Figure 4-1.4-5External stability load factors for simple walls.Figure 4-2.External analysis:nominal earth pressures, horizontal backslope with traffic4-12surcharge(afterAASHTO,2007)Figure 4-3.External analysis:earth pressure;slopingbackfill case(after AASHTO.4-132007).Figure 4-4.External analysis: earth pressure; broken backslope case (after AASHTO,2007)4-14Figure 4-5.Notationfor Coulomb active earth pressures used with wall batters, , greater4-16than100°(afterAASHTO,2007)Figure 4-6.4-18PotentialexternalfailuremechanismsforaMSEwalFigure 4-7.Calculationofeccentricity and vertical stress for bearing check,for horizontal.4-24backslopewithtrafficsurchargecondition.Figure 4-8.Calculation of eccentricity and vertical stress for bearing check, for sloping.4-25backslopecondition.Figure 4-9LocationofpotentialfailuresurfaceforinternalstabilitydesignofMSEWalls.4-34(a)inextensiblereinforcementsand(b)extensiblereinforcementsFigure 4-10.Variation of the coefficient of lateral stress ratio (K/Ka)with depth in a MSEwall (Elias and Christopher, 1997: AASHTO. 2002: after AASHTO, 2007) ..4-37Figure 4-11.Calculationofvertical stressforslopingbackfillconditionsforinternal4-38stabilityFigure 4-12.Distributionofstressfromconcentratedvertical loadforinternalandexternal4-44stability calculations..Figure 4-13.4-45Distribution of stresses from concentrated horizontal loads.4-46Figure 4-14.ReinforcementloadcontributoryheightFigure 4-15.Nominal vertical stress at thereinforcement level in the resistant zone,4-50beneath a sloping backfillFigure 4-16.Bodkin connectiondetail (lookingat cross section of segmental panel face...4-53Figure 4-17.Determination of hingeheightfor modular concrete blockfaced MSEwalls4-56(NMCA,1997)Figure 4-18Typical geometries where MSE wall compound stability is of concern:steepand tall backslope on top of the wall; tiered walls; slope at the toe of the wall.4-64andwaterattoeof theslopeFigure 4-19.Exampleof standard MSEWdesign.(MinnesotaDOT,2008)4-83Figure 5-1....-2ExamplecopingsforMSEwalls..Figure 5-2.Exampletraffic barrier for MSE walls. (a)Barrierbehind coping,(b)barrier5-5ontopofpanels,(c)barrierontopofmodularblockunitsFigure 5-3.Leveling pads, (a) Common size, b) Step detail for precast panel facing units,5-7(c)StepdetailformodularblockfacingunitsFigure 5-4..5-10Potential sources and flowpaths of water..Figure 5-5...5-11Example layout of filter at joints between segmental precast facing units.Figure 5-6.Layoutofdrainagefabricanddrainagefill atthefaceformodularblockunits(Collinet al.,2002)5-12Figure 5-7.Examplelayoutofgeotextilefilter nearthe face forwelded wirefacing units.5-135-14Figure 5-8.Example drainage blanket detail behind the retained backfill5-15Figure 5-9.Example drainage detail using a geocomposite sheet drain...FHWA NHI-10-025Table of ContentsMSEWalls andRSS-VolIIxxiiNovember2009
Figure 4-1. External stability load factors for simple walls. 4-5 Figure 4-2. External analysis: nominal earth pressures; horizontal backslope with traffic surcharge (after AASHTO, 2007) . 4-12 Figure 4-3. External analysis: earth pressure; sloping backfill case (after AASHTO, 2007). . 4-13 Figure 4-4. External analysis: earth pressure; broken backslope case (after AASHTO, 2007) . 4-14 Figure 4-5. Notation for Coulomb active earth pressures used with wall batters, , greater than 100 (after AASHTO, 2007). 4-16 Figure 4-6. Potential external failure mechanisms for a MSE wall. 4-18 Figure 4-7. Calculation of eccentricity and vertical stress for bearing check, for horizontal backslope with traffic surcharge condition . 4-24 Figure 4-8. Calculation of eccentricity and vertical stress for bearing check, for sloping backslope condition . 4-25 Figure 4-9. Location of potential failure surface for internal stability design of MSE Walls (a) inextensible reinforcements and (b) extensible reinforcements . 4-34 Figure 4-10. Variation of the coefficient of lateral stress ratio (K/Ka) with depth in a MSE wall (Elias and Christopher, 1997; AASHTO, 2002; after AASHTO, 2007) . 4-37 Figure 4-11. Calculation of vertical stress for sloping backfill conditions for internal stability. 4-38 Figure 4-12. Distribution of stress from concentrated vertical load for internal and external stability calculations. 