With this assumption, FSr is applied to both the soil and the reinforcement as part of theanalysis.Asaresult,thestability withrespecttobreakageof thereinforcementrequiresthatthe allowable reinforcement strength Tal from Chapter 3, equation 3-12must be greater thanorequal to therequired maximumdesign tension Tmaxfor each reinforcement layerSome computer programs use an assumption that the reinforcement force is a negativedriving component, thus the FS is computed as:MR(8-2)FS:M,-T,RWith this assumption, the stability factor of safety is not applied to Ts.Therefore, theallowable design strength Tal should be computed as the ultimate tensile strength TuLTdivided by the required safety factor (i.e., target stability factor of safety) along with theappropriate reduction factors RF in equation 8-12; i.e., Tal=TuLT/(FSRxRF).This providesan equivalentfactor of safety to equation 8-1, which is appropriate to account for uncertaintyin material strengths and reduction factors. The method used to develop design charts shouldlikewise be carefully evaluated to determine FS used to obtain the allowable reinforcementstrength.8.3.5Evaluation of External StabilityThe external stability of reinforced soil slopes depends on the ability of the reinforced zoneto act as a stable block and withstand all external loads without failure. Failure possibilitiesas shown in Figure 8-3 include wedge and block type sliding, deep-seated overall instability,local bearing capacity failure at the toe (lateral squeeze type failure), as well as excessivesettlement from both short- and long-term conditions.Thereinforced zonemustbe sufficientlywide at any level to resist wedgeand blocktypesliding.To evaluate sliding stability,a wedge type failure surface defined by the limits of thereinforcement can be analyzed using the conventional sliding block method of analysis asdetailed in the FHWA Soils and Foundations Workshop Reference Manual, (Samtani andNowatzki, 2006).The computer program ResSA incorporates wedge analysis of thereinforced system, using force equilibrium to analyze sliding both beyond and through thereinforced section.Conventional soil mechanics stability methods should also be used to evaluate the globalstability of the reinforced soil zone.Both rotational and wedge type failure surfacesextending behind and below the structure should be considered.Care should be taken toFHWANHI-10-0258-Reinforced Soil Slopes8-8MSE Walls and RSS -Vol IINovember2009
With this assumption, FSR is applied to both the soil and the reinforcement as part of the analysis. As a result, the stability with respect to breakage of the reinforcement requires that the allowable reinforcement strength Tal from Chapter 3, equation 3-12 must be greater than or equal to the required maximum design tension Tmax for each reinforcement layer. Some computer programs use an assumption that the reinforcement force is a negative driving component, thus the FS is computed as: MR FS (8-2) M D TS R With this assumption, the stability factor of safety is not applied to TS. Therefore, the allowable design strength Tal should be computed as the ultimate tensile strength TULT divided by the required safety factor (i.e., target stability factor of safety) along with the appropriate reduction factors RF in equation 8-12; i.e., Tal = TULT / (FSR RF). This provides an equivalent factor of safety to equation 8-1, which is appropriate to account for uncertainty in material strengths and reduction factors. The method used to develop design charts should likewise be carefully evaluated to determine FS used to obtain the allowable reinforcement strength. 8.3.5 Evaluation of External Stability The external stability of reinforced soil slopes depends on the ability of the reinforced zone to act as a stable block and withstand all external loads without failure. Failure possibilities as shown in Figure 8-3 include wedge and block type sliding, deep-seated overall instability, local bearing capacity failure at the toe (lateral squeeze type failure), as well as excessive settlement from both short- and long-term conditions. The reinforced zone must be sufficiently wide at any level to resist wedge and block type sliding. To evaluate sliding stability, a wedge type failure surface defined by the limits of the reinforcement can be analyzed using the conventional sliding block method of analysis as detailed in the FHWA Soils and Foundations Workshop Reference Manual, (Samtani and Nowatzki, 2006). The computer program ReSSA incorporates wedge analysis of the reinforced system, using force equilibrium to analyze sliding both beyond and through the reinforced section. Conventional soil mechanics stability methods should also be used to evaluate the global stability of the reinforced soil zone. Both rotational and wedge type failure surfaces extending behind and below the structure should be considered. Care should be taken to FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 8 November 2009
