Compact with a vibratory rollerorplatetype compactor for granularmaterialsor a rubber-tired or smooth drum roller for cohesive materials.When placing and compacting the reinforced fill material, care should betakentoavoidanydeformationormovementofthereinforcement.Use lightweight compaction equipment near the slopefacewith welded wiremesh systems to help maintain face alignment..CompactionControlProvide close control on the water content and density of the reinforced fill.Itshouldbe compacted to at least 95percent of the standard AASHTOT99maximumdensitywithin2percentofoptimummoisture.If the reinforced fll is a coarse aggregate, then a relative density or a methodtypecompactionspecificationshouldbeused.·Face ConstructionSlope facing requirements will depend on soil type, slope angle and thereinforcement spacing as shown in Table 8-1.If slope facing is required to prevent sloughing (i.e., slope angle β is greater than psoil)or erosion, several options are availableSufficient reinforcement lengths could beprovided for wrapped faced structures.A face wrap may not be required for slopesup to about IH:1V as indicated in Figure 8-4.In this case, the reinforcements(primary and secondary) can be simply extended to the face.For this option, a facingtreatment as detailed under Section 8.5 Treatment of Outward Face,should be appliedat sufficient intervals during construction to prevent face erosion.For wrapped or nowrap construction, the reinforcement should be maintained at close spacing (i.e.,every lift or every other lift but no greater than 16 in. (400 mm)). For armored, hardfaced systems the maximum spacing should be no greater than 32 in.(800 mm).Apositive frictional or mechanical connection should be provided between thereinforcement and armored typefacing systems.The following procedures are recommended for wrapping the face.Turn up reinforcement at theface of the slope and return the reinforcement aminimum of 3 ft (1 m) into the embankment below the next reinforcementlayer (seeFigure8-4)For steep slopes, formwork may be required to support the face duringconstruction,improvingcompaction atthefaceand providinga smootherfacefinish. Welded wire mesh is often used as a face form (see Figure 8-5). TheFHWA NHI-10-0258-Reinforced Soil Slopes8-13MSEWalls and RSS-Vol IINovember2009
- Compact with a vibratory roller or plate type compactor for granular materials or a rubber-tired or smooth drum roller for cohesive materials. - When placing and compacting the reinforced fill material, care should be taken to avoid any deformation or movement of the reinforcement. - Use lightweight compaction equipment near the slope face with welded wire mesh systems to help maintain face alignment. Compaction Control - Provide close control on the water content and density of the reinforced fill. It should be compacted to at least 95 percent of the standard AASHTO T99 maximum density within 2 percent of optimum moisture. - If the reinforced fill is a coarse aggregate, then a relative density or a method type compaction specification should be used. Face Construction - Slope facing requirements will depend on soil type, slope angle and the reinforcement spacing as shown in Table 8-1. If slope facing is required to prevent sloughing (i.e., slope angle β is greater than soil) or erosion, several options are available. Sufficient reinforcement lengths could be provided for wrapped faced structures. A face wrap may not be required for slopes up to about 1H:1V as indicated in Figure 8-4. In this case, the reinforcements (primary and secondary) can be simply extended to the face. For this option, a facing treatment as detailed under Section 8.5 Treatment of Outward Face, should be applied at sufficient intervals during construction to prevent face erosion. For wrapped or no wrap construction, the reinforcement should be maintained at close spacing (i.e., every lift or every other lift but no greater than 16 in. {400 mm}). For armored, hard faced systems the maximum spacing should be no greater than 32 in. (800 mm). A positive frictional or mechanical connection should be provided between the reinforcement and armored type facing systems. The following procedures are recommended for wrapping the face. - Turn up reinforcement at the face of the slope and return the reinforcement a minimum of 3 ft (1 m) into the embankment below the next reinforcement layer (see Figure 8-4). - For steep slopes, formwork may be required to support the face during construction, improving compaction at the face and providing a smoother face finish. Welded wire mesh is often used as a face form (see Figure 8-5). The FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 13 November 2009