4-44 Figure 4-13. Distribution of stresses from concentrated horizontal loads. 4-45 Figure 4-14. Reinforcement load contributory height . 4-46 Figure 4-15. Nominal vertical stress at the reinforcement level in the resistant zone, beneath a sloping backfill . 4-50 Figure 4-16. Bodkin connection detail (looking at cross section of segmental panel face . 4-53 Figure 4-17. Determination of hinge height for modular concrete block faced MSE walls (NMCA, 1997) . 4-56 Figure 4-18. Typical geometries where MSE wall compound stability is of concern: steep and tall backslope on top of the wall; tiered walls; slope at the toe of the wall; and water at toe of the slope . 4-64 Figure 4-19. Example of standard MSEW design. (Minnesota DOT, 2008) . 4-83 Figure 5-1. Example copings for MSE walls. 5-2 Figure 5-2. Example traffic barrier for MSE walls. (a) Barrier behind coping, (b) barrier on top of panels, (c) barrier on top of modular block units . 5-5 Figure 5-3. Leveling pads, (a) Common size, b) Step detail for precast panel facing units, (c) Step detail for modular block facing units . 5-7 Figure 5-4. Potential sources and flowpaths of water. 5-10 Figure 5-5. Example layout of filter at joints between segmental precast facing units . 5-11 Figure 5-6. Layout of drainage fabric and drainage fill at the face for modular block units. (Collin et al., 2002) . 5-12 Figure 5-7. Example layout of geotextile filter near the face for welded wire facing units . 5-13 Figure 5-8. Example drainage blanket detail behind the retained backfill . 5-14 Figure 5-9. Example drainage detail using a geocomposite sheet drain. 5-15 FHWA NHI-10-025 Table of Contents MSE Walls and RSS – Vol II xxii November 2009
Figure 5-10.Example drainage detail using a blanket drain with chimney drain (Collin et5-16al.,2002)5-17Figure 5-14.Exampledetailfor wall thatmay experience inundation5-19Figure 5-12.Example MSE wall overflow sill at top of wallFigure 5-13.5-20Exampledrainage swale near top of wall. (b)Collin et al.,2002).Figure 5-14.(a)Examplegeomembranebarrierdetails, (b)Installationofgeomembranedeicing salt runoff barrier, (c)Geomembrane installation around manhole5-22penetration.Figure 5-15.Example of undesirable water seepage through pavement due to deficient,5-23grades (Collin et al., 2002)...5-24Figure 5-16.ExampleofsurfaceflowerosionatthebottomofanMsEwallFigure 5-17.Examples of avoiding a vertical obstruction without severing soil...5-37reinforcementsFigure 5-18.Vertical obstructions in reinforced soil mass with segmental precast facing5-39unitsFigure 5-19.Example of a structural frame around vertical obstruction (a) with segmentalprecast facing - note that vertically adjacent layers of reinforcement to beseparated by a minimum of 3-in. (75 mm)of wall fill, (b)-(c)with modular5-40block face - note corner detail.Figure 5-20.Example details of reinforcements around vertical obstructions in reinforced5-41soilmasswithmodularblockunits.Figure 5-21.Navigating horizontal obstruction in MSE walls with inextensible5-43reinforcementsFigure 5-22.Navigatinghorizontal obstructioninMSEwallswithextensible5-43reinforcementFigure 5-23.5-44Exampleofbackuppanelsforlargehorizontal obstructions...5-45Figure 5-24.Examplepipepenetrations through segmental precastpanel facing units5-46Figure 5-25.Example pipepenetration through modular block facing units.Figure 5-26.5-48Example slip joints for segmental precast panel facings..5-49Figure 5-27.ExampleslipjointformodularblockwallfacingsFigure 5-28.5-50Examplelayout of geogrid reinforcement for walls with curves.5-51Figure 5-29.Example corner details5-53Figure 5-30.Conceptual connection details for a 2-stage facing systemFigure 5-31.Common joint details between segmental precast panelfacing units and CIP5-55structuresFigure 5-32.Common joint between modularblock facing units and CIP structures5-555-58Figure 5-33.ExampleofMSEwallaesthetics5-59Figure 5-34.Examples of cast-in-place abutment toMSEwall panel transitions.Figure 6-1.6-2TypesofcomplexMSEstructuresFigure 6-2.Geometry definition, location of critical failure surface and variation of K, and6-4F* parameters for analysis of a MSEW abutment on spread footingFigure 6-3.6-7Details of a typical pile supported MSE abutmentFigure 6-4.Geometry definition and typical supplemental lateral pressure distribution.6-9from deepfoundation on MSEwallfaceFigure 6-5.Example of use of a geosynthetic wrapped face wall behind an integral.6-12abutment.FHWA NHI-10-025TableofContentsxxiiiMSEWallsandRSS-VolIINovember2009
