Proposed Refinements to Design Procedures for Geosynthetic Reinforced Soil (GRS) Structures in AASHTO LRFD Bridge Design Specifications (2019) / Chapter Skim
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28 Chapter 4 RESEARCH BASIS SUPPORTING THE PROPOSED REVISIONS TO INCORPORATE SV INTO DESIGN PROCEDURES The outcomes from NCHRP Project 24-1 research study that provide justification for the recommended revisions to the AASHTO Design Method presented in Chapter 3 are provided herein. Consistent with Chapter 3, the research basis supporting the proposed revisions into current AASHTO design procedures are also grouped into five design aspects.
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29 Figure 4.1. Conceptual relationship for the interaction between neighboring reinforcement layers (i.e., interlayer interaction)
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30 Figure 4.2. Displacement ratio versus reinforcement spacing at various active reinforcement displacements: (a)
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31 The reinforcement spacing in these figures at which a significant change in the interaction between contiguous reinforcements occur corresponds to the experimentally-defined boundary for composite behavior of a geosynthetic-reinforced soil mass, Sv,c. However, this value represents a lower bound, as load in the experimental testing setup was mobilized in only one of the reinforcements.
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32 geosynthetic interface (or 0.30 m (1 ft) for a soil layer bound by two geosynthetics)
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35 4.2.3 NUMERICAL SIMULATION OUTCOMES THAT SUPPORT THE PROPOSED RECOMMENDATIONS In order to complement observations from field monitoring data, which may show significant scatter, numerical simulations were used to extend the evaluation of the effect of Sv on the magnitude and distribution of Tmax. As discussed in detail in Chapter 7 of this report, Figure 4.5 shows the effect of the reinforcement spacing on Tmax with depth, under both self-weight and traffic load conditions.
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36 numerous slip surfaces, several of which yield the same Tmax although the surfaces are different. For example, some surfaces reflect a compound mode of failure while others are strictly internal but emerging either at or above the toe of the wall.
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37 4.3.1 FIELD RESEARCH OUTCOMES THAT SUPPORT THE PROPOSED RECOMMENDATIONS The current design models for evaluating T0 are significantly different As previously indicated in Chapter 2.2 and Table 2.1, current design models differ significantly on their evaluation of T0. Specifically, AASHTO requires T0 = Tmax, while the FHWA GRS-IBS design approach assumes that the lateral pressure on the facing is comparatively small and does not require a design.
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38 Figure 4.8. Bias in the lateral stress prediction models.
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39 closely-spaced reinforcement. Specifically, from 3.3 and 3.5, the connection load T0 of a given reinforcement layer i can be expressed as follows: , = ∙ ∙ ∙ + ∆ ∙ for ≤ , = 8" [Equation 4-1]
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41 Additional evidence of the appropriateness of adopting a uniform distribution with depth for the connection load, T0, is provided by the connection loads predicted at the FM load-carrying GMSE bridge abutment. As discussed in Section 4.5 of the NCHPR Project 24-41 Final Report, the reinforcement vertical spacing for this structure was 0.40 m (16 in.)
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42 4.3.2 NUMERICAL SIMULATION OUTCOMES THAT SUPPORT THE PROPOSED RECOMMENDATIONS The numerical simulations conducted as part of the NCHRP Project 24-41 study (Section 7 of this NCHRP Project 24-41 Final Report) provide additional justification to the revisions to AASHTO design approach presented in Chapter 3.3.
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43 Figure 4.11. Distribution of connection loads predicted in mini-piers for different reinforcement vertical spacing using the same reinforcement type.
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44 (a) Under self-weight of the wall (b)
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45 (a) Under self-weight of the wall (b)
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46 Figure 4.15 shows the stress distribution before and after placement of the bridge superstructure at the FM load-carrying GMSE bridge abutment. It should be noted that the dashed lines in the figure correspond to measured values while solid lines correspond to predictions obtained using the Boussinesq approach.
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47 bed was the same in both abutments, with the bearing bed proving effective in reducing the applied slab load to about half. However, this reduction was more evident in the case of Abutment A than in Abutment B
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48 Figure 4.17. Vertical stress distribution due to slab load in abutment B of VDOT GRS-IBS.
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49 (a) Induced by bridge slab weight (b)
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50 (a) Induced by bridge slab weight (b)
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51 Figure 4.20. Variation of the reinforced soil wall deformation coefficient (δR )
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52 area)
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53 predict the lateral deformations. Otherwise, estimates of vertical settlement would need to be made, which would potentially increase the uncertainty of this method.
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54 The AASHTO/FHWA model was modified to account for the global stiffness of the structure, which was the genesis of the model development . The proposed modifications include the following: (1)
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56 The comparison shown in Figure 4.2 reveals good agreement between the lateral displacements predicted using the proposed modified method and field displacement measurements. Consequently, the modifications proposed for the equations are deemed adequate for incorporation into AASHTO for prediction of lateral displacements in GMSE structures with closely-spaced reinforcements.
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57 collected during the initial 5 years after opening to traffic and as recently as 2016 (i.e. 17 years after opening to traffic)
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59 GRS-IBS, the potential for the development of differential settlements at the end of the bridge slab could be identified, as shown in Figure 8.4.26. However, a decrease in reinforcement vertical spacing was found to have a significant impact in reducing the bump at the end of the bridge slab.
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60 Figure 4.27. Combined effect of reinforcement stiffness and spacing on the bump at the end of the bridge due to traffic load.
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