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Suggested Citation:"Chapter 1 - Background." National Academies of Sciences, Engineering, and Medicine. 2011. Proposed Specifications for LRFD Soil-Nailing Design and Construction. Washington, DC: The National Academies Press. doi: 10.17226/13327.
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Suggested Citation:"Chapter 1 - Background." National Academies of Sciences, Engineering, and Medicine. 2011. Proposed Specifications for LRFD Soil-Nailing Design and Construction. Washington, DC: The National Academies Press. doi: 10.17226/13327.
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31.1 Introduction This report presents the results of NCHRP Project 24-21, “LRFD Soil-Nailing Design and Construction Specifications.” The report contains the results of a review of the load and resistance factor design (LRFD) method used for geotechnical applications, including soil nail walls (SNWs) and the results of a comprehensive review of soil-nailing design and con- struction procedures used in current U.S. practice. Subse- quently, the report includes the basis for developing a database of soil nail pullout resistance tests, loads, and calibration results of resistance factors applicable to SNWs. A comparison of the designs of SNWs using both the LRFD and the allow- able stress design (ASD) methods for identical loads, wall geometry, and material conditions is also presented. A sum- mary of findings and suggested topics for additional research are included. Appendices include potential sections of LRFD specifications for the design and construction of SNWs, a database of soil nail pullout resistance tests, and comparative analyses. The potential LRFD specifications were developed for consideration by the American Association of State High- way and Transportation Officials (AASHTO) for future edi- tions of the LRFD Bridge Design Specifications. 1.2 Problem Statement LRFD-based design methods for steel and reinforced concrete components of bridges and structures have been used for many years in the United States (e.g., Galambos and Ravindra, 1978; AISC, 1994; and ACI, 1995). Before the 1990s, bridge components, including substructure compo- nents (e.g., bridge foundations), were designed using the ASD method, as presented in the AASHTO Standard Specifications for Highway Bridges. However, this situation changed in the early 1990s, when AASHTO developed design specifications, titled AASHTO LRFD Bridge Design Specifications (AASHTO, 1994), for highway bridges. Since the first edition, updated editions [e.g., 4th edition (AASHTO, 2007)] and interim versions of the LRFD Bridge Design Specifications have been published every few years. The main objective of the LRFD Bridge Design Specifica- tions is to promote the use of the LRFD method and thereby realize the perceived advantages of this method over the ASD method for the design of highway bridges and substructures. Some bridge substructures components [e.g., shallow foun- dations, deep foundations, and mechanically stabilized earth (MSE) walls] were addressed in the first edition of the LRFD Bridge Design Specifications, and other bridge substructures have been only progressively added to more recent editions. However, other substructure components, including SNWs, have not been included through the latest edition (i.e., 2007) of the LRFD Bridge Design Specifications. Introduced in the United States in the mid-1970s, the use of SNWs in this country has increased in the last two decades or so due, in part, to the advantages of SNWs over compara- ble retaining systems, including anchored walls, for certain subsurface and project conditions. Some of the advantages of SNWs over other systems include lower cost, faster installa- tion, use of smaller equipment, and a larger structural redun- dancy (e.g., more soil nails are installed per unit area than 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- sorship of the Federal Highway Administration (FHWA). FHWA has financed the preparation of seminal documents for the design and construction of SNWs that have helped promote this technology. In fact, nowadays, the analysis, design, and construction of SNWs in the United States are commonly performed using procedures contained in docu- ments developed on behalf of FHWA. For example, FHWA commissioned the first comprehen- sive document for the design and construction of SNWs (Elias and Juran, 1991). In 1993, FHWA sponsored a tour to Europe for FHWA engineers and U.S.-based professors and consultants to gather information on SNWs in those Euro- pean countries that were at that time leading the use of this C H A P T E R 1 Background

technology. Findings of the tour were summarized in a pub- lication (FHWA, 1993a). In 1993, FHWA also commissioned the English translation of the French national manual on soil nail technology (FHWA, 1993b), which was then one of the most advanced documents in this field. In 1994, FHWA ini- tiated Project Demonstration 103 to disseminate the use of SNWs among state departments of transportation (DOTs). As part of this effort, FHWA published “Soil Nailing Field Inspectors Manual, Project Demonstration 103” (Porterfield et al., 1994). Project Demonstration 103, whose initial contrib- utors were engineering consulting firms and research institu- tions, evolved into a manual for the design and construction of SNWs a few years later (Byrne et al., 1998). The 1998 FHWA manual presented both ASD- and LRFD-based methodolo- gies for the design of SNWs. More recently, FHWA published an updated manual on the design and construction of SNWs in the series titled “Geotechnical Engineering Circulars” (GECs) as GEC No. 7 (Lazarte et al., 2003). The 1998 FHWA manual on SNW design (Byrne et al., 1998) provided uncalibrated resistance factors for pullout resist- ance that had been developed simply by relating them to safety factors used in common SNW practice, as contained in the 16th edition of the ASD-based AASHTO Standard Speci- fications (AASHTO, 1996). GEC No. 7 (Lazarte et al., 2003) addressed only the ASD method. Therefore, a fully calibrated LRFD methodology for SNWs was lacking and hence was not included in the initial versions of the LRFD Bridge Design Spec- ifications. To allow SNWs to be included in the LRFD Bridge 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 SNWs in AASHTO design specifications), AASHTO funded this research through NCHRP. 1.3 Research Objectives NCHRP established the following objectives for this research: • Review existing procedures and specifications in current U.S. and international practice for the design and construc- tion of SNWs; • Examine existing LRFD-based guidance for the design of SNWs used in U.S. practice; and • Obtain the necessary information from soil nail load tests to develop statistically based load and resistance factors for SNWs. 1.4 Report Organization The remainder of this report is organized as follows: • Chapter 2, Research Approach, provides a description of the methodology followed to meet the research objectives; • Chapter 3, Findings and Applications, presents: – A summary of a review of the current use of the LRFD method in geotechnical design; – A summary of a review of current soil-nailing practice, focused on the U.S. practice; – An introductory discussion of load and resistance fac- tors to be used for SNW design; – A brief description of a database of soil nail load tests developed for this research; – Statistics of predicted and measured loads and resist- ances for SNW limit states; and – Calibration results of resistance factors for soil nail pull- out. • Chapter 4, Conclusions and Suggested Research, provides a summary of research findings and suggestions for future research. • Lists of references, abbreviations, and symbols are provided. Additional information is presented in the following appendices: • Appendix A: Proposed LRFD Design Specifications for Soil Nail Walls; • Appendix B: Proposed LRFD Construction Specifications for Soil Nail Walls; • Appendix C: Soil Nail Test Pullout Resistance and Load Database; and • Appendix D: Comparison of ASD- and LRFD-Based Designs of Soil Nail Walls. 4

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Proposed Specifications for LRFD Soil-Nailing Design and Construction Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Report 701: Proposed Specifications for LRFD Soil-Nailing Design and Construction contains proposed specifications for the design and construction of soil-nailed retaining structures.

The American Association of State Highway and Transportation Officials (AASHTO) Standard Bridge Specifications, the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications, and the AASHTO LRFD Bridge Construction Specifications do not include guidance for soil-nailed structures.

In the absence of AASHTO LRFD specifications, some state departments of transportation will not use soil-nailed retaining structures. Given the potential advantages of soil-nailed structures, there was a need to develop proposed standard design and construction specifications for soil-nailed structures for incorporation into the AASHTO LRFD Bridge Design and Construction Specifications.

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