welded wireface isleft in place with ungalvanized used for temporary supportandgalvanizedwireusedforpermanent supportFor grid reinforcements, a fine mesh screen or geotextile may be required attheface to retain reinforced fill materials.Slopes steeper than approximately 1:1 typically require facing support duringconstruction. Exact slope angles will vary with soil types, i.e., amount of cohesionRemovable facing supports (e.g., wooden forms) or left-in-place welded wire meshforms are typically used. Facing support may also serve as permanent or temporaryerosion protection,depending on therequirements ofthe slope.·Additional ReinforcingMaterials and Retained Backfill PlacementIf drainage layers are required, they should be constructed directly behind or on thesides of the reinforced section.Table 8-1.RSS slope facing options (after Collin, 1996)TypeofFacingWhen Geosynthetic is Wrapped at FaceSlope Face Angle andWhen Geosynthetic is not Wrapped at FaceSoil TypeVegetated FaceVegetated Face'Hard FacingHard Facing>50°Sod,GabionsWire Baskets,NotRecommendedPermanent Erosion(>~0.9H:1V)Stone,All Soil TypesBlanket wl seedShotcrete35°to50°Gabions,Sod,NotRecommendedWire Baskets,(~ 1.4H:1V to 0.9H:1V)Soil-CementPermanent ErosionStone,Clean Sands (SP)Blanket w/ seedShotcreteRounded Gravel (GP)Sod,35°to50°Soil Bio reinforcement,Gabions,Wire Baskets,(~ 1.4H:1V to 0.9H:1V)DrainageSoil-Cement,Permanent ErosionStone,Silts (ML)CompositesStone VeneerBlanket w/ seedShotcreteSandy Silts (ML)35°to 50°TemporaryHard Facing,GeosyntheticGeosynthetic(~1.4H:1V to 0.9H:1V)Erosion BlanketWrap NotNot NeededWrap NotSilty Sands (SM)w/ Seed or Sod.NeededNeededClayey Sands (SC)PermanentWell graded sands andErosion Matgravels(SW&GW)w/ Seed or Sod25°to35°TemporaryHard FacingGeosyntheticGeosynthetic(~ 2H:1V to 1.4H:1V)Erosion BlanketNot NeededWrap NotWrap Notw/ Seed or Sod,NeededNeededAll Soil TypesPermanent Erosion Matw/ Seed or SodNotes:Vertical spacing of reinforcement (primary/secondary)shall be nogreater than 16 in. (400 mm)withprimary reinforcements spaced no greater than 32 in.(800 mm)when secondary reinforcement is used2.Vertical spacing of primary reinforcement shall benogreaterthan32in.(800mm)18 in. (450 mm) high wire baskets are recommended34.1Unified SoilClassificationGeosynthetic or natural horizontal drainage layers to intercept and drain the saturated soil at the face of5.the slope.FHWANHI-10-0258-Reinforced Soil SlopesMSE Walls and RSS -- Vol II8-14November2009
welded wire face is left in place with ungalvanized used for temporary support and galvanized wire used for permanent support. For grid reinforcements, a fine mesh screen or geotextile may be required at the face to retain reinforced fill materials. Slopes steeper than approximately 1:1 typically require facing support during construction. Exact slope angles will vary with soil types, i.e., amount of cohesion. Removable facing supports (e.g., wooden forms) or left-in-place welded wire mesh forms are typically used. Facing support may also serve as permanent or temporary erosion protection, depending on the requirements of the slope. Additional Reinforcing Materials and Retained Backfill Placement If drainage layers are required, they should be constructed directly behind or on the sides of the reinforced section. Table 8-1. RSS slope facing options (after Collin, 1996). Slope Face Angle and Soil Type Type of Facing When Geosynthetic is not Wrapped at Face When Geosynthetic is Wrapped at Face Vegetated Face1 Hard Facing2 Vegetated Face1 Hard Facing2 > 50o (> ~0.9H:1V) All Soil Types Not Recommended Gabions Sod, Permanent Erosion Blanket w/ seed Wire Baskets,3 Stone, Shotcrete 35o to 50o (~ 1.4H:1V to 0.9H:1V) Clean Sands (SP)4 Rounded Gravel (GP) Not Recommended Gabions, Soil-Cement Sod, Permanent Erosion Blanket w/ seed Wire Baskets,3 Stone, Shotcrete 35o to 50o (~ 1.4H:1V to 0.9H:1V) Silts (ML) Sandy Silts (ML) Soil Bio reinforcement, Drainage Composites5 Gabions, Soil-Cement, Stone Veneer Sod, Permanent Erosion Blanket w/ seed Wire Baskets,3 Stone, Shotcrete 35o to 50o (~ 1.4H:1V to 0.9H:1V) Silty Sands (SM) Clayey Sands (SC) Well graded sands and gravels (SW & GW) Temporary Erosion Blanket w/ Seed or Sod, Permanent Erosion Mat w/ Seed or Sod Hard Facing, Not Needed Geosynthetic Wrap Not Needed Geosynthetic Wrap Not Needed 25o to 35o (~ 2H:1V to 1.4H:1V) All Soil Types Temporary Erosion Blanket w/ Seed or Sod, Permanent Erosion Mat w/ Seed or Sod Hard Facing Not Needed Geosynthetic Wrap Not Needed Geosynthetic Wrap Not Needed Notes: 1. Vertical spacing of reinforcement (primary/secondary) shall be no greater than 16 in. (400 mm) with primary reinforcements spaced no greater than 32 in. (800 mm) when secondary reinforcement is used. 2. Vertical spacing of primary reinforcement shall be no greater than 32 in. (800 mm). 3. 18 in. (450 mm) high wire baskets are recommended. 4. Unified Soil Classification 5. Geosynthetic or natural horizontal drainage layers to intercept and drain the saturated soil at the face of the slope. FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 14 November 2009
