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3 CHAPTER 1 Background versions of the LRFD Bridge Design Specifications have been 1.1 Introduction published every few years. This report presents the results of NCHRP Project 24-21, The main objective of the LRFD Bridge Design Speciﬁca- “LRFD Soil-Nailing Design and Construction Speciﬁcations.” tions is to promote the use of the LRFD method and thereby The report contains the results of a review of the load and realize the perceived advantages of this method over the ASD resistance factor design (LRFD) method used for geotechnical method for the design of highway bridges and substructures. applications, including soil nail walls (SNWs) and the results Some bridge substructures components [e.g., shallow foun- of a comprehensive review of soil-nailing design and con- dations, deep foundations, and mechanically stabilized earth struction procedures used in current U.S. practice. Subse- (MSE) walls] were addressed in the ﬁrst edition of the LRFD quently, the report includes the basis for developing a database Bridge Design Speciﬁcations, and other bridge substructures of soil nail pullout resistance tests, loads, and calibration have been only progressively added to more recent editions. results of resistance factors applicable to SNWs. A comparison However, other substructure components, including SNWs, of the designs of SNWs using both the LRFD and the allow- have not been included through the latest edition (i.e., 2007) able stress design (ASD) methods for identical loads, wall of the LRFD Bridge Design Speciﬁcations. geometry, and material conditions is also presented. A sum- Introduced in the United States in the mid-1970s, the use mary of ﬁndings and suggested topics for additional research of SNWs in this country has increased in the last two decades are included. Appendices include potential sections of LRFD or so due, in part, to the advantages of SNWs over compara- speciﬁcations for the design and construction of SNWs, a ble retaining systems, including anchored walls, for certain database of soil nail pullout resistance tests, and comparative subsurface and project conditions. Some of the advantages of analyses. The potential LRFD speciﬁcations were developed SNWs over other systems include lower cost, faster installa- for consideration by the American Association of State High- tion, use of smaller equipment, and a larger structural redun- way and Transportation Ofﬁcials (AASHTO) for future edi- dancy (e.g., more soil nails are installed per unit area than tions of the LRFD Bridge Design Speciﬁcations. ground anchors). The use of SNWs as a permanent retaining structure in transportation projects became more common in the late 1980s and early 1990s thanks largely to the spon- 1.2 Problem Statement sorship of the Federal Highway Administration (FHWA). LRFD-based design methods for steel and reinforced FHWA has ﬁnanced the preparation of seminal documents concrete components of bridges and structures have been for the design and construction of SNWs that have helped used for many years in the United States (e.g., Galambos and promote this technology. In fact, nowadays, the analysis, Ravindra, 1978; AISC, 1994; and ACI, 1995). Before the design, and construction of SNWs in the United States are 1990s, bridge components, including substructure compo- commonly performed using procedures contained in docu- nents (e.g., bridge foundations), were designed using the ASD ments developed on behalf of FHWA. method, as presented in the AASHTO Standard Speciﬁcations For example, FHWA commissioned the ﬁrst comprehen- for Highway Bridges. However, this situation changed in the sive document for the design and construction of SNWs early 1990s, when AASHTO developed design speciﬁcations, (Elias and Juran, 1991). In 1993, FHWA sponsored a tour to titled AASHTO LRFD Bridge Design Speciﬁcations (AASHTO, Europe for FHWA engineers and U.S.-based professors and 1994), for highway bridges. Since the ﬁrst edition, updated consultants to gather information on SNWs in those Euro- editions [e.g., 4th edition (AASHTO, 2007)] and interim pean countries that were at that time leading the use of this
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4 • Examine existing LRFD-based guidance for the design of technology. Findings of the tour were summarized in a pub- lication (FHWA, 1993a). In 1993, FHWA also commissioned SNWs used in U.S. practice; and • Obtain the necessary information from soil nail load tests the English translation of the French national manual on soil nail technology (FHWA, 1993b), which was then one of the to develop statistically based load and resistance factors for most advanced documents in this ﬁeld. In 1994, FHWA ini- SNWs. tiated Project Demonstration 103 to disseminate the use of SNWs among state departments of transportation (DOTs). 1.4 Report Organization As part of this effort, FHWA published “Soil Nailing Field Inspectors Manual, Project Demonstration 103” (Porterﬁeld The remainder of this report is organized as follows: et al., 1994). Project Demonstration 103, whose initial contrib- • Chapter 2, Research Approach, provides a description of utors were engineering consulting ﬁrms and research institu- tions, evolved into a manual for the design and construction of the methodology followed to meet the research objectives; • Chapter 3, Findings and Applications, presents: SNWs a few years later (Byrne et al., 1998). The 1998 FHWA manual presented both ASD- and LRFD-based methodolo- – A summary of a review of the current use of the LRFD gies for the design of SNWs. More recently, FHWA published method in geotechnical design; an updated manual on the design and construction of SNWs – A summary of a review of current soil-nailing practice, in the series titled “Geotechnical Engineering Circulars” (GECs) focused on the U.S. practice; as GEC No. 7 (Lazarte et al., 2003). – An introductory discussion of load and resistance fac- The 1998 FHWA manual on SNW design (Byrne et al., 1998) tors to be used for SNW design; provided uncalibrated resistance factors for pullout resist- – A brief description of a database of soil nail load tests ance that had been developed simply by relating them to developed for this research; safety factors used in common SNW practice, as contained in – Statistics of predicted and measured loads and resist- the 16th edition of the ASD-based AASHTO Standard Speci- ances for SNW limit states; and ﬁcations (AASHTO, 1996). GEC No. 7 (Lazarte et al., 2003) – Calibration results of resistance factors for soil nail pull- addressed only the ASD method. Therefore, a fully calibrated out. • Chapter 4, Conclusions and Suggested Research, provides LRFD methodology for SNWs was lacking and hence was not included in the initial versions of the LRFD Bridge Design Spec- a summary of research ﬁndings and suggestions for future iﬁcations. To allow SNWs to be included in the LRFD Bridge research. • Lists of references, abbreviations, and symbols are provided. Design Speciﬁcations and to further promote the use of SNWs by all state DOTs, particularly among those that have not applied this technology (in part because of the absence of Additional information is presented in the following SNWs in AASHTO design speciﬁcations), AASHTO funded appendices: this research through NCHRP. • Appendix A: Proposed LRFD Design Speciﬁcations for Soil Nail Walls; 1.3 Research Objectives • Appendix B: Proposed LRFD Construction Speciﬁcations NCHRP established the following objectives for this research: for Soil Nail Walls; • Appendix C: Soil Nail Test Pullout Resistance and Load • Review existing procedures and speciﬁcations in current Database; and • Appendix D: Comparison of ASD- and LRFD-Based U.S. and international practice for the design and construc- tion of SNWs; Designs of Soil Nail Walls.