《高等土力学》课程教学资源（书籍文献）Geotechnical engineering circular No.4（Ground anchors and anchored systems）.pdf_P21-P25

CHAPTER 1INTRODUCTION1.1PURPOSEThe purpose of this document is to provide state-of-the-practice information on ground anchors andanchored systems for highway applications.Ground anchors discussed in this document are cementgrouted, prestressed tendons that are installed in soil or rock.Anchored systems discussed includeflexible anchored walls, slopes supported using ground anchors, slope and landslide stabilizationsystems, and structures that incorporate tiedown anchors.The intended audience includesgeotechnical, structural, and highway design and construction specialists involved with the design,construction,contracting,and inspectionofthesesystems.Ground anchors and anchored systems have become increasingly more cost-effective throughimprovements in design methods, construction techniques, anchor component materials, and on-siteacceptance testing.This has resulted in an increase in the use of both temporary and permanentanchors.The reader should recognize that, as a result of theevolving nature of anchoring practicethe information presented herein is not intended to be prescriptive. Design, construction, and loadtesting methods are described that are currently used in U.S.practice.1.2ANCHOREDSYSTEMSERVICELIFEThe focus of this document is on design methods and procedures for permanent ground anchors andanchored systems.Permanent anchored systems are generally considered to have a service life of 75to1ooyears.However,anchored systems arealsocommonlyused fortemporaryapplications.Theservice life of temporary earth support systems is based on the time required to support the groundwhile the permanent systems are installed. This document has adopted the American Association ofState Highway and Transportation Officials (AASHTO) guidance which considers temporarysystemsto bethosethatareremoved or becomeinoperativeupon completion of thepermanentsystems.The time period for temporary systems is commonly stated to be18 to 36months but maybeshorterorlongerbased on actual projectconditions.Furthermore this document has subdivided temporary systems into“support of excavation"(SOE)temporary systems andcriticaltemporary systems.In general the owner will determine whichtemporary systems are to be designated as critical. Often that decision is based on the owner's needto restrict lateral movement of the support system to minimize ground movements behind the supportsystem. In this document, it is recommended that critical temporary systems be designed to the samecriteriausedforpermanentanchoredsystems.ConverselySOEanchored systemsarecommonlydesigned to lessrestrictive criteria than permanent anchored systems.The owner commonlyassignstheresponsibilityfordesign and performance of sOE anchored systemsto the contractor.Thedesign of these SOE anchored systems is often based more on system stability than on minimizingground movements.1

1 CHAPTER 1 INTRODUCTION 1.1 PURPOSE The purpose of this document is to provide state-of-the-practice information on ground anchors and anchored systems for highway applications. Ground anchors discussed in this document are cement grouted, prestressed tendons that are installed in soil or rock. Anchored systems discussed include flexible anchored walls, slopes supported using ground anchors, slope and landslide stabilization systems, and structures that incorporate tiedown anchors. The intended audience includes geotechnical, structural, and highway design and construction specialists involved with the design, construction, contracting, and inspection of these systems. Ground anchors and anchored systems have become increasingly more cost-effective through improvements in design methods, construction techniques, anchor component materials, and on-site acceptance testing. This has resulted in an increase in the use of both temporary and permanent anchors. The reader should recognize that, as a result of the evolving nature of anchoring practice, the information presented herein is not intended to be prescriptive. Design, construction, and load testing methods are described that are currently used in U.S. practice. 1.2 ANCHORED SYSTEM SERVICE LIFE The focus of this document is on design methods and procedures for permanent ground anchors and anchored systems. Permanent anchored systems are generally considered to have a service life of 75 to 100 years. However, anchored systems are also commonly used for temporary applications. The service life of temporary earth support systems is based on the time required to support the ground while the permanent systems are installed. This document has adopted the American Association of State Highway and Transportation Officials (AASHTO) guidance which considers temporary systems to be those that are removed or become inoperative upon completion of the permanent systems. The time period for temporary systems is commonly stated to be 18 to 36 months but may be shorter or longer based on actual project conditions. Furthermore this document has subdivided temporary systems into “support of excavation” (SOE) temporary systems and “critical” temporary systems. In general the owner will determine which temporary systems are to be designated as critical. Often that decision is based on the owner’s need to restrict lateral movement of the support system to minimize ground movements behind the support system. In this document, it is recommended that critical temporary systems be designed to the same criteria used for permanent anchored systems. Conversely, SOE anchored systems are commonly designed to less restrictive criteria than permanent anchored systems. The owner commonly assigns the responsibility for design and performance of SOE anchored systems to the contractor. The design of these SOE anchored systems is often based more on system stability than on minimizing ground movements