identify any weak soil layers in the retained fill and natural soils behind and/or foundationsoil below the reinforced soil zone. Evaluation of potential seepage forces is especiallycritical for global stability analysis.Compound failure surfaces initiating externally andpassing through or between reinforcement sections should also be evaluated, especially forcomplex slope or soil conditions.Extending the lengths of lower level reinforcements mayimprove the overall global stability,however,special considerations for the orientation ofthereinforcement in the analysis must be considered based on the foundation conditions, asdetailed in Chapter 9.B)A)SLIDINGINSTABILITYDEEP SEATED OVERALLINSTABILITYSOFTSOILFIRMSOILc)LOCAL BEARING CAPACITYD)EXCESSIVE SETTLEMENT（LATERAL SQUEEZEFAILURBFigure 8-3.Externalfailuremodesforreinforcedsoilslopes.FHWA NHI-10-0258-Reinforced Soil Slopes8-9MSEWallsandRSS-VolIINovember2009
identify any weak soil layers in the retained fill and natural soils behind and/or foundation soil below the reinforced soil zone. Evaluation of potential seepage forces is especially critical for global stability analysis. Compound failure surfaces initiating externally and passing through or between reinforcement sections should also be evaluated, especially for complex slope or soil conditions. Extending the lengths of lower level reinforcements may improve the overall global stability, however, special considerations for the orientation of the reinforcement in the analysis must be considered based on the foundation conditions, as detailed in Chapter 9. FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 9 November 2009 Figure 8-3. External failure modes for reinforced soil slopes
Evaluation of deep-seated failure does not automatically check for bearing capacity of thefoundation or failure at the toe of the slope. High lateral stress in a confined soft stratumbeneath the embankment could lead to a lateral squeeze type failure.The shear forcesdeveloped underthe embankment should becompared to thecorresponding shear strength ofthe soil. Approaches discussed by Jurgenson (1934), Silvestri (1983), and Bonaparte et al.(1987), and Holtz et al. (2008) are appropriate. The approach by Silvestri is demonstrated inexampleproblemE.10 inAppendixE.Settlement should be evaluated for both total and differential movement.While settlement ofthereinforced slope is not of concern, adjacent structures or structures supported by the slopemay not tolerate such movements.Differential movements can also affect decisions onfacing elements as discussed in Section 8.4.In areas subject to potential seismic activity, a simple pseudo-static type analysis should beperformed using a seismic coefficient obtained from Division 1A of the AASHTO StandardSpecifications for Highway Bridges (AASHTO, 2002),AASHTO LRFD Bridge DesignSpecifications(AASHTO,2007)or using local practice.Reinforced slopes are flexiblesystems and, unless used for bridge abutments, they are not laterally restrained. For freestanding abutments that can tolerate lateral displacement of up to 10A in., Division 1A -Seismic Design, Article 6.4.3 Abutments (AASHTO, 2002) and Appendix A11.1.1.2(AASHTO, 2007) both imply that a seismic design acceleration Am = A/2 and acorresponding horizontal seismic coefficient Kh =A/2 can be used for seismic design.Appropriately a seismic design acceleration of A/2 is recommended for reinforced soilslopes,unlessthe slope supports structuresthat cannottoleratesuchmovementsIf any of the external stability safety factors are less than the required factor of safety,thefollowingfoundation improvement options could be considered:Excavateand replace soft soil..Flatten the slope..Construct a berm at the toe of the slope to provide an equivalent flattened slope.Theberm could be placed as a surcharge at the toe and removed after consolidation of thesoil has occurred..Stage construct the slope toallowtimefor consolidation of thefoundation soils. Embed the slope below grade (> 3 ft), or construct a shear key at the toe of the slope(evaluate based on active-passive resistance). Use ground improvement techniques (e.g., wick drains, stone columns, etc.)Additional information on ground improvement techniques can be found in the FHWAGround Improvement Methods reference manuals NHI-06-019 and NHI-06-020 (Elias et al.,2006).FHWA NHI-10-0258-Reinforced Soil Slopes8-10MSEWallsandRSS-VolIINovember2009
Evaluation of deep-seated failure does not automatically check for bearing capacity of the foundation or failure at the toe of the slope. High lateral stress in a confined soft stratum beneath the embankment could lead to a lateral squeeze type failure. The shear forces developed under the embankment should be compared to the corresponding shear strength of the soil. Approaches discussed by Jurgenson (1934), Silvestri (1983), and Bonaparte et al. (1987), and Holtz et al. (2008) are appropriate. The approach by Silvestri is demonstrated in example problem E.10 in Appendix E. Settlement should be evaluated for both total and differential movement. While settlement of the reinforced slope is not of concern, adjacent structures or structures supported by the slope may not tolerate such movements. Differential movements can also affect decisions on facing elements as discussed in Section 8.4. In areas subject to potential seismic activity, a simple pseudo-static type analysis should be performed using a seismic coefficient obtained from Division 1A of the AASHTO Standard Specifications for Highway Bridges (AASHTO, 2002), AASHTO LRFD Bridge Design Specifications (AASHTO, 2007) or using local practice. Reinforced slopes are flexible systems and, unless used for bridge abutments, they are not laterally restrained. For free standing