Proposed Refinements to Design Procedures for Geosynthetic Reinforced Soil (GRS) Structures in AASHTO LRFD Bridge Design Specifications (2019) / Chapter Skim
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14 Chapter 3 PROPOSED REVISIONS TO INCORPORATE SV INTO CURRENT AASHTO DESIGN PROCEDURES 3.1 OVERVIEW The qualitative and quantitative outcomes of this research, stemming from laboratory testing, field instrumentation data, numerical analysis, and comprehensive interpretation of previous work by others, was used to develop the design recommendations proposed herein. This section summarizes these recommendations and provides guidance for modification of appropriate sections of AASHTO to recognize the benefits of using closely-spaced reinforcements in both GMSE and load-carrying GMSE bridge abutments (including those currently identified as GRS-IBS structures)
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15 0.8 (i.e., the reinforcement pullout factor F*
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16 However, the ultimate strength value was defined based on 3D column tests, which may not have boundary conditions that are representative of walls supporting bridges (e.g., confinement of the loading plate and 3D vs 2D loading)
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17 In summary, adopting closely-spaced reinforcement in design was found to have significant impact on design on a number of aspects, including the distribution of maximum reinforcement tension with depth (a comparatively uniform profile was identified) , as well as the magnitude of the horizontal structure deformations (a comparatively smaller magnitude)
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18 Figure 3.1. Total tensile capacity (shaded area)
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19 AASHTO as 1.3 or 1.5, depending on the criticality of the structure. The importance of such assessment is demonstrated by Leshchinsky (2009)
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20 the NCHRP Project 24-41 study concluded that close vertical spacing in extensible geosyntheticreinforced walls, having coverage of 100%, affects the distribution of Tmax. over the depth of a wall.
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21 3.3 EFFECT OF SV ON T0 MAGNITUDE AND DISTRIBUTION A number of revisions are recommended herein to account for the impact of reinforcement vertical spacing, Sv, and the effect of closely-spaced reinforcements on the magnitude and distribution of T0 in particular. The specific revisions recommended for incorporation into the current AASHTO design procedures are: • Consistent with the limits defined for determining Tmax, For the purposes of defining T0, a vertical spacing Sv,nc = 0.4 m (16 in.)
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22 C11.10.6.2.2 For closely-spaced geosynthetic reinforcement, T0 will be equal to the maximum nominal reinforcement tension as calculated and the vertical spacing defined as in 11.10.6.2.2. Based on research in NCHRP Project 24-41 the magnitude of the connection load, T0 is recommended as follows: , = .
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23 by appropriate lateral earth pressure coefficient, leads to the horizontal stress used to predict the reinforcement load. The rate at which the stress dissipates with depth in the proposed approach is defined by a 2D pyramid with side slopes of 2(v)
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24 3.5 EFFECT OF SV ON VERTICAL AND LATERAL DEFORMATIONS The results of the parametric numerical evaluation (Section 7 of the NCHRP Project 21-41 Final Report) indicated a significant influence of reinforcement vertical spacing on the lateral deformation response of GMSE structures in general, and load-carrying GMSE bridge abutments in particular.
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25 δmax = the maximum displacement (units of H) , H = the height of the wall (ft or m)
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26 In cases where the vertical settlement is unknown, [INSERT 3RD BULLET ABOVE] C11.10.11 Lateral displacements shall be evaluated following the recommendations in Article 11.10.4.2.
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27 11.10.4.1 Add the following after last paragraph in current article: Differential settlement shall also be part of the analysis of abutment transitions in accordance with Article 10.5.2.4. For MSE walls with the bridge abutment supported by deep foundations, the analysis shall include the total settlement of the embankment behind the bridge including the compression of the fill following Article 10.6.2.4, as compared to the settlement of the footing supported by the deep foundation based on Articles 10.7.2.3 -10.7.2.5 for piles or Articles 10.8.2.2 – 10.8.2.4 for drilled shafts.
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