SEEDEDTOPSOILINFILL SOILSWIREMESHBASKETFACING-STRUTBRACEGEOTEXTILEEILTERORREINFORCEMENTEROSIONCONTROLMATA)0.51STONE ARMOR FACING (BY DTHERS)AASHTOM288ClaSS2S-INCH(MAXTOHGeotextile FilterAGGREGATESIZESURGESTONE(TYP)TAR0.25′ (MNFINISHED0.5′ (MIN.)(TYP.)4.0°ORJREYSEReinforcement3.0ORASB)BY DESIGNFigure 8-5.Example of welded wire mesh detail for temporary (during construction) orpermanentfacesupportshowinga)smoothinclinedface,andb)steppedface.8.5TREATMENTOFOUTWARDFACE8.5.1GrassTypeVegetationStability of a slope can be threatened by erosion due to surface water runoff. Erosion controland revegetation measures must, therefore, be an integral part of all reinforced slope systemdesigns and specifications.If not otherwise protected, reinforced slopes should be vegetatedafter construction to prevent or minimize erosion due to rainfall and runoff on the faceVegetation requirements will vary by geographic and climatic conditions and are, therefore,project specific.For the unwrapped face (the soil surface exposed), erosion control measures are necessary toprevent raveling and sloughingoftheface.A wrappedfacehelpsreduceerosion problems;however, treatments are still required on the face to shade geosynthetic soil reinforcementand prevent ultraviolet light exposure that will degrade the geosynthetic over time. In eithercase, conventional vegetated facing treatments generally rely on low growth, grass typeFHWA NHI-10-0258 -Reinforced Soil Slopes8-15MSEWallsandRSS-VolIINovember2009
SEEDED TOPSOIL A) B) AASHTO M288 Class 2 Geotextile Filter Reinforcement Figure 8-5. Example of welded wire mesh detail for temporary (during construction) or permanent face support showing a) smooth inclined face, and b) stepped face. 8.5 TREATMENT OF OUTWARD FACE 8.5.1 Grass Type Vegetation Stability of a slope can be threatened by erosion due to surface water runoff. Erosion control and revegetation measures must, therefore, be an integral part of all reinforced slope system designs and specifications. If not otherwise protected, reinforced slopes should be vegetated after construction to prevent or minimize erosion due to rainfall and runoff on the face. Vegetation requirements will vary by geographic and climatic conditions and are, therefore, project specific. For the unwrapped face (the soil surface exposed), erosion control measures are necessary to prevent raveling and sloughing of the face. A wrapped face helps reduce erosion problems; however, treatments are still required on the face to shade geosynthetic soil reinforcement and prevent ultraviolet light exposure that will degrade the geosynthetic over time. In either case, conventional vegetated facing treatments generally rely on low growth, grass type FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 15 November 2009
vegetation with more costly flexible armor occasionally used where vegetation cannot beestablished. Due to the steep grades that can be achieved with reinforced soil slopes, it canbe difficult to establish and maintain grass type vegetative cover.The steepness of the gradelimits the amount of water absorbed by the soil before runoff occurs.Although rootpenetration should not affect the reinforcement, the reinforcement may restrict root growth,depending on the reinforcement type. This can have an adverse influence on the growth ofsome plants.Grass is also frequently ineffective where slopes are impacted by waterways.A synthetic (permanent) erosion control mat is normally used to improve the performance ofgrass cover.This mat must also be stabilized against ultra-violet light and should be inert tonaturally occurring soil-born chemicals and bacteria.The erosion control mat serves to: 1)protect the bare soil face against erosion until the vegetation is established, 2) assist inreducing runoff velocity for increased water absorption by the soil,thus promoting long-termsurvival of the vegetative cover, and 3)reinforce the surficial root system of the