CHAPTER 1INTRODUCTION1.1OBJECTIVESNew methods and technologies of retention and steepened-slope construction continue to bedeveloped, often by specialty contractors and suppliers, to solve problems in locations ofrestricted Right-of-Way (ROW),at marginal sites with difficult subsurface conditions andother environmental constraints,and to expedite construction.Professionals charged withtheresponsibility for planning, designing, and implementing improvements and additions in suchlocations should understand the application, limitations and costs associated with a host ofmeasuresandtechnologies available.This manual was prepared to assist design engineers, specification writers, estimators,construction inspectors and maintenance personnel with the selection, design, constructionandmaintenance ofMechanically Stabilized Earth Walls(MSEW)andReinforcedSoiSlopes (RSS)The design, construction and monitoring techniques for these structures have evolved overthe last three decades as a result of efforts by researchers, material suppliers and governmentagencies to improve some single aspect of the technology or the materials used.This manualis a comprehensive document that integrates all design, construction, materials, contracting,and monitoring aspects required for successful project implementation.This manual has been developed in support of FHWA educational programs on the design,construction, and maintenance of MSE wall and RSS structures construction.Its principalfunction is to serve as a reference source to the materials presented.The manual serves asFHWA's primary technical guideline on the use of thesetechnologies on transportationfacilities.1.1.1 ScopeThe manual addresses in a comprehensive manner the following areas:: Overview of MSE development and the cost, advantages, and disadvantages of usingMSEstructures:Available MSE systems and applications to transportation facilities.Basicsoil-reinforcementinteraction.:DesignofroutineandcomplexMSEwalls.FHWA NHI-10-0241- Introduction1-1MSEWallsandRSS-VolINovember2009
CHAPTER 1 INTRODUCTION 1.1 OBJECTIVES New methods and technologies of retention and steepened-slope construction continue to be developed, often by specialty contractors and suppliers, to solve problems in locations of restricted Right-of-Way (ROW), at marginal sites with difficult subsurface conditions and other environmental constraints, and to expedite construction. Professionals charged with the responsibility for planning, designing, and implementing improvements and additions in such locations should understand the application, limitations and costs associated with a host of measures and technologies available. This manual was prepared to assist design engineers, specification writers, estimators, construction inspectors and maintenance personnel with the selection, design, construction and maintenance of Mechanically Stabilized Earth Walls (MSEW) and Reinforced Soil Slopes (RSS). The design, construction and monitoring techniques for these structures have evolved over the last three decades as a result of efforts by researchers, material suppliers and government agencies to improve some single aspect of the technology or the materials used. This manual is a comprehensive document that integrates all design, construction, materials, contracting, and monitoring aspects required for successful project implementation. This manual has been developed in support of FHWA educational programs on the design, construction, and maintenance of MSE wall and RSS structures construction. Its principal function is to serve as a reference source to the materials presented. The manual serves as FHWA's primary technical guideline on the use of these technologies on transportation facilities. 1.1.1 Scope The manual addresses in a comprehensive manner the following areas: C Overview of MSE development and the cost, advantages, and disadvantages of using MSE structures. C Available MSE systems and applications to transportation facilities. C Basic soil-reinforcement interaction. C Design of routine and complex MSE walls. FHWA NHI-10-024 1 – Introduction MSE Walls and RSS – Vol I 1 – 1 November 2009
DesignofMSEwallsforextremeevents.Design detailing of MSE walls.Design of steepenedRSS.Specificationsandcontracting approaches forbothMSEwallsandRSS constructionConstructionmonitoringand inspection.Design examples.A separate companion manual addresses long-termcorrosion of metallic reinforcementsandlong-termdegradation of polymericreinforcements.SectionsoftheCorrosion/Degradation manual address the background of full-scale,long-termevaluation programs and the procedures required to develop, implement, and evaluatethem.These procedures have been developed to provide practical information on thistopic for MSE users for non-corrosion or polymer specialists, who are interested indeveloping long-term monitoring programs for these types of structures.As an integral part of this Manual, several example calculations are appended thatdemonstrate individual design aspects.1.1.2 SourceDocumentsThis Mechanically Stabilized Earth Walls (MSE) and Reinforced Soil Slopes (RSS), Designand Construction Guidelines Manual is an update of the current FHWA NHI-00-043 (Elias etal.,2001), has evolvedfrom thefollowingAASHTO