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10 Chapter 2 OVERVIEW OF CURRENT DESIGN PROCEDURES FOR GEOSYNTHETIC-REINFORCED SOIL STRUCTURES WITH CLOSELY- SPACED REINFORCEMENT 2.1 BACKGROUND ON CURRENT DESIGN PROCEDURES The design of geosynthetic-reinforced soil structures involving closely-spaced reinforcements for transportation infrastructure in the US has been conducted using two different approaches: (1) the âGRS-IBSâ approach (also identified herein as the âcomposite designâ approach), as detailed in FHWA-HRT-11-026 (FHWA 2012) and recently in FHWA-HRT-17-080 (FHWA 2018); and, (2) the âsimplified method,â also known as the âtie back wedgeâ approach (identified herein as the âcurrent AASHTOâ approach), as detailed in AASHTO 2017 and FHWA 2009. The GRS-IBS approach focuses on the design of a subset of load-carrying GMSE bridge abutments (i.e. GRS- IBS) that are characterized by having closely-spaced reinforcements and other prescriptive construction requirements. The current AASHTO approach, included in the AASHTO 2017 LRFD Bridge Design Specifications and the FHWA (2009) guidelines can be adopted for both closely- and comparatively widely-spaced reinforcements, but does not account for potential added benefits of the closely-spaced nature of the reinforcement layout. Both the AASHTO (2017) and FHWA (2009) documents focus on conventional GMSE structures, with specific additional considerations for the case of load-carrying GMSE bridge abutments. The primary philosophy that guided the development of AASHTOâs current approach was to establish a unified design framework that allows designers to evaluate reinforced wall alternatives for different reinforcement materials (e.g. steel or geosynthetic reinforcement) systematically, based on performance and cost. This philosophy ultimately led to a significant increase in the number of reinforced walls relative to conventional systems. Using the same philosophy of establishing unified design frameworks whenever feasible, the design recommendations proposed in Chapter 3 aim at developing a singular design framework that allows designers to evaluate reinforced wall alternatives (including load-carrying MSE bridge abutments) systematically for different reinforcement vertical spacing (e.g. closely- or widely-spaced reinforcement) based on performance and cost. It should be noted that the current AASHTO approach was validated through research performed on full-scale structures, centrifuge models and numerical models, as part of the FHWA study documented in Volume II of FHWA Report No. FHWA-RD-89-043 (Christopher et al. 1989) and in Christopher (1993). It was acknowledged that the method was inherently conservative to ensure acceptable performance. While this conservatism results in excessive reinforcement in terms of limit state performance, it has resulted in tolerable deformations of the structure under working load conditions. That is, the resulting amount of reinforcement would enable failure loads well in excess of the predicted values. However, deformations occurring under operating conditions have proven to be adequately small. It was suggested that additional studies be performed on the influence of external loading conditions and additional consideration be given to the influence of varying reinforcement vertical spacing through changes in its layer spacing. Additional studies have also been suggested on coupling the effects of reinforcement creep with
11 those of soil creep. Further justification, including calibration of the design approach, is provided by Allen et al. (2001). While a maximum reinforcement spacing value was established at that time to facilitate construction and control face deformations, methods were not identified to capitalize on additional advantages of closely-spaced reinforcement. Much of the effort was also placed on the design of roadway walls, with only limited research conducted on bridge abutments and piers. However, this procedure has been used to safely design a number of GMSE walls supporting bridge structures (e.g. the Founders Meadows Bridge abutment as reported in Section 4.5 of the NCHRP Project 24-41 Final Report). The GRS-IBS method was reportedly developed to take advantage of closely-spaced geosynthetic reinforcement, recognizing that decreased spacing provides increased confinement, reduced lateral deformation, suppressed dilation, and a reduction in connection stresses (e.g. FHWA 2012). An increased density of reinforcement layers also increases the inherent redundancy of the structure thus resulting in better performance. The method was specifically developed for bridge support, as these advantages would be most beneficial for this application and would also address a need for replacement of smaller bridge structures. FHWA developed both empirical and analytical design models for geosynthetic-reinforced soil structures supporting bridge loads (FHWA 2012, FHWA 2018). FHWA also calibrated the reliability of these models using performance test data, which have been correlated against results from laboratory and field monitoring programs (Nicks et al. 2013). However, the results of those studies are only deemed valid for the conditions specifically simulated in that research, as that study did not consider generic boundaries for which improved performance occurs due to closely-spaced reinforcement. 2.2 COMPARISON OF CURRENT DESIGN PROCEDURES To develop the philosophy for the final design approach proposed herein, the Research Team first reviewed the fundamental differences between the currently available AASHTO approach and the existing approach developed specifically for the design of GRS-IBS structures. While there are a number of details that vary between these two methods, there is a distinct difference in the design premise used in the development of these two approaches. The GRS-IBS design method outlined by FHWA (2012, 2018) is specifically for load-carrying geosynthetic-reinforced soil abutments. The method only provides minimal design information for free standing GMSE walls (e.g. wing walls) and no design details are provided for walls supporting embankment type surcharge loads (albeit reinforcement spacing will also influence those designs). On the other hand, the current AASHTO wall design approach was essentially developed for free standing structures to which surcharge loads could be applied (e.g. footing supporting bridge, wall supporting embankment slopes). This fundamental difference of decoupling the bridge from the supporting structure in the latter case does not account for considerations such as mechanisms of load transfer immediately below the applied vertical load, the need to accurately predict vertical movements, and the restraining of lateral deformations of the bridge on the load- carrying GMSE system.
