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129 CHAPTER 4 CONCLUSIONS AND SUGGESTED RESEARCH CONCLUSIONS vertical is recommended for a segmental abutment wall facing. A typical setback of 5 to 6 mm between succes- A design method and construction guidelines for GRS sive courses of facing blocks is recommended for 200 abutments with a flexible facing have been developed in the mm (8 in.) height blocks. course of this study. The design method adopted the format · The reinforcement length may be "truncated" in the bot- and methodology of the NHI manual for the design of MSE tom portion of the wall provided that the foundation is bridge abutments. Fourteen specific refinements and revi- "competent." The recommended configuration of the sions of the NHI design method are presented, and the basis truncation is: reinforcement length = 0.35 H at the foun- for each refinement and revision is provided. The refine- dation level (H = total height of the abutment wall) and ments and revisions are based on findings from previous case increases upward at a 45 deg angle. The allowable bear- histories, full-scale loading experiments, and finite element ing pressure of the sill, as determined in the three-step analysis of GRS abutments, as well as the authors' experi- procedure, should be reduced by 10 percent for trun- ences and knowledge on GRS structures in general and GRS cated-base walls. Permitting truncated reinforcement abutments in particular. typically will translate into significant savings when The construction guidelines were established based on the excavation is involved in the construction of the load- guidelines for segmental GRS walls as provided by various bearing wall of a bridge abutment. agencies (including AASHTO, NCMA, FHWA, CTI, SAGP, · A recommended "sill clear distance" between the back and JR) as well as the authors' observations and experiences face of the facing and the front edge of the sill is 0.3 m with the construction of GRS walls and abutments. The (12 in.). The recommended clear distance is a result of construction guidelines focus on GRS abutments with a finite element analysis with the consideration that the segmental concrete block facing. As the literature on con- soil immediately behind the facing is usually of a lower struction of GRS abutments with other forms of flexible fac- compacted density because a heavy compactor is not ing is rather limited, only the basic construction guidelines permitted close to the wall face. for three types of flexible facing--geotextile-wrapped, tim- · For most bridge abutments, a relatively high-intensity ber, and natural rock--are presented. load is applied close to the wall face. To ensure that the The major refinements and revisions to the NHI design foundation soil beneath the abutment will have a suffi- method are as follows: cient safety margin against bearing failure, a revision is made to check the contact pressure over a more critical · The allowable bearing pressure of a bridge sill on the region within the "influence length" D1 (as defined in load-bearing wall (the lower wall) of a GRS abutment Chapter 3) behind the wall face or the reinforcement is determined as a function of the friction angle of the length in the lower wall, whichever is smaller. In the fill, reinforcement vertical spacing, sill width, and sill current NHI manual, the contact pressure is the average type (isolated sill or integrated sill). A simple three- pressure over the entire reinforced zone (with eccen- step procedure is provided for determination of the tricity correction). allowable bearing pressures under various design con- · If the bearing capacity of the foundation soil support- ditions. ing the bridge abutment is found only marginally · The default value for reinforcement vertical spacing is acceptable or somewhat unacceptable, it is recom- set at 0.2 m. To ensure satisfactory performance and an mended that a reinforced soil foundation (RSF) be adequate margin of stability, reinforcement spacing used to increase bearing capacity and reduce potential greater than 0.4 m is not recommended for GRS abut- settlement. A typical RSF is formed by excavating a ments under any conditions. pit 0.5 * L deep (L = reinforcement length in the load- · To provide improved appearance and greater flexibility bearing wall) and replacing it with compacted road in construction, a front batter of 1/35 to 1/40 from the base material reinforced by the same reinforcement to