MSEW

1
MSEW RSS Design, Specifications, and
Construction

• Brian Collins, PE
• MDT Geotechnical Section
• Missoula District

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Acronyms

• MSEW Mechanically Stabilized Earth Wall
• SRW Segmental Retaining Wall (a.k.a. modular
  block wall)
• MBW Modular Block Wall (a.k.a. segmental
  retaining wall)
• RSS Reinforced Soil Slope

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Presentation Overview

• What are MSEW and RSS?
• Types of Retaining Walls
• Advantages
• Disadvantages
• Design
• Specifications
• Construction

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MSEW Mechanically Stabilized Earth Wall
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RSS Reinforced Soil Slope
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So, whats the difference between MSEW and RSS?
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Reinforced slopes steeper than 70 degrees are
considered walls (per FHWA Design Manual).
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Reinforced Backfill

• MSE walls - Use select backfill for reinforced
  fill (lt15 fines,
  nonplastic A-1-a)
• Reinforced Soil Slopes - Use low plasticity
  granular material for reinforced fill
  (lt50 fines, PIlt20)

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Types of Walls

• 1 - Fill Walls

2 - Cut Walls
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Types of Walls

• Cut Walls
• Braced
• Tie Backs
• Soil Nails

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Types of Walls

• Fill Walls
• Cantilever
• Gravity
• Reinforced Soil

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• MSEW Applications
• ROW restrictions
• Bridge abutments
• Wing walls
• Areas with poor foundation soils
• Areas with slope stability problems
• In place of reinforced concrete or gravity wall
• RSS Applications
• Unstable or failed slopes
• Slopes susceptible to surface erosion
• ROW restrictions

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Vicinity White Coyote Rd South Ravalli
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South Ravalli Medicine Tree
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South Helena Interchange
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Lost Trail Pass (Hwy 93 at MT/ID border)
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Beartooth Highway
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North Arlee Vic. White Coyote
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US Hwy 2 Goat Lick
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Garryowen Lodge Grass
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Bighorn River Bridge - Hardin
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Advantages of MSEW

• Simple and relatively fast to construct
• Do not require special skills
• Require less site preparation
• Need less space in front of wall for construction
  operations
• Reduce ROW acquisitions
• Tolerant to settlement (relatively)
• Cost effective
• Can be constructed to heights in excess of 80 feet

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Advantages of RSS

• Usually the most economical alternative when a
  soil slope does not fit
• Lower quality material can be used for reinforced
  fill compared to what would be required for a
  wall
• Reduce ROW acquisitions
• RSS with vegetated faces will blend with the
  natural environment better than a wall
• Safer than a flatter soil slope with the same
  factor of safety

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Disadvantages of MSEW RSS

• Require a relatively large space behind wall/face
  for reinforcement (fill walls)
• MSEW require special borrow (A-1-a) for
  reinforced fill
• MSEW are proprietary designs
• Specifications and contracting practices have not
  been standardized
• MSEW design requires shared responsibility
  between contractor/supplier and owner (MDT)
• Montana Contractors dont have much experience
  with MSEW or RSS

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DESIGN
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MSEW Design
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Preliminary Design

• Consider all possible alternatives
• Choose a system (MSEW or RSS)
• Consider facing options
• Develop performance criteria (loads, height,
  embedment, etc.)
• Consider effects on site on corrosion/degradation
  of reinforcements

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Facing Options
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MSEW Design

• Sizing for external stability
• Sliding
• Overturning
• Bearing capacity
• Global stability
• Sizing for internal stability
• Internal failure surfaces
• Maximum tensile forces in reinforcement
• Pullout
• Seismic stability
• Connection strength

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External Stability - MSEW
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Internal Stability - MSEW

• POTENTIAL INTERNAL FAILURE SURFACES
• INEXTENSIBLE ETENSIBLE

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Internal Stability - MSEW

• MAXIMUM TENSILE FORCE IN REINFORCEMENT
• Tmax sHSv
• Tmax maximum tension in each reinforcement
  layer
• sH horizontal stress at each reinforcement
  level
• Sv vertical reinforcement spacing
• PULLOUT
• Tmax (1/FSPO) F g Zp Le C Rc a