12 Differences between the two methods have been detailed in a number of publications, most notably the publication by Nicks et al. (2013a). The key differences between the two design methods are shown in Table 2.1. External stability considers the reinforced soil as a coherent mass and is largely the same in both approaches, except that the GRS-IBS design approach eliminates the requirement to calculate the eccentricity related to overturning. The changes involving internal stability are significant and attributed to closely-spaced reinforcement layers required in GRS-IBS (i.e. Sv less than or equal to 0.3 m (12 in.)) combined with high quality backfill requirements. Wu et al. (2013) documents the anticipated response, identified as composite behavior of the reinforced soil mass, and provides justification for the layer spacing requirements and proposed model to predict lateral deformations. The justification for the absence of pullout requirement is based on a composite hypothesis in which the composite mass moves as a whole due to the interaction between reinforcement and soil (Wu and Pham, 2014). Justification for the absence of connection strength requirement and for test data to support this design requirement is provided by Iwamoto et al. (2013) and adopted in the GRS-IBS approach. Table 2.1. Comparison between GMSE and GRS-IBS design methods (adapted after Nicks et al. 2013a) Design Check GMSE (AASHTO and FHWA GEC-11) FHWA GRS-IBS Method Required reinforcement vertical spacing, Sv Sv ⤠0.8 m (32 in.), but no lower limit. Can be comparatively small, but provides no benefits to adopt closely-spaced reinforcement in the design. That is, design strength is strictly linear with reinforcement vertical spacing, as follows. = Sv ⤠0.3 m (12 in.), with bearing bed reinforcement (minimum of 5 layers) placed beneath beam seat in between primary reinforcement at Sv ⤠½ primary spacing. Design strength relates non- linearly with reinforcement vertical spacing, as follows: = 0.7 Reinforced fill (range) 100% < 100 mm (4 in.) to 100% < No. 4, sand to cobles with up to 15% finer than 0.075 mm (US No. 200 sieve) and PI ⤠6. 100% < 50 mm (2 in.), clean, open-graded crushed gravel and well-graded sandy gravel. Reinforcement length, B B ⥠6.7 m (22 ft.) for bridge structures, uniform. Allows different reinforcement length values, with 0.4H at the base with an overall average of 0.7H. B = 0.3H or 1.8 m (6 ft.) at the base, non-uniform with a minimum of 0.7H in upper reinforcement layers. Includes a reinforced soil foundation, which increases base width B to (B + 0.25B). Vertical load capacity Vertical load capacity is not established. However, provides allowable load for serviceability limit and strength limit state, factored bearing resistance, based on limited field performance tests and monitored structures. Vertical load capacity is established empirically (based on performance test results) or analytically. The capacity uses the same allowable load limit from AASTHO and GEC-11, as follows: , = 0.7 , = , = ,3.5 Deformation Chart provided for estimation of lateral movement during construction. Post- construction deformations not available. Empirical deformation requirements provided (based on performance test and monitored structures). - Vertical Strain, εv ⤠0.5%, and - Lateral Strain, εL ⤠1.0%
13 Reinforcement rupture = . . , = 0.92.5 With Tallow ⤠T@ε = 2% Reinforcement pullout Required No criterion provided Connection strength Required No criterion provided Friction angle ⤠40 deg (Instrumented structures indicates that design model underestimate reinforcement loads at higher friction angles) ⥠38 deg As defined by minimum required value reported by Adams et al. (2012) Limiting eccentricity Required No criterion Taking into account the genesis of the current design guidelines as documented in this Chapter, the remainder of this document provides recommendations for incorporating the benefits of closely-spaced reinforcement into AASHTO. Specifically, Chapter 3 provides the actual revisions into current AASHTO design procedures, while Chapter 4 documents the basis from these recommendations, pointing to specific research findings documented in Sections 2 through 7 of the NCHRP Project 24-41 Final Report.