Figure 5-10. Example drainage detail using a blanket drain with chimney drain (Collin et al., 2002) . 5-16 Figure 5-14. Example detail for wall that may experience inundation. 5-17 Figure 5-12. Example MSE wall overflow sill at top of wall. 5-19 Figure 5-13. Example drainage swale near top of wall. ((b) Collin et al., 2002) . 5-20 Figure 5-14. (a) Example geomembrane barrier details, (b) Installation of geomembrane deicing salt runoff barrier, (c) Geomembrane installation around manhole penetration. 5-22 Figure 5-15. Example of undesirable water seepage through pavement due to deficient grades (Collin et al., 2002). 5-23 Figure 5-16. Example of surface flow erosion at the bottom of an MSE wall . 5-24 Figure 5-17. Examples of avoiding a vertical obstruction without severing soil reinforcements. 5-37 Figure 5-18. Vertical obstructions in reinforced soil mass with segmental precast facing units . 5-39 Figure 5-19. Example of a structural frame around vertical obstruction (a) with segmental precast facing - note that vertically adjacent layers of reinforcement to be separated by a minimum of 3-in. (75 mm) of wall fill, (b)-(c) with modular block face – note corner detail . 5-40 Figure 5-20. Example details of reinforcements around vertical obstructions in reinforced soil mass with modular block units. 5-41 Figure 5-21. Navigating horizontal obstruction in MSE walls with inextensible reinforcements. 5-43 Figure 5-22. Navigating horizontal obstruction in MSE walls with extensible reinforcement . 5-43 Figure 5-23. Example of backup panels for large horizontal obstructions . 5-44 Figure 5-24. Example pipe penetrations through segmental precast panel facing units . 5-45 Figure 5-25. Example pipe penetration through modular block facing units . 5-46 Figure 5-26. Example slip joints for segmental precast panel facings . 5-48 Figure 5-27. Example slip joint for modular block wall facings . 5-49 Figure 5-28. Example layout of geogrid reinforcement for walls with curves. 5-50 Figure 5-29. Example corner details. 5-51 Figure 5-30. Conceptual connection details for a 2-stage facing system . 5-53 Figure 5-31. Common joint details between segmental precast panel facing units and CIP structures. 5-55 Figure 5-32. Common joint between modular block facing units and CIP structures . 5-55 Figure 5-33. Example of MSE wall aesthetics . 5-58 Figure 5-34. Examples of cast-in-place abutment to MSE wall panel transitions. 5-59 Figure 6-1. Types of complex MSE structures . 6-2 Figure 6-2. Geometry definition, location of critical failure surface and variation of Kr and F* parameters for analysis of a MSEW abutment on spread footing . 6-4 Figure 6-3. Details of a typical pile supported MSE abutment . 6-7 Figure 6-4. Geometry definition and typical supplemental lateral pressure distribution from deep foundation on MSE wall face . 6-9 Figure 6-5. Example of use of a geosynthetic wrapped face wall behind an integral abutment. 6-12 FHWA NHI-10-025 Table of Contents MSE Walls and RSS – Vol II xxiii November 2009
6-12Figure 6-6.Exampleabutment seatdetail6-14Figure 6-7.Design rules fora2-tier superimposed MSE wall systemFigure 6-8.Dimensioning of MSE wallwithunevenreinforcement lengths6-176-18Figure 6-9.Back-to-back MSE wallsFigure 6-10.Generic cross-section of a shored MSE (SMSE)wall system for steep terrains..6-19(Morrison et al., 2006)Figure 6-11.Minimumrecommended geometryof a shored MSE wall system in steepterrains, (a)with extension of upper two rows of reinforcement, and (b) withthe upper two rows connected to the shoring wall (Morrison et al., 2006).... 