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NCHRP Project 12-59(01) Seismic Design and Construction of Geosynthetic-Reinforced Soil (GRS) Bridge Abutments with Modular-Block Facing INTRODUCTION A geosynthetic-reinforced soil (GRS) mass is formed by placing closely-spaced layers of polymeric geosynthetic reinforcement in a soil mass during soil placement. The reinforcement in a GRS mass serves primarily to improve engineering properties of soil. The concept of GRS has been used successfully over the past few decades in many transportation facilities, including retaining walls, embankments, roadways, and steepened slopes. Tests and in-service installations have shown that GRS systems, particularly GRS walls with modular-block facing, are structurally sound, easy and fast to construct, and low cost compared to other designs. Interest in using GRS design for bridge abutments and approaches, in particular, has grown but a lack of rational and reliable design and construction guidelines for such structures has impeded more widespread adoption. NCHRP Report 556: Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing, was produced as a first step effort toward developing such guidelines. The research described in that report addressed static loading conditions only. NCHRP Project 12-59(01), the subject of this report, was undertaken to develop design and construction guidelines for applications in seismically active regions. The research described here focused on single-span, simply-supported bridges subjected to seismic forces. Current seismic design methods for reinforced soil retaining walls â both pseudo-static methods and displacement methods â have been developed for situations where the self-weight of the soil is the predominant load. For a GRS bridge abutment, however, the abutmentâs top surface is intended to provide a foundation of the bridge superstructure. The GRS abutment will be expected not only to maintain its stability as a soil mass but also to bear the additional large sustained and seismic loads associated with the bridge superstructure.
2 The objective of this research was to extend the earlier research reported in NCHRP Report 556 to consider seismic loading conditions and thereby provide a more comprehensive basis for developing rational guidelines for design and construction of GRS abutments and approaches with modular-block facing. This research began with a comprehensive literature review on seismic performance of reinforced-soil structures. The review, presented in Chapter 1, included reports of seismic performance of reinforced-soil abutments and relevant design methods and construction guidelines and specifications. The review informed development of proposed allowable stress design (ASD) and LRFD design methods for GRS bridge abutments subject to seismic loading. These step-by-step methods are described in Chapters 2 and 3, respectively. The methods are based on current AASHTO bridge- design specifications and published guidelines for mechanically stabilized earth as well as results presented in NCHRP Report 556. A large shake-table test was conducted to measure a model abutmentâs response to dynamic loading, and these measurements in turn were used to validate and refine the proposed design and construction guidelines. Chapters 4 and 5 present the design of the bridge isolation system for the shake table test and the construction and testing of the model abutment. The full-scale test was performed at the Engineering Research and Development Center of U.S. Armyâs Construction Engineering Research Laboratory (ERDC-CERL) in Urbana, Illinois, using their Triaxial Earthquake and Shock Simulator (TESS). Results of the shake table tests, described in Chapter 6, agreed well with predictions and exhibited little to no damage to the abutment until lateral accelerations reached 0.67g, at which point several of the concrete masonry unit (CMU) blocks began to exhibit some cracking, primarily at the bottom corners of the model abutment. Negligible horizontal and small vertical movements of the model sill were recorded. The model was deemed fully functional after the test had progressed to loading at 1.0 g. The testing demonstrated that design to ensure
3 appropriate vibratory isolation of the bridge superstructure from the foundation abutment is important to ensure good structural performance. Observations from the shake-table testing and published data on seismic behavior of GRS walls were used to validate a finite element representation of GRS abutments with flexible facing. Chapter 7 describes the extensive parametric studies that were made, using recorded actual acceleration histories from the Kobe and Northridge earthquakes, to characterize the influence of (a) soil placement condition, (b) bridge height, (c) bridge span, (d) geosynthetic reinforcement stiffness, and (e) geosynthetic reinforcement spacing as design variables. These studies considered maximum and permanent lateral deformations of abutment wall, maximum and permanent lateral deformations of the sill, maximum and permanent lateral deformations of bridge, and maximum acceleration of abutment wall and the bridge as parameters to be controlled. The parametric studies indicate that GRS abutments would have sustained small settlements (less than 5 cm) while sustaining significant permanent lateral displacements (up to 20 cm) under extreme earthquake loads. However, the parametric analyses showed that when one of the two abutments deformed forwardâin the longitudinal direction of the bridgeâthe other abutment on the opposite side of the bridge deformed backward, i.e., the two abutments along with the bridge superstructure would deform in a near simple shear manner unlikely to create significant additional stresses in the bridge during an earthquake. Different earthquake spectra might cause different results. In the parametric analysis, a 7.5-cm wide expansion joint was assumed to be present at each end of the bridge beam. These expansion joints were designed to accommodate thermal expansion of the single span bridge and to allow the bridge to oscillate horizontally via elastomeric bearing pads. The 7.5 cm expansion gaps allow for deformation of the elastomeric pads laterally up to 7.5 cm in any horizontal direction under extreme loads without loss of functionality. The parametric studies showed that expansion gaps always remained open during seismic loading (i.e., gap width > 0), indicating that the bridge was never in contact with the abutment backwall.
4 Chapter 8 presents construction guidelines for GRS abutments subject to seismic loading. These guidelines for earthwork construction control are essentially the same as those proposed in NCHRP Report 556 for static loading situations. The construction guidelines focus on GRS abutments with segmental concrete block facing and include only basic guidance for abutments with other forms of flexible facing. Chapter 9 summarizes principal results of the research. These results may have significant value for practitioners considering the use of GRS bridge abutments with modular block facing and for researchers seeking to explore further the likely behavior of such abutments subjected to seismic loads. Evidence from this study and the preceding research described in NCHRP Report 556 indicates that GRS abutments designed according to ASD methods, modified to include expansion joints and elastomeric bearings and well constructed, can withstand large ground accelerations and maintain bridge support. However, the testing conducted in this study provides only a limited basis for drawing general conclusions. Additional testing and analyses are needed to confirm the reliability of guidelines developed in this research and extend their range of applicability.