vegetativecover.Once vegetation is established on the face, it must be protected to ensure long-term survival.Maintenance issues, such as mowing (if applicable), must also be carefully considered.Theshorter, weaker root structure of most grasses may not provide adequate reinforcement anderosion protection.Grass is highly susceptible to fire, which can also destroy the syntheticerosion control mat.Downdrag from snow loads or upland slides may also strip matting andvegetation off the slope face.The low erosion tolerance combined with other factorspreviously mentioned creates a need to evaluate revegetation measures as an integral part ofthedesign.Slope face protection shouldnot be left to the construction contractor or vendor'sdiscretion.Guidance should beobtained from maintenanceand regional landscapinggroupsin the selection of the most appropriate low maintenance vegetation.8.5.2SoilBioengineering(WoodyVegetation)An alternative to low growth, grass type vegetation is the use of soil bioengineering methodsto establish hardier, woody type vegetation in the face of the slope (Sotir and Christopher,2000).Soil bioengineering uses living vegetation purposely arranged and imbedded in theground to prevent shallow mass movement and surficial erosion. However, the use of soilbioengineering in itself is limited to stable slopemasses.Combining this highly erosivesystem with geosynthetic reinforcement produces a very durable, low maintenance structurewith exceptional aesthetic and environmental qualities.Appropriately applied, soil bioengineering offers a cost-effective and attractive approach forstabilizing slopes against erosion and shallow mass movement, capitalizing on the benefitsand advantages that vegetation offers.The value of vegetation in civil engineering and theFHWA NHI-10-0258-Reinforced Soil Slopes816MSE Walls and RSS - Vol IINovember2009
vegetation with more costly flexible armor occasionally used where vegetation cannot be established. Due to the steep grades that can be achieved with reinforced soil slopes, it can be difficult to establish and maintain grass type vegetative cover. The steepness of the grade limits the amount of water absorbed by the soil before runoff occurs. Although root penetration should not affect the reinforcement, the reinforcement may restrict root growth, depending on the reinforcement type. This can have an adverse influence on the growth of some plants. Grass is also frequently ineffective where slopes are impacted by waterways. A synthetic (permanent) erosion control mat is normally used to improve the performance of grass cover. This mat must also be stabilized against ultra-violet light and should be inert to naturally occurring soil-born chemicals and bacteria. The erosion control mat serves to: 1) protect the bare soil face against erosion until the vegetation is established; 2) assist in reducing runoff velocity for increased water absorption by the soil, thus promoting long-term survival of the vegetative cover; and 3) reinforce the surficial root system of the vegetative cover. Once vegetation is established on the face, it must be protected to ensure long-term survival. Maintenance issues, such as mowing (if applicable), must also be carefully considered. The shorter, weaker root structure of most grasses may not provide adequate reinforcement and erosion protection. Grass is highly susceptible to fire, which can also destroy the synthetic erosion control mat. Downdrag from snow loads or upland slides may also strip matting and vegetation off the slope face. The low erosion tolerance combined with other factors previously mentioned creates a need to evaluate revegetation measures as an integral part of the design. Slope face protection should not be left to the construction contractor or vendor's discretion. Guidance should be obtained from maintenance and regional landscaping groups in the selection of the most appropriate low maintenance vegetation. 8.5.2 Soil Bioengineering (Woody Vegetation) An alternative to low growth, grass type vegetation is the use of soil bioengineering methods to establish hardier, woody type vegetation in the face of the slope (Sotir and Christopher, 2000). Soil bioengineering uses living vegetation purposely arranged and imbedded in the ground to prevent shallow mass movement and surficial erosion. However, the use of soil bioengineering in itself is limited to stable slope masses. Combining this highly erosive system with geosynthetic reinforcement produces a very durable, low maintenance structure with exceptional aesthetic and environmental qualities. Appropriately applied, soil bioengineering offers a cost-effective and attractive approach for stabilizing slopes against erosion and shallow mass movement, capitalizing on the benefits and advantages that vegetation offers. The value of vegetation in civil engineering and the FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 16 November 2009
