Design of Flexible Retaining Walls

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Design of Flexible Retaining Walls

• Prof. Jie Han, Ph.D., PE
• The University of Kansas

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Outline of Presentation

• Design of Cantilever Sheet Pile Walls
• Design of Anchored Sheet Pile Walls
• Design of Braced Sheet Pile Walls
• Design of Slurry Walls
• Other Related Design

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Flexible Retaining Walls
H
H
H
Anchor
Cantilever sheet pile wall (lt 3m)
Anchored sheet pile wall (gt 3m)
Braced sheet pile wall (gt 3m)
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• Design of
• Cantilever Sheet Pile Walls

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Wall Rotation
Pivot or rotation point
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Theoretical Earth Pressure Distribution for Sands
H
Active
Sand
Passive
Passive
Active
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Actual Earth Pressure Distribution
?a
?p
?p
8
Simplified Earth Pressure Distribution for Sands
?a
?p
?p
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Earth Pressure Distribution for Sands (Details)

• Use a modified
• passive earth pressure
• coefficient Kp by a factor
• of 1.5 to 2.0
• Lines CD and DF have
• the same slope and the
• slope can be determined
• by
• ? ?? (Kp- Ka)

A
?a
B
C
D
E
?p
?p
F
G
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Earth Pressure Distribution for Sands (Details)

• Determine z1 by
• z1 pa(B)/?
• z2 and z3 can be solved
• by
• Pa(AC)Pp(CE)- Pp(EF)
• MC(AC)Pp(EF)(z22z3/3)
• - Pp(CE)(z2/2)

A
?a
Pa(AC)
B
C
Pp(CE)
MC(AC)
D
E
?p
Pp(EF)
?p
F
G
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Maximum Moment

• The maximum moment
• occurs where the shear
• equals zero
• The maximum moment
• should be less than the
• allowable flexural
• capacity

Mmax
zm
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Earth Pressure Distribution for Two Sand Layers
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Theoretical Earth Pressures for Clays
Active and passive earth pressures

• Short-term analysis
• - Based on undrained strengths (csu and ?0)
• Ka Kp 1
• Long-term analysis
• - Based on drained strengths (cc and ? ?)

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Simplified Earth Pressure Distribution for Clays
Hc
Hc
H
H
4su-??H
4su??H
Short-term
Long-term
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• Design of
• Anchored Sheet Pile Walls

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Simplified Earth Pressure Distribution for Sands

• The embedment depth
• can be computed by
• MOPp(H-zfzp) Paza0
• The required anchorage
• resistance
• F Pa Pp

zF
F
O
za
H
Pa
zC
C
zp
D
Pp
??(Kp-Ka)(D-zC)
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Simplified Earth Pressure Distribution for Clays

• The embedment depth
• can be computed by
• MOPp(H-zfD/2) Paza0
• The required anchorage
• resistance
• F Pa Pp

zF
F
O
za
H
Pa
??z
C
Pp
D
zp
(4c-??z)
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Sheet-Pile Anchorages
Final ground
Anchor rod
Anchor rod
Excavation of trench to pour a deadman
Original ground
Piles
Cast-in-place deadman
Piles used as anchors
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Sheet-Pile Anchorages
Steel rod
Auger hole
d
z1
Pressure grout anchor in soil or rock
L
Soil or rock (tieback) anchor
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Load Capacity of Deadman Anchor
Passive zone
Active zone
Pp
Pa
F
?p
F (Pp - Pa)L/FS
L
FS 1.25 to 1.5
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Sheet-Pile Anchorages
Auger hole
d
z1
L
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• Design of
• Braced Sheet Pile Walls

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Construction Sequence of Braced Wall
Earth pressure
Strut 1
Excavate
Add 1st strut
Excavate next depth
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Construction Sequence of Braced Wall
Strut 2
3rd excavation
Add 2nd strut
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Simplified Earth Pressure Distribution
0.65?HKa
?H 4c
(0.2 to 0.4)?H
0.25H
0.25H
H
0.50H
0.75H
0.25H
(a) Sand
(b) Soft to medium clay
(c) Stiff-fissured clay
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Consistency vs. Unconfined Compression Strength
of Clays
qu
Consistency
tsf
kPa
Very soft
0 0.25
0 24
Soft
0.25 0.50
24 48
Medium
0.50 1.00
48 96
Stiff
1.00 2.00
96 192
Very stiff
2.00 4.00
192 383
Hard
gt 4.00
gt 383
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Forces on Struts in Braced Sheet Pile Walls
FA
FA
Pa1
A
Hinge
zA
z1
FB
B


FC
C
FD
FD
D
FAzAPa1z1
FB1Pa1 - FA
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Location of First Strut
Tension crack
The first strut location should not exceed the
depth of the potential tension crack
hc
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• Slurry Walls

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Construction of Slurry Walls

• Trench excavation
• Slurry for maintaining
• stability of excavation
• Backfilled with clay or
• concrete

Slurry
Filter skin
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Design of Slurry Walls (Clay)
For clay without slurry, the critical depth is
With slurry in the trench (clay and undrained)
Clay
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Design Example
Clay su 35kPa, ? 18.2kN/m2
FS 1.5
4 (35)
Solve for height
H
1.5 (18.2-?s)
?s
?s (kN/m3)
H (m)
(g/cm3)
1.1
10.79
12.6
1.15
11.28
13.5
1.2
11.77
14.5
1.25
12.26
15.7
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Design of Slurry Walls (Sand)
With slurry in the trench (cohesionless soil)
Pa Pw
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• Other Issues Related to Excavations

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Wall Movement
Settlement, s
Deflection, d
Heave, sh
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Settlement Calculation

• Steps
• Compute the wall
• deflection based earth
• pressures and wall
• properties, ?
• 2. Compute the volume
• of deflection, Vs
• 3. Compute distance of
• settlement influence, D
• D Htan(450-?/2)
• H H Hp
• Hp B if ?0 or Hp o.5B
• if ?gt0. Bexcavation width

D
s
d
H
sh
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Settlement Calculation
Steps 4. Compute the surface Settlement at the
wall, sw 5. Assume a parabolic variation of
s, i.e.
D
x
s
sw
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Base Stability
qs
The ultimate bearing Capacity
B
0.707B
Ht
su
Applied pressure
qb
Factor of Safety
FS qult/q gt1.2 to 1.5
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Instability due to Seepage
The factor of safety against seepage
Ht
hw
hs
?s
Clay
Sand