6-21Figure 6-12Location of potential failure surface for internal stability design of shoredMSEwalls(a)extensiblereinforcements,(b)inextensiblereinforcements6-23(Morrisonetal.,2006)Figure 6-13.Distribution of stress from concentrated vertical load for internal and external.....6-23stabilitycalculations(Morrisonetal.,2006).Figure 6-14.Computation for Tmax and evaluation of pullout resistance (after Morrison et..6-25al.,2006)...Figure 6-15.Example global stability and compound failure surfaces (Morrison et al.,6-262006)Figure 6-16.Minimumrecommended geometryof a stablefeatureMSE(SFMSE)wall6-28system..7-3Figure 7-1.Definition of heights for seismic analyses..7-6Figure 7-2.Useofa slope stabilityapproachto compute seismic earth pressureFigure 7-3.Procedurefordeterminationofky(Andersonetal.,2008).7-7Figure 7-4..7-8Boundary between WUS and CEUS (Anderson et al. 2008)7-10Figure 7-5.Seismicinternal stabilityofaMSEwall.Figure 7-6.Comparison of static and dynamic impactforcewith1-inch (25mm)7-15maximum displacement (Bligh et al., 2009)FHWANHI-10-025Tableof ContentsMSEWallsandRSS-VolIIxxivNovember2009
Figure 6-6. Example abutment seat detail . 6-12 Figure 6-7. Design rules for a 2-tier superimposed MSE wall system . 6-14 Figure 6-8. Dimensioning of MSE wall with uneven reinforcement lengths . 6-17 Figure 6-9. Back–to–back MSE walls. 6-18 Figure 6-10. Generic cross-section of a shored MSE (SMSE) wall system for steep terrains (Morrison et al., 2006) . 6-19 Figure 6-11. Minimum recommended geometry of a shored MSE wall system in steep terrains, (a) with extension of upper two rows of reinforcement, and (b) with the upper two rows connected to the shoring wall (Morrison et al., 2006) . 6-21 Figure 6-12. Location of potential failure surface for internal stability design of shored MSE walls (a) extensible reinforcements, (b) inextensible reinforcements (Morrison et al., 2006) . 6-23 Figure 6-13. Distribution of stress from concentrated vertical load for internal and external stability calculations (Morrison et al., 2006) . 6-23 Figure 6-14. Computation for TMAX and evaluation of pullout resistance (after Morrison et al., 2006) . 6-25 Figure 6-15. Example global stability and compound failure surfaces (Morrison et al., 2006) . 6-26 Figure 6-16. Minimum recommended geometry of a stable feature MSE (SFMSE) wall system . 6-28 Figure 7-1. Definition of heights for seismic analyses .7-3 Figure 7-2. Use of a slope stability approach to compute seismic earth pressure.7-6 Figure 7-3. Procedure for determination of ky (Anderson et al., 2008) .7-7 Figure 7-4. Boundary between WUS and CEUS (Anderson et al. 2008) .7-8 Figure 7-5. Seismic internal stability of a MSE wall. 7-10 Figure 7-6. Comparison of static and dynamic impact force with 1-inch (25 mm) maximum displacement (Bligh et al., 2009) . 7-15 FHWA NHI-10-025 Table of Contents MSE Walls and RSS – Vol II xxiv November 2009
CHAPTER 8REINFORCEDSOILSLOPESPROJECTEVALUATION8.1INTRODUCTIONWhere right of way is available and the cost of a MSE wall is high, a steepened slope shouldbe considered.In this chapter the background and design requirements for evaluating areinforced soil slope (RsS)alternative are reviewed.Step-by-step design procedures arepresented inChapter9.Section 8.2reviews thetypes of systemsand thematerials ofconstruction.Section 8.3 provides a discussion of the internal stability design approach foruse of reinforcement as compaction aids, steepening slopes and slope repair.Computerassisted design programs are also reviewed. The section concludes with a discussion ofexternalstabilityrequirements.Section8.4reviewstheconstruction sequence.Section8.5covers treatment of the outward face of the slope to prevent erosion.Section 8.6 coversdesign details of appurtenant features including traffic barrier and drainage considerations.Finally, section 8.7 presents several case histories to demonstrate potential cost savings.8.2REINFORCEDSOILSLOPESYSTEMS8.2.1 Types of SystemsReinforced soil systems consist of planar reinforcements arranged in nearlyhorizontal planesin the reinforced fill to resist outward movement of the reinforced fill mass.Facingtreatments ranging from vegetation to flexible armor systems are applied to preventunraveling and sloughing of the face. These systems are generic in nature and canincorporate any of a variety of reinforcements and facing systems. Design assistance