and FHWAreferences:AASHTO LRFD Bridge Design Specifications, 4th Edition, 2007, with 2008 and 2009Interim RevisionsEarthRetaining Structures,FHWA-NHI-07-071 (Tanyu et al.,2008)AASHTO LRFD Bridge Construction Specifications, 2nd Edition, 2004, with 2006, 2007,2008, and 2009 Interim RevisionsGeosynthetic Design and Construction Guidelines, FHWA NHI-07-092 (Holtz et al.,2008)Guidelines for Design, Specification, and Contracting of Geosynthetic MechanicallyStabilized Earth Slopes on Firm Foundations, FHWA-SA-93-025 (Berg, 1993)Reinforced Soil Structures - Volume I, Design and Construction Guidelines - Volume II,Summary of Research and Systems Information, FHWA RD 89-043 (Christopher et al.1990)Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design andConstruction Guidelines, FHWA, (Elias and Christopher, 1997)Additional guidance,where not availablefromother sources, was specifically developed forthis manual.FHWA NHI-10-0241-Introduction1-2MSEWallsandRSS-VolINovember2009
C Design of MSE walls for extreme events. C Design detailing of MSE walls. C Design of steepened RSS. C Specifications and contracting approaches for both MSE walls and RSS construction. C Construction monitoring and inspection. C Design examples. C A separate companion manual addresses long-term corrosion of metallic reinforcements and long-term degradation of polymeric reinforcements. Sections of the Corrosion/Degradation manual address the background of full-scale, long-term evaluation programs and the procedures required to develop, implement, and evaluate them. These procedures have been developed to provide practical information on this topic for MSE users for non-corrosion or polymer specialists, who are interested in developing long-term monitoring programs for these types of structures. As an integral part of this Manual, several example calculations are appended that demonstrate individual design aspects. 1.1.2 Source Documents This Mechanically Stabilized Earth Walls (MSE) and Reinforced Soil Slopes (RSS), Design and Construction Guidelines Manual is an update of the current FHWA NHI-00-043 (Elias et al., 2001), has evolved from the following AASHTO and FHWA references: C AASHTO LRFD Bridge Design Specifications, 4th Edition, 2007, with 2008 and 2009 Interim Revisions C Earth Retaining Structures, FHWA-NHI-07-071 (Tanyu et al., 2008) C AASHTO LRFD Bridge Construction Specifications, 2nd Edition, 2004, with 2006, 2007, 2008, and 2009 Interim Revisions C Geosynthetic Design and Construction Guidelines, FHWA NHI-07-092 (Holtz et al., 2008) C Guidelines for Design, Specification, and Contracting of Geosynthetic Mechanically Stabilized Earth Slopes on Firm Foundations, FHWA-SA-93-025 (Berg, 1993) C Reinforced Soil Structures - Volume I, Design and Construction Guidelines - Volume II, Summary of Research and Systems Information, FHWA RD 89-043 (Christopher et al. 1990) C Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines, FHWA, (Elias and Christopher, 1997) Additional guidance, where not available from other sources, was specifically developed for this manual. FHWA NHI-10-024 1 – Introduction MSE Walls and RSS – Vol I 1 – 2 November 2009
1.1.3TerminologyCertain interchangeable terms will be used throughout this manual.For clarity, they aredefined as follows:Inclusion is a generic term that encompasses all man-made elements incorporated in the soilto improve its behavior.Examples of inclusions are steel strips, geotextile sheets, steel orpolymeric grids, steel nails, and steel tendons between anchorage elements. The termreinforcement is used only for those inclusions where soil-inclusion stress transfer occurscontinuously along the inclusion.Mechanically Stabilized Earth Wall (MSE wall or MSEW) is a generic term that includesreinforced soil (a term used when multiple layers of inclusions act as reinforcement in soilsplaced as fill). Reinforced Earthis a trademark for a specific reinforced soil system.Reinforced Soil Slopes(RSS) are a form of reinforced soil that incorporate planarreinforcing elements in constructed earth-sloped structures with face inclinations of less than70 degrees.Geosynthetics is a generic term that encompasses flexible polymeric materials used ingeotechnical engineering such as geotextiles, geomembranes, geonets, and geogrids.Facing is a component of the reinforced soil system used to prevent the soil from ravelingout between the rows of reinforcement.Common facings include precast concrete panels,dry cast modular blocks, gabions, welded wire mesh, shotcrete, timber lagging and panels,polymeric cellular confinement systems, and wrapped sheets of geosynthetics.The facingalso plays a minor structural role in the stability of the structure.For RsS structures itusually consists of welded wire mesh, geosynthetic wrap-around, and/or some type of erosioncontrolmaterial.Retained backfill is the fill material located behind the mechanically stabilized soil zoneReinforced fill is the fill material in which the reinforcements are placedGeneric cross sections of MSE structures are shown in Figures 1-1 and 1-2FHWA NHI-10-0241-Introduction1-3MSEWallsandRSS-VolINovember2009