rolewoodyvegetation plays inthestabilization of slopes hasgained considerablerecognitionin recent years (Gray and Sotir, 1995).Woody vegetation improves the hydrology andmechanicalstabilityofslopesthroughrootreinforcementandsurfaceprotection.Theuseofdeeply-installed and rooted woody plant materials, purposely arranged and imbedded duringslopeconstruction offers:.Immediateerosioncontrolforslopes,stream,and shoreline.Improvedface stabilitythroughmechanical reinforcementby rootsReduced maintenance costs, with less need to return to revegetate or cut grass.: Modification of soil moisture regimes through improved drainage and depletion ofsoil moisture and increase of soil suctionby root uptake and transpiration:Enhancedwildlifehabitatandecologicaldiversity.. Improved aesthetic quality and naturalizationThe biological and mechanical elements must be analyzed and designed to work together inan integrated and complementary manner to achieve the required project goals. In additionto using engineering principles to analyze and design the slope stabilization systems, plantscience and horticulture are needed to select and establish the appropriate vegetation for rootreinforcement,erosioncontrol,aestheticsandtheenvironment.Numerousareas ofexpertisemust integrate to provide the knowledge and awareness required for success. RsS systemsrequire knowledge of the mechanisms involving mass and surficial stability of slopes.Likewise when the vegetative aspects are appropriate to serve as reinforcements and drainsan understanding of the hydraulic and mechanical effects of slope vegetation is necessaryFigure 8-6 shows a cross section of the components of a vegetated reinforced slope (VRsS)system. The design details for face construction include vegetation selection, placement, anddevelopment as well as several agronomic and geotechnical design issues (Sotir andChristopher, 2000).FrontFaceWrapFaceErosion Control FabricRootedCompected SelectFillMaterialPlants7CompectedBackfillFigure8-6.Componentsof avegetated reinforced slopePrimaryNo Soale(VRSS) systemReinforcementFHWA NHI-10-0258-Reinforced Soil Slopes8-17MSEWallsandRSS-Vol IINovember2009
role woody vegetation plays in the stabilization of slopes has gained considerable recognition in recent years (Gray and Sotir, 1995). Woody vegetation improves the hydrology and mechanical stability of slopes through root reinforcement and surface protection. The use of deeply-installed and rooted woody plant materials, purposely arranged and imbedded during slope construction offers: Immediate erosion control for slopes; stream, and shoreline. Improved face stability through mechanical reinforcement by roots. Reduced maintenance costs, with less need to return to revegetate or cut grass. Modification of soil moisture regimes through improved drainage and depletion of soil moisture and increase of soil suction by root uptake and transpiration. Enhanced wildlife habitat and ecological diversity. Improved aesthetic quality and naturalization. The biological and mechanical elements must be analyzed and designed to work together in an integrated and complementary manner to achieve the required project goals. In addition to using engineering principles to analyze and design the slope stabilization systems, plant science and horticulture are needed to select and establish the appropriate vegetation for root reinforcement, erosion control, aesthetics and the environment. Numerous areas of expertise must integrate to provide the knowledge and awareness required for success. RSS systems require knowledge of the mechanisms involving mass and surficial stability of slopes. Likewise when the vegetative aspects are appropriate to serve as reinforcements and drains, an understanding of the hydraulic and mechanical effects of slope vegetation is necessary. Figure 8-6 shows a cross section of the components of a vegetated reinforced slope (VRSS) system. The design details for face construction include vegetation selection, placement, and development as well as several agronomic and geotechnical design issues (Sotir and Christopher, 2000). FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 17 November 2009 Figure 8-6. Components of a vegetated reinforced slope (VRSS) system