is oftenavailable through many of the reinforcement suppliers, which often have proprietarycomputerprograms.This manual does not cover reinforcing the base section of an embankment for constructionover soft soils, which is a different type reinforcement application. The user is referred to theFHWA Geosynthetics Design and Construction Guidelines (Holtz et al., 2008) for thatapplication.An extension of this application is to lengthen reinforcement at thebase of theembankment to improve the global stability of a reinforced soil slope.This application willbe covered, however, steepening a slope significantly increases the potential for bearingcapacity failure over soft soils and extensive geotechnical exploration along with rigorousanalysis is required.FHWA NHI-10-0258-Reinforced Soil Slopes8-1MSE Walls and RSS -Vol IINovember2009
CHAPTER 8 REINFORCED SOIL SLOPES PROJECT EVALUATION 8.1 INTRODUCTION Where right of way is available and the cost of a MSE wall is high, a steepened slope should be considered. In this chapter the background and design requirements for evaluating a reinforced soil slope (RSS) alternative are reviewed. Step-by-step design procedures are presented in Chapter 9. Section 8.2 reviews the types of systems and the materials of construction. Section 8.3 provides a discussion of the internal stability design approach for use of reinforcement as compaction aids, steepening slopes and slope repair. Computer assisted design programs are also reviewed. The section concludes with a discussion of external stability requirements. Section 8.4 reviews the construction sequence. Section 8.5 covers treatment of the outward face of the slope to prevent erosion. Section 8.6 covers design details of appurtenant features including traffic barrier and drainage considerations. Finally, section 8.7 presents several case histories to demonstrate potential cost savings. 8.2 REINFORCED SOIL SLOPE SYSTEMS 8.2.1 Types of Systems Reinforced soil systems consist of planar reinforcements arranged in nearly horizontal planes in the reinforced fill to resist outward movement of the reinforced fill mass. Facing treatments ranging from vegetation to flexible armor systems are applied to prevent unraveling and sloughing of the face. These systems are generic in nature and can incorporate any of a variety of reinforcements and facing systems. Design assistance is often available through many of the reinforcement suppliers, which often have proprietary computer programs. This manual does not cover reinforcing the base section of an embankment for construction over soft soils, which is a different type reinforcement application. The user is referred to the FHWA Geosynthetics Design and Construction Guidelines (Holtz et al., 2008) for that application. An extension of this application is to lengthen reinforcement at the base of the embankment to improve the global stability of a reinforced soil slope. This application will be covered; however, steepening a slope significantly increases the potential for bearing capacity failure over soft soils and extensive geotechnical exploration along with rigorous analysis is required. FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 1 November 2009
An alternate slope reinforcement technique,the“Deep Patch"method, is used for stabilizingand potentially repairing roadway fill slopes on secondary roads where removal andreplacement are not feasible (e.g., in mountainous terrain).In this method, reinforcements(typicallygeogrids)areplaced in theupperportion of the slopeto essentiallytie it back.Anempirical design approach has been developed by the U.S.Department of Agriculture(USDA)Forest Service technology in partnership with FHWA Federal Lands Highway(Musser and Denning, 2005)..The method is not explicitly included in the design sections ofChapters 8 and 9 as this approach is often considered as a temporary repair method to retardthe movement of a slope until a more permanent solution can be implemented; however, abrief description of the method is included in AppendixF.8.2.2ConstructionMaterialsReinforcementtypes.Reinforced soil slopes can be constructed with any of thereinforcements described