1.1.3 Terminology Certain interchangeable terms will be used throughout this manual. For clarity, they are defined as follows: Inclusion is a generic term that encompasses all man-made elements incorporated in the soil to improve its behavior. Examples of inclusions are steel strips, geotextile sheets, steel or polymeric grids, steel nails, and steel tendons between anchorage elements. The term reinforcement is used only for those inclusions where soil-inclusion stress transfer occurs continuously along the inclusion. Mechanically Stabilized Earth Wall (MSE wall or MSEW) is a generic term that includes reinforced soil (a term used when multiple layers of inclusions act as reinforcement in soils placed as fill). Reinforced Earth® is a trademark for a specific reinforced soil system. Reinforced Soil Slopes (RSS) are a form of reinforced soil that incorporate planar reinforcing elements in constructed earth-sloped structures with face inclinations of less than 70 degrees. Geosynthetics is a generic term that encompasses flexible polymeric materials used in geotechnical engineering such as geotextiles, geomembranes, geonets, and geogrids. Facing is a component of the reinforced soil system used to prevent the soil from raveling out between the rows of reinforcement. Common facings include precast concrete panels, dry cast modular blocks, gabions, welded wire mesh, shotcrete, timber lagging and panels, polymeric cellular confinement systems, and wrapped sheets of geosynthetics. The facing also plays a minor structural role in the stability of the structure. For RSS structures it usually consists of welded wire mesh, geosynthetic wrap-around, and/or some type of erosion control material. Retained backfill is the fill material located behind the mechanically stabilized soil zone. Reinforced fill is the fill material in which the reinforcements are placed. Generic cross sections of MSE structures are shown in Figures 1-1 and 1-2. FHWA NHI-10-024 1 – Introduction MSE Walls and RSS – Vol I 1 – 3 November 2009
FacingReinfoedeFRetainedBackfillReinforcement(Typ.)DrainReinforcedZoLevelingFoundation SoilPadFigure 1-1.Genericcross sectionofanMSEstructure1.2HISTORICALDEVELOPMENTRetaining structures are essential elements of every highway design.Retaining structures areused not only for bridge abutments and wing walls but also for slope stabilization and tominimize right-of-way for embankments. For many years, retaining structures were almostexclusively made of reinforced concrete and were designed as gravity or cantilever wallswhich are essentially rigid structures and cannot accommodate significant differentialsettlements unless founded on deep foundations. With increasing height of soil to be retainedand poor subsoil conditions, the cost of reinforced concrete retaining walls increases rapidlyMechanically Stabilized Earth Walls (MSEWs) and Reinforced Soil Slopes (RSSs) are cost-effective soil-retaining structures that can tolerate much larger settlements than reinforcedconcrete walls. By placing tensile reinforcing elements (inclusions) in the soil, the strengthof the soil can be improved significantly.Use of a facing system to prevent soil ravelingbetween the reinforcing elements allows very steep slopes and vertical walls to beconstructed safely.FHWA NHI-10-0241- IntroductionMSEWallsandRSS-VolI1-4November2009
Figure 1-1. Generic cross section of an MSE structure. 1.2 HISTORICAL DEVELOPMENT Retaining structures are essential elements of every highway design. Retaining structures are used not only for bridge abutments and wing walls but also for slope stabilization and to minimize right-of-way for embankments. For many years, retaining structures were almost exclusively made of reinforced concrete and were designed as gravity or cantilever walls which are essentially rigid structures and cannot accommodate significant differential settlements unless founded on deep foundations. With increasing height of soil to be retained and poor subsoil conditions, the cost of reinforced concrete retaining walls increases rapidly. Mechanically Stabilized Earth Walls (MSEWs) and Reinforced Soil Slopes (RSSs) are costeffective soil-retaining structures that can tolerate much larger settlements than reinforced concrete walls. By placing tensile reinforcing elements (inclusions) in the soil, the strength of the soil can be improved significantly. Use of a facing system to prevent soil raveling between the reinforcing elements allows very steep slopes and vertical walls to be constructed safely. FHWA NHI-10-024 1 – Introduction MSE Walls and RSS – Vol I 1 – 4 November 2009
SecondaryReinforcement(Typ.)ErosionPrimaryReinforcementProtectionRetainedReinforced FillBackfillTDrainC(a)ErosionProtectionSurficialReinforcement(Typ.)(b)Figure 1-2.Genericcrosssections ofreinforced slopestructures,reinforcementsusedto(a)increase stability of a slope; and (b)provide improved compaction andsurficial stability at edge of slopes (after Berg et al., 1990)FHWA NHI-10-0241-Introduction1-5MSEWallsandRSS-VolINovember2009
(a) (b) Figure 1-2. Generic cross sections of reinforced slope structures, reinforcements used to: (a) increase stability of a slope; and (b) provide improved compaction and surficial stability at edge of slopes (after Berg et al., 1990). FHWA NHI-10-024 1 – Introduction MSE Walls and RSS – Vol I 1 – 5 November 2009