Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing (2006)

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

130 be used in the load-bearing wall at 0.3-m vertical spacing. • Both a minimum ultimate tensile strength and a mini- mum tensile stiffness of the reinforcement should be specified to ensure sufficient tensile resistance at the service loads, to provide adequate ductility, and to ensure a sufficient safety margin against rupture fail- ure. A recommended procedure for determining the required minimum tensile stiffness (at 1.0 percent strain) and the minimum ultimate tensile strength are stipulated. • It is recommended to extend the reinforcement lengths in both the upper and lower walls, at least the top three layers of each wall, to about 1.5 m beyond the end of the approach slab to promote integration of the abutment walls with the approach embankment and the load-bear- ing abutment, so as to eliminate the bridge “bumps”—a chronic problem in many bridges. • Connection strength is not a design concern as long as the reinforcement spacing is kept not more than 0.2 m, the selected fill is compacted to meet the specification stipulated in the recommended construction guidelines, and the applied pressure does not exceed the recom- mended design pressures in the recommended design method. SUGGESTED RESEARCH It is suggested that a series of full-scale experiments of GRS bridge abutments be conducted to confirm the overall validity of the recommended design and construction guidelines. The most important features to be confirmed by the experiments follow: • Allowable design pressure. The allowable design sill pressure as affected by sill width, sill clear distance, soil friction angle, reinforcement spacing, truncated base, and sill type (especially an integrated sill) should be confirmed. Some attention should also be given to the effect of construction sequence (i.e., the potential bene- fits of constructing the upper wall before placing the bridge superstructure). • Long-term performance of GRS bridge abutments in the in-service condition. It is suggested that the design method and construction guidelines be implemented in bridges on secondary roads with simple (yet reliable) instruments to monitor their long-term performance. • Offset between the facing blocks and the centerline of bridge bearings. The analysis conducted in this study has indicated that an offset less than 0.9 m (3 ft) (as sug- gested by the NHI manual) would not cause any adverse effects. The effect of an offset of 0.8 m (e.g., 0.2 m of block depth + 0.3 m of clear distance + 0.3 m of half- sill-width) may be selected for testing. • Preloading and/or pre-stressing of GRS abutments. Simple measures of preloading and/or prestressing has been shown effective at increasing significantly the load-carrying capacity of a GRS abutment and reducing the sill settlement from two- to fivefold (depending on the fill placement condition) and the lateral movement by about threefold. A cost-effective procedure and proper specifications for preloading and/or prestressing have not been established.

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