In this document, the basic design recommendations pertain to both permanent anchored systems andcritical temporary systems. In this document, the term “permanent anchored systems" or “permanentapplications"include critical temporary systems.Whenever appropriate in this document, discussionis provided concerning the differences in design requirements for SOE systems and permanentsystems.The following components of an anchored system design are generally less restrictivefortemporary SOE systems as compared to permanent systems:(1) selection of timber lagging,(2)allowable stresses in structural components; (3) factors of safety: (4) design for axial load, (5) surcharge loads used to evaluate wall loadings; (6) seismic design criteria, and (7) anchor loadtesting.1.3BACKGROUNDThe first use of ground anchors in the U.S. was for temporary support of excavation systems. Thesesystems were typically designed and constructed by specialty contractors.The use of permanentground anchors for public sector projects in the U.S. did not become common until the late 1970sandtoday,representa commontechnique forearthretention and slopestabilizationfor highwayapplications.In certain design and construction conditions,anchored systems offer severaladvantages over more conventional systems that have resulted in economic and technical benefitsFor example, benefits of anchored walls over concrete gravity retaining walls for support of ahighway cut include:unobstructed workspace for excavations;ability to withstand relatively large horizontal wall pressures without requiring a significantincrease in wall cross section;elimination of the need to provide temporary excavation support since an anchored wall canbe incorporated into the permanent structureeliminationofneedforselectbackfill:elimination of need fordeepfoundation support:reduced constructiontime,andreducedright-of-way(ROW)acquisitionIn 1979, the U.S. Department of Transportation Federal Highway Administration (FHWA) Office ofTechnology Applications authorized a permanent ground anchor demonstration project.Theobjective of the project was to provide highway agencies with adequate information to promoteroutine use of permanent ground anchors and anchored walls.The purpose of the demonstrationproject was to: (1) study existing ground anchor technology and installation procedures;(2)determine areas where additional work was required; (3)update existing technology,(4)develop abasic design manual; and (5) solicit installations on highway projects. Between 1979 and 1982, twoFHWA research reports were completed (Permanent Ground Anchors"FHWA Report NosFHWA-RD- 81-150, 151, and 152 and "Tiebacks"FHWA Report No.FHWA-RD-82-047) and pilottest projects were begun by highway agencies.A design manual was developed by FHWA in 1984,2

2 In this document, the basic design recommendations pertain to both permanent anchored systems and critical temporary systems. In this document, the term “permanent anchored systems” or “permanent applications” include critical temporary systems. Whenever appropriate in this document, discussion is provided concerning the differences in design requirements for SOE systems and permanent systems. The following components of an anchored system design are generally less restrictive for temporary SOE systems as compared to permanent systems: (1) selection of timber lagging; (2) allowable stresses in structural components; (3) factors of safety; (4) design for axial load; (5) surcharge loads used to evaluate wall loadings; (6) seismic design criteria; and (7) anchor load testing. 1.3 BACKGROUND The first use of ground anchors in the U.S. was for temporary support of excavation systems. These systems were typically designed and constructed by specialty contractors. The use of permanent ground anchors for public sector projects in the U.S. did not become common until the late 1970s and today, represent a common technique for earth retention and slope stabilization for highway applications. In certain design and construction conditions, anchored systems offer several advantages over more conventional systems that have resulted in economic and technical benefits. For example, benefits of anchored walls over concrete gravity retaining walls for support of a highway cut include: • unobstructed workspace for excavations; • ability to withstand relatively large horizontal wall pressures without requiring a significant increase in wall cross section; • elimination of the need to provide temporary excavation support since an anchored wall can be incorporated into the permanent structure; • elimination of need for select backfill; • elimination of need for deep foundation support; • reduced construction time; and • reduced right-of-way (ROW) acquisition. In 1979, the U.S. Department of Transportation Federal Highway Administration (FHWA) Office of Technology Applications authorized a permanent ground anchor demonstration project. The objective of the project was to provide highway agencies with adequate information to promote routine use of permanent ground anchors and anchored walls. The purpose of the demonstration project was to: (1) study existing ground anchor technology and installation procedures; (2) determine areas where additional work was required; (3) update existing technology; (4) develop a basic design manual; and (5) solicit installations on highway projects. Between 1979 and 1982, two FHWA research reports were completed (“Permanent Ground Anchors” FHWA Report Nos. FHWA-RD- 81-150, 151, and 152 and “Tiebacks” FHWA Report No. FHWA-RD-82-047) and pilot test projects were begun by highway agencies. A design manual was developed by FHWA in 1984

