New Method for Prediction of Loads in Steel Reinforced Soil Walls T. M. Allen, M.ASCE 1 ; Richard J. Bathurst 2 ; Robert D. Holtz, F.ASCE 3 ; Wei F. Lee 4 ; and D. Walters 5 Abstract: The paper describes a new working stress design methodology introduced by the writers for geosynthetic reinforced soil walls (K-Stiffness Method) that is now extended to steel reinforced soil walls. A large database of full-scale steel reinforced soil walls (a total of 20 fully instrumented wall sections) was used to develop the new design methodology. The effects of global wall stiffness, soil strength, reinforcement layer spacing, and wall height were investigated. Results of simple statistical analyses using the ratio of measured to predicted peak reinforcement loads (i.e., method bias) demonstrate the improved prediction accuracy. The AASHTO Simplified Method results in an average method bias of 1.1 with a coefficient of variation (COV) of 45%, whereas the proposed K-Stiffness Method results in an average bias of 0.95 and a COV of 32%. Soil strength was found to have limited influence on reinforcement loads for steel reinforced soil walls, especially for high shear strength soils, while global wall stiffness and wall height had a major influence on reinforcement loads. DOI: 10.1061/(ASCE)1090-0241(2004)130:11(1109) CE Database subject headings: Retaining walls; Earth reinforcement; Design; Geosynthetics; Soil structure; Stiffness. Introduction Accurate prediction of reinforcement loads and their distribution within the backfill is necessary to produce cost effective, inter- nally stable reinforced soil wall designs. The predicted reinforce- ment loads are used in design to evaluate the reinforcement strength and spacing requirements as well as the length required to resist pullout. The three methods used in North American practice for esti- mating reinforcement loads in geosynthetic or steel reinforced soil walls include the Coherent Gravity Method (AASHTO 1996), the Federal Highway Administration (FHWA) Structure Stiffness Method (Christopher et al. 1990), and the Simplified Method (Elias et al. 2001; AASHTO 2002). The development of these design methods and their accuracy with respect to predicted rein- forcement loads are summarized by Allen et al. (2001) for steel reinforced soil walls. All of these methods are semiempirical, using limit equilibrium concepts to develop the design model, but using working stress observations to adjust the models to fit the magnitude and distribution of reinforcement loads measured or estimated in full-scale structures. This approach has worked rea- sonably well for typical steel reinforced soil walls, but has worked poorly for predicting loads in geosynthetic reinforced structures (Bell et al. 1983; Rowe and Ho 1993; Allen et al. 2002). Current methods for steel reinforced structure design have been developed by focusing on the observed behavior of certain types of steel reinforcement, potentially limiting the applicability of the method to different reinforcement types or geometry. The FHWA Structure Stiffness and Simplified Method provide differ- ent empirically derived envelopes for different reinforcement types in an attempt to broaden their applicability to a wider range of reinforcement types. Data presented by Allen et al. (2001) indicate that differentiating between the reinforcement types in this manner may not capture the true causes of the differences in the measured loads in walls built with the various reinforcement types and geometry possible. A desirable feature of any new design methodology is a seam- less transition between geosynthetic and steel reinforced walls. Doing so will keep the design approach for reinforced soil walls simple, allow different reinforcement options to be evaluated within a common framework, provide confidence that the ap- proach can be applied to a wide range of geosynthetic and metal- lic soil reinforcement products, and keep the level of safety con- sistent among all reinforced soil wall types. This paper demonstrates how the K-Stiffness Method proposed by the writers for geosynthetic reinforced soil walls (Allen et al. 2003) can be extended to steel reinforced walls with granular (noncohesive) backfills. Summary of Case Histories Evaluated Key properties, parameters, and source references for the case histories in this paper are summarized in Table 1. Additional de- tails for each of these case histories, including wall and reinforce- 1 State Geotechnical Engineer, Washington State Dept. of Transporta- tion, Olympia, WA 98504-7365. 2 Professor, GeoEngineering Centre at Queen’s-RMC, Civil Engineer- ing Dept., Royal Military College of Canada, Kingston, Ontario, Canada K7K 7B4 (corresponding author), E-mail: bathurst-r@rmc.ca 3 Professor, Dept. of Civil Engineering, Univ. of Washington, More Hall: FX 10, Box 352700, Seattle, WA 98195-2700. 4 PhD, Geotechnical Associate Researcher, R&D Section, Taiwan Construction Research Institute, Hsin Tien City, Taiwan. 5 PhD Candidate, GeoEngineering Centre at Queen’s-RMC, Dept. of Civil Engineering, Queen’s Univ., Kingston, Ontario, Canada K7L 3N6. Note. Discussion open until April 1, 2005. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on January 10, 2002; approved on January 12, 2004. This paper is part of the Journal of Geotechnical and Geoenvironmental Engineering, Vol. 130, No. 11, November 1, 2004. ©ASCE, ISSN 1090- 0241/2004/11-1109–1120/$18.00. JOURNAL OF GEOTECHNICAL AND GEOENVIRONMENTAL ENGINEERING © ASCE / NOVEMBER 2004 / 1109