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Internal Stability - MSEW

• SEISMIC STABILITY

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Internal Stability - MSEW

• CONNECTION STRENGTH
• The allowable connection strength is the lesser
  of
• The design allowable strength of the
  reinforcement
• The connection strength developed by friction or
  structural means
• A reduced connection strength where bodkin joints
  or geotextile seams are used

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RSS Design
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Failure Modes for Reinforced Soil Slopes
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External Failure Modes for Reinforced Slopes
SLIDING INSTABILITY
DEEP SEATED OVERALL STABILITY
LOCAL BEARING CAPACITY FAILURE
EXCESSIVE SETTLEMENT
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Analysis of Reinforced Soil Slopes
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ReSSA
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What is the most important aspect of design for
both mechanically stabilized earth walls and
reinforced soil slope?
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• DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE
  DRAINAGE DRAINAGE DRAINAGE DRAINAGE DRAINAGE

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Why is drainage so important?
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from Simac (2006)
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from Simac (2006)
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from Simac (2006)
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from Simac (2006)
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20
20
5
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20
H25
Hw20
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LATERAL EARTH PRESSURES
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Lateral Earth Pressure on Back of MSEW due to Soil

• sh g H Ka
• Fa ½ g H2 Ka
• g 125 pcf for retained soil
• H 25 ft
• Ka tan2(45-f/2) tan2(45-34/2) .283
• Fa ½(125pcf)(25ft)2(0.283) 11,055 lb/ft

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Lateral Earth Pressure on Back of MSEW due to
Water

• su gw Hw
• Fw ½ gw Hw2
• g 62.4 pcf
• Hw 20 ft
• Fw ½(62.4pcf)(20ft)2 12,480 lb/ft

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Total Lateral Earth Force on Back of MSEW

• FT Fa Fw
• 11,055 lb/ft 12,480 lb/ft
• 23,535 lb/ft

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Rules of Thumb for Design

• 1. K.I.S.S. Keep It Simple Stupid (Michael
  Scott). Dont create a wall or slope that will
  be difficult to construct

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Use only one or two types of reinforcement
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Keep the lengths and spacing of reinforcement
relatively uniform
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Rules of Thumb for Design

• Maximum vertical spacing between reinforcement
  layers 2 ft
• Minimum length of reinforcement 70 of wall
  height (AASHTO)
• Include a drainage system (behind the
  reinforcement)

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Drainage
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Rules of Thumb for Design

• Minimum embedment below grade for MSEW frost
  depth (3-5 ft in Montana)
• Dont forget global stability

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SPECIFICATIONS
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3 Contractual Methods for MSE

1. Agency or supplier design.
2. End result design approach.
3. Standard designs.

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Advantages

• Agency or supplier design
• Much longer design period
• In house expertise
• End result design approach
• Design performed by experienced staff
• System components routinely used together
• Lessens design costs and manpower required for
  owner
• Standard designs
• Agency has more responsibility for details
• Systems and components approved before bid
• Agency makes design decisions
• Filters out substandard work

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Disadvantages

• Agency or supplier design
• Requires more time from geotechnical staff
• Reduces competitive bidding by wall manufacturers
• Problems designing certain design details due to
  patents
• End result design approach
• Difficult for owner to review and approve new
  technology
• Short time frame to deal with complex phasing or
  special details
• Standard designs
• Lots of time and effort required to develop
  generic designs
• Can only be used when site conditions meet
  certain criteria

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MDTs Approach to MSEWs

• End Result Design Approach
• MDT completes subsurface investigation
• MDT Geotechnical Section performs feasibility and
  global stability calculations for the proposed
  wall layout
• Specify beginning and end of wall stationing
• Specify top of wall elevation and finished grade
  elevation in front of wall
• Specify minimum wall embedment depth
• Specify minimum reinforcement length
• Specify reinforced backfill properties and design
  parameters
• Specify required safety factors
• Specify maximum spacing of reinforcement
• Specify settlement criteria
• Specify facing options