which was updated in 1988 (FHWA-DP-68-1R, 1988), as part of the demonstration project. Duringthe demonstration project, five U.S. highway projects with permanent anchored systems wereinstrumented and performance data were gathered (see FHWA-DP-90-068-003, 1990).Today,ground anchorsand anchored systemshavebecomean integral componentofhighwaydesign intheU.S.This document has been written, in part, to update the FHWA (1988) design manual titled"Permanent Ground Anchors".That document provides an introduction to basic ground anchorconcepts and provides the practicing highway engineer with sufficient information to contractforpermanent ground anchors and anchored systems.This document draws extensively from FHWA(1988)in describing issues such as subsurface investigation and laboratory testing, basic anchoringprinciples, ground anchor load testing, and inspection of construction materials and methods used foranchored systems.Since1988, advances have been made in designmethods resulting from anchoredsystem performancedata and from new construction materials, methods,and equipment.Results of research activities conducted since 1989 are also included in this document.Mostrecently,researchwasconductedunderaFHWAresearchcontractonthedesignandperformanceofground anchors and anchored soldierbeam and timberlagging walls.As part of that researchproject, performance data on two full-scale anchored walls and four large-scale model anchoredwalls were collected and analyzed. The settlement, axial load, and downdrag force on soldier beams,and lateral wall movements of the wall systems were evaluated (see FHWA-RD-98-066, 1998 andFHWA-RD-98-067, 1998).Several of the analysis methods and design procedures that wererecommended based on the results of the research(seeFHWA-RD-97-130, 1998)are adopted herein.Procedures used for ground anchor acceptance testing have also been improved since FHWA (1988)was published.The AASHTO Task Force 27report"In-Situ Soil Improvement Techniques"(1990)included both a generic construction specification for permanent ground anchors and a groundanchor inspection manual.Those documents form the basisfortheconstruction standards developedby many highway agencies. The Post-Tensioning Institute (PTI) document titled “Recommendationsfor Prestressed Rock and Soil Anchors" (PTI, 1996) is a document commonly referenced that wasdeveloped collectively by owners, design consultants, specialty contractors, and material suppliersAASHTO Task Force 27 (1990) and PTI (1996) were used as the basis for the chapters of thisdocumentongroundanchoracceptancetestingandgroundanchorcorrosionprotection.Informationfromthosedocuments was alsoused to develop thegeneric ground anchor specification provided inappendixE.3

3 which was updated in 1988 (FHWA-DP-68-1R, 1988), as part of the demonstration project. During the demonstration project, five U.S. highway projects with permanent anchored systems were instrumented and performance data were gathered (see FHWA-DP-90-068-003, 1990). Today, ground anchors and anchored systems have become an integral component of highway design in the U.S. This document has been written, in part, to update the FHWA (1988) design manual titled "Permanent Ground Anchors". That document provides an introduction to basic ground anchor concepts and provides the practicing highway engineer with sufficient information to contract for permanent ground anchors and anchored systems. This document draws extensively from FHWA (1988) in describing issues such as subsurface investigation and laboratory testing, basic anchoring principles, ground anchor load testing, and inspection of construction materials and methods used for anchored systems. Since 1988, advances have been made in design methods resulting from anchored system performance data and from new construction materials, methods, and equipment. Results of research activities conducted since 1989 are also included in this document. Most recently, research was conducted under a FHWA research contract on the design and performance of ground anchors and anchored soldier beam and timber lagging walls. As part of that research project, performance data on two full-scale anchored walls and four large-scale model anchored walls were collected and analyzed. The settlement, axial load, and downdrag force on soldier beams, and lateral wall movements of the wall systems were evaluated (see FHWA-RD-98-066, 1998 and FHWA-RD-98-067, 1998). Several of the analysis methods and design procedures that were recommended based on the results of the research (see FHWA-RD-97-130, 1998) are adopted herein. Procedures used for ground anchor acceptance testing have also been improved since FHWA (1988) was published. The AASHTO Task Force 27 report "In-Situ Soil Improvement Techniques” (1990) included both a generic construction specification for permanent ground anchors and a ground anchor inspection manual. Those documents form the basis for the construction standards developed by many highway agencies. The Post-Tensioning Institute (PTI) document titled “Recommendations for Prestressed Rock and Soil Anchors" (PTI, 1996) is a document commonly referenced that was developed collectively by owners, design consultants, specialty contractors, and material suppliers. AASHTO Task Force 27 (1990) and PTI (1996) were used as the basis for the chapters of this document on ground anchor acceptance testing and ground anchor corrosion protection. Information from those documents was also used to develop the generic ground anchor specification provided in appendix E