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10 RESEARCH APPROACH A series of load-carrying capacity analyses of GRS abutments with a segmental concrete block facing The following tasks and the associated research approach were also conducted to determine the allowable bear- were undertaken to achieve the objective of this study: ing pressures of sills under various design conditions. · Task 3: Conduct Full-Scale Loading Experiments · Task 1: Perform Literature Study Two full-scale experiments of GRS abutments with a An extensive literature study was performed to synthe- segmental concrete block facing were performed at the size the measured performance and observed behavior Turner-Fairbank Highway Research Center in McLean, from case histories of well-instrumented GRS bridge- Virginia, under the supervision of Michael Adams. The supporting structures. Both in-service structures and full- test abutments were instrumented to monitor their per- scale experiments from around the world were included formance in response to increasing loads applied to the in the literature study. In addition, a literature study on sill. The measured results of the experiments were ana- construction guidelines and specifications of GRS walls lyzed by the finite element analysis code, DYNA3D, for used in the United States and abroad was conducted. further verification of the analytical model. The full- The findings of the study were used as the framework of scale experiments were also evaluated by the MSEW the recommended construction guidelines. program, an analysis/design computer program based · Task 2: Conduct Analytical Study on the design method presented in the NHI manual. A finite element computer code, DYNA3D, written by · Task 4: Develop Design and Construction Guidelines Hallquist and Whirley in 1989 (along with LS-DYNA, A design method for GRS abutments with a flexible a PC version of DYNA3D) was selected for this study. facing was developed in the course of this study. The The code was selected primarily because of its capabil- design method adopted the format and methodology of ity to predict different failure modes of GRS abutments the design method for MSE bridge abutments in the with a segmental concrete block facing. The analytical NHI manual. Fourteen refinements and revisions of the study includes the following: NHI design methods were proposed. The refinements and Extensive verification of the capability of DYNA3D revisions were based on measured performance of case and LS-DYNA to analyze performance and failure histories, findings of the analytical study, and the authors' conditions of GRS bridge-supporting structures with experience with GRS walls and abutments. a segmental concrete block facing was conducted. Construction guidelines for GRS abutments with dif- The structures analyzed include the spread footing ferent forms of flexible facing were also developed. The tests by Briaud and Gibbens (1994), the spread foot- construction guidelines were based primarily on the ing tests on reinforced sands by Adams and Collin guidelines provided by provided by: the American Asso- (1997), the FHWA Turner-Fairbank GRS bridge pier ciation of State Highway and Transportation Officials, in Virginia (Adams, 1997), the Garden experimental AASHTO (1998), the National Concrete Masonry Asso- embankment in France (Gotteland et al., 1997), and ciation, NCMA (1997), the Federal Highway Admin- the two full-scale GRS bridge abutment loading exper- istration, FHWA (Elias and Christopher, 1997), the iments conducted as part of this study. Colorado Transportation Institute, CTI (Wu, 1994), the A parametric study on the performance characteristics Swiss Association of Geotextile Professionals, SAGP of GRS bridge abutments as affected by (a) soil place- (1981), and the Japan Railways, JR (1998), as well as ment condition, (b) reinforcement stiffness/strength, the authors' experience with GRS walls and abutments. (c) reinforcement spacing, (d) truncation of reinforce- The recommended construction guidelines addressed site ment near wall base, (e) sill width, and (f) the clear and foundation preparation, reinforcement selection and distance between the front edge of sill and back face placement, backfill selection and placement, facing selec- of wall facing. tion and placement, drainage, and construction sequence.