Design of Mechanically Stabilized Earth Walls

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Design of Mechanically Stabilized Earth Walls
Prof. Jie Han, Ph.D., PE The University of Kansas
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Outline of Presentation

• Introduction
• External Stability Analysis
• Internal Stability Analysis
• Local Stability Analysis

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Illustrated Cross Section
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Publications

• FHWA Demo 82 - Federal Highway Administration
• Demonstration Project 82 Manual (1997)/AASHTO
• MSEW/MesaPro Software
• NCMA - National Concrete Masonry Association
• Design Manual (Second Edition, 1997)
• SRWall - NCMA Segmental Retaining Wall Software

This presentation will focus on FHWA DEMO 82
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Wall Systems and Reinforcement
Wall Systems

• Segmental (modular) block walls
• Panel walls

Reinforcement

• Inextensible - metal reinforcement (steel strips
  or mesh)
• Extensible - geosynthetic reinforcement
  (geotextile or geogrid)

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Reinforced Fill
Free draining backfill - cohesion, c 0
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Potential Failure Plane Extensible
Reinforcement
w 0 100
Rankine failure plane
b
a 450f/2
La
Le
w gt 100
w
H
Coulomb failure plane
a f f(f, b, d, w)
a
d b
L
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Potential Failure Plane - Inextensible
Reinforcement
0.3H1
H1/2
La
Le
H1
H
H1/2
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Requirements for Reinforcement Length

• L/H gt 0.7
• L gt 8 ft
• Le gt 3 ft

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Recommended Factors of Safety
External

• Sliding FS ? 1.5
• Eccentricity at base e ? L/6 in soil e ? L/4
  in rock
• Bearing capacity FS ? 2.5
• Deep seated stability FS ? 1.3
• Compound stability FS ? 1.3

Internal

• Pullout FS ? 1.5
• Rupture see detailed discussion
• Internal sliding FS ? 1.5

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• External Stability Analysis

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External Stability - Force Diagram (Static)
W2
b
W1
Ps
H1
b
H1/3
L
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Coefficient of External Active Earth Pressure
(Static)
w 0 100
w gt 100
d ?
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External Stability - Base Sliding
Sliding resisting force
FSsliding
Sliding driving force
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External Stability - Overturning
Resisting moment about toe
FSoverturning
Driving moment about toe
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External Stability - Bearing Capacity
W2
Meyerhof-type distribution
L - 2e
b
qa
W1
Ps
H1
Ultimate bearing capacity, qult
b
Footing width, L-2e and no embedment effect
H1/3
L
Factor of safety
FS qult/qa
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• Internal Stability Analysis

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Force Diagram (Static)
S1/2hs
q gS
hs
z
qr gz
H
pa1KagH
pa2KagS
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Coefficient of Internal Active Earth Pressure
w 0 100
w gt 100
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Variation of Internal Pressure Ratio with Depth
1.0
1.2
1.7
2.5
K/Ka
Metal bar mats welded wire grids
Metal strips
6m
Depth, z
Geosynthetics
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Internal Stability - Internal Sliding
b
Ps
h
b
h/3
L
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Maximum Tension in Reinforcement
Pa
Sv
Tmax
Tmax Pa
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Internal Stability - Rupture or Strength
Metal reinforcement
Long Term Allowable Strength Ta FS x A0 x fy ?
Tmax FS 0.55 for strip or 0.48 for grid A0 No
of longitudinal bars x As, As - bar section area
Geosynthetics
Long Term Design Strength Tal Tult/(RF) RF
RFcreep x RFdurability x RFinstallation damage FS
Tal/Tmax ? 1.5
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Internal Stability - Pullout
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• Local Stability Analysis

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Connection Test
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Local Stability - Facing Connection
Rupture
CRu Ttultconn/Tlot
Tultconn TultCRu
Ttultconn(Tult/Tlot)
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Local Stability - Facing Connection
Pullout or Displ. _at_ 0.6
CRs Ttconn/Tlot
Tconn TultCRs Ttconn(Tult/Tlot)
(FS Tc/Tmax gt 1.5)
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Local Stability - Bulging or Shear

• Maximum reinforcement Spacing

2 x block depth 2 ft. for most blocks
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Local Stability - Crest Toppling

• One-half reinforcement spacing

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• Case Studies

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Case Study -1
Wall Height H 20 ft Backslope b 00 Batter w
0.450 Friction Angle (All) f 300 MESA4
Geogrids Reinforcement Length L 15 ft Spacing s
2 ft Interaction Coefficient Ci 0.58
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Results of Case Study -1A(Static)
External FS Base Sliding
2.58 Eccentricity(e/L) 0.096 Overturning
5.06 Bearing 5.50 Internal Rupture
1.07 Pullout 4.23 Internal Sliding
1.72 Local Connection Rupture 1.23 Connection
PO/Service 1.11
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Case Study -2
Wall Height H 20 ft Backslope b 260 Batter w
0.450 Friction Angle (All) f 300 MESA4
Geogrids Reinforcement Length L 15 ft Spacing s
2 ft Interaction Coefficient Ci 0.58
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Results of Case Study -2A (Static)
External FS Base Sliding
1.46 Eccentricity(e/L) 0.120 Overturning
2.58 Bearing 3.33 Internal Rupture
0.90 Pullout 5.29 Internal Sliding
0.92 Local Connection Rupture 1.04 Connection
PO/Service 0.94