in Chapter 2.While discrete strip type reinforcing elements can beused, a majority of the systems are constructed with continuous sheets of geosynthetics (i.e.,geotextiles or geogrids)or wire mesh.Small, discrete micro reinforcing elements such asfibers, yarns, and microgrids located very close to each other have also been used. However,the design is based on more conventional unreinforced designs with cohesion added by thereinforcement (which is not covered in this manual).Reinforced Fill Requirements.Reinforced fill requirements for reinforced soil slopes arediscussed in Chapter 3.Because a flexible facing (e.g.wrapped facing) is normally used,minor face distortion that may occur due to reinforced fill settlement, freezing and thawing,or wetting and drying can be tolerated.Thus, lower quality reinforced fill than recommendedfor MSE walls can be used. The recommended reinforced fill is limited to low-plasticity,granular material (i.e.,PI≤20 and≤ 50 percent finer than a No.200 US sieve (0.075mm))However, with good drainage, careful evaluation of soil and soil-reinforcement interactioncharacteristics, field construction control, and performance monitoring (see Chapter 11),most indigenous soil can beconsidered.8.3DESIGNAPPROACH8.3.1ApplicationConsiderationsAs reviewed in Chapter 2,there are two main purposes for using reinforcement in slopes!.Improved stabilityforsteepened slopesand sloperepair.. Compaction aids, for support of construction equipment and improved face stability.FHWA NHI-10-0258- Reinforced Soil Slopes8-2MSEWalls andRSS-Vol IINovember2009
An alternate slope reinforcement technique, the “Deep Patch” method, is used for stabilizing and potentially repairing roadway fill slopes on secondary roads where removal and replacement are not feasible (e.g., in mountainous terrain). In this method, reinforcements (typically geogrids) are placed in the upper portion of the slope to essentially tie it back. An empirical design approach has been developed by the U.S. Department of Agriculture (USDA) Forest Service technology in partnership with FHWA Federal Lands Highway (Musser and Denning, 2005). The method is not explicitly included in the design sections of Chapters 8 and 9 as this approach is often considered as a temporary repair method to retard the movement of a slope until a more permanent solution can be implemented; however, a brief description of the method is included in Appendix F. 8.2.2 Construction Materials Reinforcement types. Reinforced soil slopes can be constructed with any of the reinforcements described in Chapter 2. While discrete strip type reinforcing elements can be used, a majority of the systems are constructed with continuous sheets of geosynthetics (i.e., geotextiles or geogrids) or wire mesh. Small, discrete micro reinforcing elements such as fibers, yarns, and microgrids located very close to each other have also been used. However, the design is based on more conventional unreinforced designs with cohesion added by the reinforcement (which is not covered in this manual). Reinforced Fill Requirements. Reinforced fill requirements for reinforced soil slopes are discussed in Chapter 3. Because a flexible facing (e.g. wrapped facing) is normally used, minor face distortion that may occur due to reinforced fill settlement, freezing and thawing, or wetting and drying can be tolerated. Thus, lower quality reinforced fill than recommended for MSE walls can be used. The recommended reinforced fill is limited to low-plasticity, granular material (i.e., PI ≤ 20 and ≤ 50 percent finer than a No. 200 US sieve {0.075 mm}). However, with good drainage, careful evaluation of soil and soil-reinforcement interaction characteristics, field construction control, and performance monitoring (see Chapter 11), most indigenous soil can be considered. 8.3 DESIGN APPROACH 8.3.1 Application Considerations As reviewed in Chapter 2, there are two main purposes for using reinforcement in slopes: Improved stability for steepened slopes and slope repair. Compaction aids, for support of construction equipment and improved face stability. FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 2 November 2009