
) 1 +
sin 32 +
64
3
sin(32 − 10)
sin 75 −
64
3
sin(10 + 75)
= 0.4683
Resultant active force = =
1
2
(0.4683)(18.5)(6.5)2 = 183 𝑘𝑁
𝜃 = 15 +
2
3
32 = 36.3
Example 5-6
20
Flexible Retaining Walls
21
Mode of Failure in Flexible Walls
22
Cantilever Walls
Mostly used to support sallow cuts (less than 4m)
Using the approximate pressure distribution will simplify the solution to find do . The actual depth of
embedment d 1.2do
A factor of safety is applied to the passive pressure.
23
Active side
depth g’ ’ ka a(soil) a(surch.)
kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 16 0 0.333 0 3.33
4 16 64 0.333 21.3 3.33
L 10.2 64+10.2L 0.333 21.3+3.4L 3.33
Passive side
depth g’ ’ kp p
m kN/m
3
kN/m
2
kN/m
2
0 16 0 2 0
1 16 16 2 32
L 10.2 16+10.2L 2 32+20.4L
area No. Force Aram moment
kN m kN.m
1 42.6 L+4/3 42.6L+56.8
2 21.3L L/2 10.7L
2
3 1.7L
2
L/3 0.57L
3
4 13.3+3.33L 2+L/2 1.67L
2
+13.3L+26.6
5 -16.0 L+1/3 -16L-5.33
6 -32L L/2 -16L
2
7 -10.2L
2
L/3 -3.4L
3
SM -2.83L
3
-3.63L
2
+39.9L+78.1 =0
L = 4 m
d = 1.2(1+4) = 6 m
active side
passive
side
Determine the required depth of embedment of the
piling to ensure a factor of safety of 1.5 with respect
to gross passive resistance
Example 5-7
24
Anchored Walls
25
Active side
depth g’ ’ ka a(soil) a(surch.)
kN/m
3
kN/m
2
kN/m
2
kN/m
2
0 16 0 0.333 0 3.33
4 16 64 0.333 21.3 3.33
L 10.2 64+10.2L 0.333 21.3+3.4L 3.33
Passive side
depth g’ ’ kp p
m kN/m
3
kN/m
2
kN/m
2
0 16 0 2 0
1 16 16 2 32
L 10.2 16+10.2L 2 32+20.4L
area No. Force Aram moment
kN m kN.m
1 42.6 2.17 92.3
2 21.3L L/2+3.5 10.65L
2
+74.5L
3 1.7L
2
2L/3+3.5 1.13L
3
+5.95L
2
4 13.3+3.33L 1.5+L/2 1.67L
2
+11.6L+20.0
5 -16.0 3.17 -50.7
6 -32L L/2+3.5 -16L
2
-112L
7 -10.2L
2
2L/3+3.5 -6.8L
3
-35.7L
2
Tie force(T) -T 0.0 0.0
SF -8.5L
2
-7.37L+39.9 -T = 0 SM -5.67L
3
-33.4L
2
-25.9L+61.6 =0
substitue L = 0.97 m L = 0.97 m
T = 24.8 kN/m d = (1+0.97) 2 m
active side
passive side
Example 5-8
26
Rigid Walls
27
Mode of Failure in Rigid Walls
28
Pressure Distribution acting on rigid walls
Based on Rankine Lateral Pressure (frictionless wall)
29
Example
Check the stability of the cantilever concrete
retaining wall against overturning, sliding and
bearing capacity. Assume concrete unit weight
of 23.58 kN/m3
FS (overturning) 2
FS (sliding) 1.5
FS (bearing capacity) 3
30
Forces Acting on the Wall
Forces that resist sliding are the passive force Pp plus the friction force between the foundation of the wall and the
soil underneath. The friction force can be calculated as follows
1 2
2 3
tan
For a unit length of wall,
tan
is the friction angle between the base of the wall and the soil ( to )
C is the cohesion bet
friction normal cs
base
friction friction base cs
soil
cs
C
A B
F A V BC
d
d
d
1 2
2 3
ween the base of the wall and the soil ( to )
soil
C
31
Example
Check the stability of the cantilever concrete
retaining wall against overturning, sliding and
bearing capacity. Assume concrete unit weight
of 23.58 kN/m3
FS (overturning) 2
FS (sliding) 1.5
FS (bearing capacity) 3
32
Overturning Moments,
SMo
Resisting Moments,
SMR
Sliding forces,
SFS
Resisting force,
SFR
c
Pa
ph
pv
H’
H’/3
Pp
( )
R
overturning
o
M
FS
M
( )
R
sliding
S
F
FS
F
Forces Acting on the Wall
33
Sloped ground in the active side
34
Factor of safety against overturning
The passive force
effect on overturning
is usually ignored
35
Factor of Safety Against Sliding
Forces that resist sliding are the passive force Pp plus the friction force between the
foundation of the wall and the soil underneath. The friction force can be calculated as
follows
1 2
2 3
tan
For a unit length of wall,
tan
is the friction angle between the base of the wall and the soil ( to )
C is the cohesion bet
friction normal cs
base
friction friction base cs
soil
cs
C
A B
F A V BC
d
d
d
1 2
2 3
ween the base of the wall and the soil ( to )
soil
C
36
2 6
R o
M MB B
e
V
(max)
(min)
6
1
toe
heel
V e
q
B B
max
(bearing capacity) 3u
q
FS
q
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