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52.1 Introduction To achieve the objectives established for this project, the fol- lowing research approach and research tasks were established: ⢠Review existing procedures for the design and construction of SNWs according to the ASD and LRFD methodologies; ⢠Compile load-test data from several sources; ⢠Develop resistance factors through calibration of load-test data using appropriate reliability-based methods; ⢠Compare designs of SNWs prepared with LRFD and ASD methods; and ⢠Prepare LRFD design and construction specifications for SNWs to be considered by AASHTO for future editions of the AASHTO LRFD Bridge Design Specifications. Each of these tasks is described in further detail in the fol- lowing paragraphs. 2.2 Review of Design and Construction Procedures for Soil Nailing First a review of the LRFD method as applied to geotechni- cal applications and retaining structures was made. As part of this review, an evaluation was performed of LRFD methodolo- gies developed for other bridge substructure components that shared some common aspects with SNWs. Subsequently, a review was conducted of existing procedures for the design and construction of SNWs focused on U.S. practice. Relevant state- of-the-art publications related to the design of SNWs, includ- ing Byrne et al. (1998), Lazarte et al. (2003), and other recent national and international references (e.g., Clouterre, 2000) addressing SNW design were reviewed. 2.3 Compilation of Soil Nail Load-Test Data A database of soil nail load-test results was compiled to provide data for the calibration of resistance factors for soil nail pullout. Sources of information included load-test results from (i) files owned by the research team, (ii) members of ADSC: The International Association of Foundation Drilling, (iii) other SNW contractors, (iv) DOTs, (v) research institu- tions, and (vi) published journals and reports. Chapter 3 pro- vides a description of the data and contains a discussion of data adequacy for calibration purposes. 2.4 Development of Resistance Factors through Calibration of Load-Test Data Resistance factors for the design of SNWs were developed and calibrated applying reliability methods and using the val- ues contained in the soil nail test database. The calibration was conducted using the procedures presented in the publi- cation âDevelopment of Geotechnical Resistance Factors and Downdrag Load Factors for LRFD Foundation Strength Limit State Designâ (Allen, 2005). Chapter 3 provides the results of the calibration. 2.5 Comparisons of Designs Based on the LRFD and ASD Methods Calibrated resistance factors were used in LRFD-based designs of various SNWs. These designs were compared with designs obtained using the ASD method for the same SNWs and load conditions. Differences of key design parameters in SNWs design were assessed and potential advantages of the LRFD-based methodology were quantified. Proposed changes to be considered in future editions of the LRFD Bridge Design Specifications were identified. 2.6 Proposed LRFD Design and Construction Specifications LRFD-based specifications for the design and construction of SNWs were developed as part of this research. Appendices A and B, respectively, contain the proposed design and construc- tion specifications, which are formatted according to the latest LRFD Bridge Design Specifications (AASHTO, 2007). C H A P T E R 2 Research Approach