abutments that can tolerate lateral displacement of up to 10A in., Division 1A – Seismic Design, Article 6.4.3 Abutments (AASHTO, 2002) and Appendix A11.1.1.2 (AASHTO, 2007) both imply that a seismic design acceleration Am = A/2 and a corresponding horizontal seismic coefficient Kh =A/2 can be used for seismic design. Appropriately a seismic design acceleration of A/2 is recommended for reinforced soil slopes, unless the slope supports structures that cannot tolerate such movements. If any of the external stability safety factors are less than the required factor of safety, the following foundation improvement options could be considered: Excavate and replace soft soil. Flatten the slope. Construct a berm at the toe of the slope to provide an equivalent flattened slope. The berm could be placed as a surcharge at the toe and removed after consolidation of the soil has occurred. Stage construct the slope to allow time for consolidation of the foundation soils. Embed the slope below grade (> 3 ft), or construct a shear key at the toe of the slope (evaluate based on active-passive resistance). Use ground improvement techniques (e.g., wick drains, stone columns, etc.) Additional information on ground improvement techniques can be found in the FHWA Ground Improvement Methods reference manuals NHI-06-019 and NHI-06-020 (Elias et al., 2006). FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 10 November 2009
8.4CONSTRUCTIONSEOUENCEAs the reinforcement layers are easily incorporated between the compacted lifts of fill,construction of reinforced slopes is very similar to normal slope construction.The elementsof construction consist of simply:1. Placing the soil2.Placingthereinforcement3.Constructing the faceThe following is the usual construction sequence as shown in Figure 8-4:.Site PreparationClear and grub site.Remove all slide debris (for slope reinstatement projects)-Preparea level subgradeforplacement ofthefirst level of reinforcement-Proof-roll subgrade at the base of the slope with a roller or rubber-tiredvehicle.Observeand approvefoundationpriortofill placement.Placedrainage features (e.g.,basedrain and/or backdrain)as required?Reinforcing Layer PlacementReinforcement should be placed with the principal strength directionperpendiculartothefaceofthe slope.Secure reinforcement with retaining pins to prevent movement during fillplacement.Aminimumoverlapof 6 in.(150mm)isrecommended alongtheedgesperpendicular to the slope for wrapped face structures. Alternatively with gridreinforcement,theedges may beclipped or tied together.Whenreinforcements are not required for face support, no overlap is required andedges should be butted.Reinforcedfill PlacementPlace fill to the required lift thickness on the reinforcement using a front endloaderordozeroperatingonpreviouslyplacedfillornatural ground.Maintain a minimum of 6 in. (150 mm) of fill between the reinforcement andthe wheels or tracks of construction equipment.FHWA NHI-10-0258- Reinforced Soil Slopes8-11MSE Walls and RSS - Vol IINovember2009
8.4 CONSTRUCTION SEQUENCE As the reinforcement layers are easily incorporated between the compacted lifts of fill, construction of reinforced slopes is very similar to normal slope construction. The elements of construction consist of simply: 1. Placing the soil 2. Placing the reinforcement 3. Constructing the face The following is the usual construction sequence as shown in Figure 8-4: Site Preparation - Clear and grub site. - Remove all slide debris (for slope reinstatement projects). - Prepare a level subgrade for placement of the first level of reinforcement. - Proof-roll subgrade at the base of the slope with a roller or rubber-tired vehicle. - Observe and approve foundation prior to fill placement. - Place drainage features (e.g., basedrain and/or backdrain) as required. Reinforcing Layer Placement - Reinforcement should be placed with the principal strength direction perpendicular to the face of the slope. - Secure reinforcement with retaining pins to prevent movement during fill placement. - A minimum overlap of 6 in. (150 mm) is recommended along the edges perpendicular to the slope for wrapped face structures. Alternatively with grid reinforcement, the edges may be clipped or tied together. When reinforcements are not required for face support, no overlap is required and edges should be butted. Reinforced fill Placement - Place fill to the required lift thickness on the reinforcement using a front end loader or dozer operating on previously placed fill or natural ground. - Maintain a minimum of 6 in. (150 mm) of fill between the reinforcement and the wheels or tracks of construction equipment. FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 11 November 2009
WRAPPED FACE CONSTRUCTIONNO WRAP CONSTRUCTIONCOMPACTFILLSECONDARY&FACEREINFORCEMENTAt"...1-USCUOHTUSE ORONARTACHONCOUPACTION EOUIPCNTA)LIFT 1 PLUSREINFORCEMENTFOR LIFT 2OPTIONALFACECONSTRUCTION:OVEREXTENDFILLCOMPACTANDCUTBACKORUSEAFORMPRIMARYREINFORCEMENTB)SECOND PRIMARY REINFORCEMENT LAYBREROSIONCONTROLMATc)COMPLETIONORSECONDSTAGESHRUBSCOMPACTIONFORMROCKFACE..D)FACING ALTERNATIVESFigure 8-4.ConstructionofreinforcedsoilslopesFHWANHI-10-0258-Reinforced Soil Slopes8-12MSEWallsandRSS-VolIINovember2009
Figure 8-4. Construction of reinforced soil slopes. FHWA NHI-10-025 8 – Reinforced Soil Slopes MSE Walls and RSS – Vol II 8 – 12 November 2009