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MDTs Approach to RSSs

• MDT completes subsurface investigation
• MDT Geotechnical performs complete design
  calculations
• Specify RSS layout
• Specify reinforcement locations, lengths and
  minimum property requirements
• Specify reinforced fill material properties

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MSEWCONSTRUCTION
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Keep in mind
a retaining wall is a structure, not an
embankment, inspect it as you would a bridge
abutment.
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Required Contractor Submittals

• Design Submittal calculations working
  drawings
• 30 days before start of wall excavation
• submit to MDT Geotechnical Section
• allow 21 calendar days for review
• Backfill Source Approval
• Submit sample 30 days before beginning
  construction
• Reinforcement Source Approval
• Submit Manufacturers Certificate of Compliance
• Submit sample 5 feet in length by the full
  reinforcement width
• Allow 30 days for evaluation

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Excavation

• Complete as shown on approved on working drawings
• Contractor is responsible for any temporary
  excavation support (as required)
• Always perform excavations in accordance with
  OSHA requirements

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Foundation Preparation

• Grade the foundation level for the length of
  reinforcement plus 1 foot
• Compact the foundation level with a smooth-drum
  vibratory roller
• Proofroll subgrade remove and replace any
  unsuitable foundation material with select gravel
• Keep the foundation excavation dry at all times

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05 MAY 2006
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18 MAY 2006
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02 JUNE 2006
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02 JUNE 2006
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19 JUNE 2006
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19 JULY 2006
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Spend time to ensure the foundation is ready
from Simac (2006)
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Leveling Pad
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Leveling Pad

• Used to align facing panels/blocks and provide
  temporary foundation support elevation should
  be within 1/8 tolerance
• Minimum dimensions 6 thick x 12 wide
• Concrete or crushed gravel
• Allow concrete to cure min. 12 hrs

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Modular Block MSE Walls

• The first course of blocks is critical
• Level front to back and side to side
• Ensure proper alignment
• Dont leave gaps between blocks
• Level front to back on courses above geogrid and
  shim as needed

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Modular Block MSE Walls
(from Keystone Construction Manual)
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Modular Block Connections
(from Keystone Construction Manual)
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Modular Block Connections
(from Mesa Installation Manual)
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Unit / Drainage Material

• All modular blocks must be filled
• Usually ¾-minus well-graded crushed stone
• Provides additional bond between courses of
  blocks
• Provides friction between geogrid reinforcement
  and blocks
• Increases weight of wall facing
• Provides drainage behind wall face
• Prevents loss of material in gaps between blocks

(from Keystone Construction Manual)
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Place unit fill and backfill after each lift
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Backfill Placement

• Place following each course of facing
• Maximum 8 lifts
• Do not leave voids below the reinforcement behind
  the wall face
• Compact all backfill, there is no such thing as
  self-compacting fill

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Wall failure due to no compaction within 3-5 feet
of wall face
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Compaction

• Compact to 95 of MT-230 density at 2 of
  optimum moisture content
• Use a rolling pattern if backfill material has
  too much oversized gravel to perform proctor test
• A large smooth-drum vibratory roller should be
  used away from the wall face
• Use lightweight (walk behind) compaction
  equipment within 4 feet of wall face
• Do not use sheepsfoot rollers

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Geogrid Placement
Uniaxial geogrid Machine/Longitudinal Direction
(MD) no splices or overlaps allowed Cross-Machin
e/ Transverse Direction (XD, XMD) Ribs Junctions
/nodes
Shims may be required at front face of blocks
use scrap ribs of geogrid
(from Mesa Installation Manual)
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Geogrid Placement
(from Mesa Installation Manual)
Dont shim right at front face of blocks too
much stress
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Pull geogrid tight (remove wrinkles and folds)
and hold in place with pins, stakes, soil piles,
etc.
(from Mesa Installation Manual)
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Geogrid Cutoff Lengths
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Geogrid Placement - Curves
(from Mesa Installation Manual)
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Installation of Guardrail Posts
(from Mesa Installation Manual)
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RECo Wall System
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System Components

