MSE Wall Design
MSE Wall as a Gravity Wall
• An MSE wall is a gravity wall.
Heavy walls are good
External Stability• Bearing resistance• Settlement• Sliding• Overturning• Overall Stability
External Stability
Bearing
Bearing Failure
External Stability
Settlement
1.2-7
Flexibility of Wall Systems
TYPE OF FACING LIMIT OF DIFFERENTIAL SETTLEMENT
CIP Gravity Wall 1/500 2 In. in 100 Ft.
Welded Wire Facing 1/50 2 Ft. in 100 Ft.5’ x 5’ Panels with ¾” Joints 1/100 1 Ft. in 100 Ft.5’ x 10’ Panels with ¾” Joints 1/200 6 In. in 100 Ft.Modular Block Walls 1/200 6 In. in 100 Ft.Full Height Facing Panels 1/500 2 In. in 100 Ft.
External Stability
Sliding
Sliding Failure
External Stability
Overturning
External Stability
Overall Stability
Forces
Water Pressure
Earth Pressure
Surcharge
Earthquake
ForcesBack Slope Surcharge
ForcesSurcharge
Drainage
• If the wall backfill “fills” with water ….• Weight of soil increases• Shear strength of backfill decreases• Pressures on the wall increase• Greater tendency for fill to settle• Potential for movement of soils as water exits
wall or slope.• Buoyancy needs to be considered
• Appropriate drainage systems are critical !!
Drainage System Installation
Why Care about Drainage?
An example…
20’
5’
20’
20’
5’
Hw=
20’
LATERAL EARTH PRESSURES
H=25’
Design Concepts
Topic 2
Internal Stability Design of MSE Walls
What is a Failure Surface ??
Geosynthetic Reinforced Wall
H
L
Active
Zone
Resistant
Zone
Potential
Failure
Surface
Soil ReinforcementReinforced
Soil Mass
Metal Reinforced Wall
How do Reinforcements Hold Up the Wall ?
L
Soil
Reinforcement
“Active” Force
Resistant Length, LeActive
Length, La
Internal Stability Reinforcement Failure
Modes
• Pull out• Reinforcement tensile strength• Connection
Internal Stability Reinforcement Failure Modes – CORROSION/DEGRADATION
• STEEL REINFORCEMENT• Area of steel is increased to account for the
anticipated corrosion loss over the design life• GEOSYNTHETIC REINFORCEMENT
• Ultimate tensile strength increased to account for strength loss due to installation damage, creep, and durability
The internal stability of this wall seems OK!
• Compare the internal stability of the Design MSE wall and the As-Built MSE Wall
Design Checks
Continuous Geogrid Reinforcement
H =
20
ft
L = 20 ft
2
1
Section
Backfill
Medium sand with 15% (max) non-plastic fines at
95% compaction
Spacing
18”
Chimney Drain
4 ft
3 ft
31
Turf reinforcement mat, topsoil, and seed
Design
H =
25
ft
L = 20 ft
1.5
1
Plan Section
L = 15 ft
Backfill
Medium sand with 35% plastic fines at 95% compaction
Spacing 36”
Geogrid
Reinforcement
Note: Geogrid Coverage = 50%
2 ft
2 ft
21
As-Built
Durability
• MSE Walls• Corrosion of steel reinforcement• Degradation of geosynthetic
reinforcement• Durability of the facing
1.2-29
MSE Facing Durability