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

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CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH PROBLEM STATEMENT Soil is generally weak in tension and relatively strong in compression and shear. The concept of reinforcing a soil mass by incorporating a material that is strong in tensile resistance is similar to that of reinforced concrete. The rein- forcing mechanisms of reinforced soil and reinforced con- crete, however, are somewhat different. In reinforced soil, the bonding between the soil and the reinforcement is derived from soil-geosynthetic interface friction, and in some cases also from adhesion and passive resistance. Through the inter- face friction, the reinforcement restrains lateral deformation of the soil next to the reinforcement and therefore increases the stiffness and strength of the soil mass. Over the past two decades, geosynthetic-reinforced soil (GRS), a reinforced soil mass that uses layers of geosynthet- ics as reinforcement, has been employed in the construction of many earth structures, including retaining walls, embank- ments, slopes, and shallow foundations. In actual construction, GRS structures have demonstrated many distinct advantages over their conventional counterparts. GRS structures are typ- ically more ductile, more flexible (hence more tolerant to dif- ferential settlement), more adaptable to low-quality backfill, easier to construct, and more economical. They also require less overexcavation. In recent years, applications of the GRS technology to bridge-supporting structures have gained increasing attention. Depending on the facing rigidity, GRS bridge-supporting structures can be grouped into two types: “rigid” facing and “flexible” facing structures. A “rigid” facing is typically a continuous reinforced concrete panel, either precast or cast- in-place. A “flexible” facing, on the other hand, typically takes the form of wrapped geosynthetic sheets, dry-stacked con- crete modular blocks, timbers, natural rocks, or gabions. In contrast to a “flexible” facing, a “rigid” facing offers a sig- nificant degree of “global” bending resistance along the entire height of the facing panel, and thus offers greater resis- tance to “global” flexural deformation caused by lateral earth pressure exerted on the facing. Since 1994, the Japan Railway has constructed a large number of full-height concrete facing GRS bridge abut- ments and piers (Tateyama et al., 1994; Kanazawa et al., 1994; Tatsuoka et al., 1997) using a rigid wall GRS system 8 developed by Tatsuoka and his associates at the University of Tokyo. These GRS bridge-supporting structures were constructed in two stages. The first stage involves con- structing a wrapped-faced GRS wall with the aid of gabions, and the second stage involves casting in-place a full-height reinforced concrete facing over the wrapped face. Field measurement has shown that these structures experienced lit- tle deformation under service loads and have performed far better than conventional reinforced concrete retaining walls and abutments in the 1995 Japan Great Hansin earthquake that measured 7.2 on the Richter scale (Tatsuoka et al., 1997). Most recently, Tatsuoka and his associates developed a pre- load-prestress technique to improve the performance of the GRS bridge-supporting structures (Tatsuoka et al., 1997; Uchimura et al., 1998). Despite their success, the “rigid” facing GRS bridge-supporting structures have only found applications in Japan, mostly because of their cost and longer construction time compared to GRS walls with a “flexible” facing. GRS bridge-supporting structures with a flexible facing have been the subject of several studies (e.g., Gotteland et al., 1997; Adams, 1997; Ketchart and Wu, 1997; Miyata and Kawasaki, 1994; Werner and Resl, 1986; and Benigni et al., 1996), and recently have seen actual applications in the United State and abroad, including the Vienna railroad embankment in Austria (Mannsbart and Kropik, 1996), the New South Wales GRS bridge abutments in Australia (Won et al., 1996), the Black Hawk bridge abutments in Colorado, (Wu et al., 2001), and the Founders/Meadows bridge abut- ments in Colorado, (Abu-Hejleh et al., 2000). These struc- tures have shown great promise in terms of ductility, flexi- bility, constructability, and costs. Figure 1-1 shows the schematic diagram of a typical GRS bridge abutment with a segmental concrete block facing. The abutment has four major components: (1) a GRS load-bearing wall (the lower wall), (2) a back wall (the upper wall), which may or may not be a reinforced soil wall, (3) a bridge sill, i.e., a footing to support bridge loads, and (4) a segmental con- crete block wall facing. The bridge sill can be either integrated with the upper wall face (referred to as an “integrated sill”) or isolated from the upper wall as a separate footing (referred to as an “isolated sill”). The bridge sill shown in Figure 1-1 is an integrated sill.

GRS abutments with a flexible facing have some distinct advantages over the conventional reinforced concrete abut- ments. The advantages include the following: • GRS abutments are more flexible, hence more tolerant to foundation settlement and to seismic loading. • When properly designed and constructed, GRS abut- ments are remarkably stable. GRS abutments also have higher ductility (i.e., are less likely to experience a sud- den catastrophic collapse) than conventional reinforced concrete abutments. • When properly designed and constructed, GRS abut- ments can alleviate the bridge “bumps” commonly occurring at the two ends of a bridge supported by con- ventional reinforced concrete abutments, especially when they are on piles. • GRS abutments do not require embedment into the foundation soil for stability. This advantage is espe- cially important when an environmental problem, such as excavation into previously contaminated soil, is involved. • The lateral earth pressure behind a GRS abutment wall is much smaller than that in a conventional reinforced concrete abutment. • Construction of GRS abutments is rapid and requires only “ordinary” construction equipment. 9 • GRS abutments are generally much less expensive to construct than their conventional counterparts. It has generally been assumed that the design methods and construction guidelines of GRS retaining walls are readily applicable to GRS bridge abutments. The approach has raised concerns as GRS abutments are generally sub- jected to a relatively high-intensity load that is fairly close to the wall face. Basic design guidelines for Mechanically Stabilized Earth (MSE) bridge abutments have been pro- vided by the National Highway Institute (NHI) reference manual, entitled Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guide- lines, (Elias et al., 2001). The NHI manual also gives a design example for an abutment reinforced with steel strips. Design and construction guidelines for GRS abutments with a flexible facing that are based on sound engineering research are not available. RESEARCH OBJECTIVE The objective of this study was to develop rational design and construction guidelines for GRS bridge abutments with a flexible facing. The objective has been successfully achieved. modular block facing geosynthetic reinforcement lo ad -b ea rin g w al l (lo we r w all ) ba ck w al l (up pe r w all ) sill Figure 1-1. Typical GRS bridge abutment with a segmental concrete block facing.

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