• Concrete leveling pad
• Precast concrete facing panels
• Facing panel joint materials
• Rubber bearing blocks
• Rubber shims
• Filter cloth
• Reinforcing strips or ladder strips
• Fasteners
• Select granular backfill

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Steel ladder reinforcing or steel strip
reinforcing
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Construction Procedures

• Prepare subgrade (foundation) and install
  drainage system
• Install leveling pad
• Set and brace first course alternating full-
  and half-height panels
• Use wooden wedges and clamps to hold panels in
  position
• Attach filter cloth with adhesive (Do not put
  adhesive in gap between panels)

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(from RECo Installation Manual)
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Construction Procedures, contd

• Place backfill in lifts up to 1-2 above the
  lowest level of reinforcing
• Connect reinforcing elements
• Place bearing blocks and set second course of
  full panels
• Repeat cycle
• Place backfill
• Compact
• Place reinforcement
• Install bearing blocks
• Install panels
• Place geotextile filter

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(from RECo Installation Manual)
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(from RECo Installation Manual)
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BACKFILL LEVELS
(from RECo Installation Manual)
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Construction Procedures, contd

• After each course is completed, remove wooden
  wedges in the course 3 levels below
• Set top panels and finish backfilling and
  compaction
• Remove all wedges and clamps
• Install any concrete coping, traffic barriers, or
  CIP concrete as required

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GABION WALLS
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GABION WALLS
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Gabion Assembly
(from Maccaferri Installation Manual)
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Gabion Assembly, contd
(from Maccaferri Installation Manual)
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Gabion Internal Bracing
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Gabion Installation

• Prepare subgrade (foundation) and install
  drainage system
• Assemble gabion baskets
• Place a number of gabion baskets in position and
  tie adjoining baskets together
• Tie adjacent courses of gabions together after
  tensioning
• Fill gabions while keeping gabions stretched
  longitudinally to maintain alignment

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Fill each course of gabions in stages
(from Maccaferri Installation Manual)
Overfill each basket by 50-75 mm to allow for
settlement
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Gabion Wall Construction, contd

• Place reinforcing mesh on the top surface of a
  finished layer at the elevations shown on the
  plans
• Wire the next course of gabions in place at the
  front of the wall before placing backfill
• Place backfill and compact

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Gabion Basket Connections
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Welded Wire Wall System
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Wall Parts
(from Hilfiker Construction Guide)
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Welded Wire Wall Construction

• Prepare subgrade (foundation)
• Install drainage system
• STEP 1 Install first row of mats

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STEP 2 Install backing mat
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STEP 3 Place backfill and geotextile. Then
place rock in face.
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STEP 45 Start next course. Repeat steps 1-3.
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Welded Wire Walls
(from Hilfiker Construction Guide)
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Welded Wire Walls
(from Hilfiker Construction Guide)
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Welded Wire Walls curves corners
(from Hilfiker Construction Guide)
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Welded Wire Walls backing mats
(from Hilfiker Construction Guide)
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Welded Wire Walls using hardware cloth
(from Hilfiker Construction Guide)
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(from Hilfiker Construction Guide)
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Guardrail Penetration
(from Hilfiker Construction Guide)
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Culvert Through Wall
(from Hilfiker Construction Guide)
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Culvert Through Wall, contd
(from Hilfiker Construction Guide)
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Culvert Through Wall, contd
(from Hilfiker Construction Guide)
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MSE Construction - Summary

• Review all plans and specifications
• Review approved working drawings
• Review materials requirements and approval
  submittals
• Spend time to ensure the foundation is ready
  before starting the wall
• Verify that the correct type and length of
  reinforcement is installed at the locations shown
  on the plans
• Test backfill for compaction and gradation
  requirements during construction
• Verify that the wall batter is within the
  tolerance listed in the specifications
• Keep water away from the work area

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Questions?