National Academies Press: OpenBook

Rock-Socketed Shafts for Highway Structure Foundations (2006)

Chapter: Chapter One - Introduction

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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
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Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 3
Page 4
Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 4
Page 5
Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 5
Page 6
Suggested Citation:"Chapter One - Introduction." National Academies of Sciences, Engineering, and Medicine. 2006. Rock-Socketed Shafts for Highway Structure Foundations. Washington, DC: The National Academies Press. doi: 10.17226/13975.
×
Page 6

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3BACKGROUND Highway bridges represent a large investment in the U.S. transportation infrastructure, and structural foundations account for a significant percentage of total bridge costs. Current foundation engineering practice in the transporta- tion industry represents a dramatic advancement compared with 25 years ago. Development of this topic is illustrated by considering that NCHRP Synthesis 42: Design of Pile Foundations (Vesic 1977) does not mention rock-socketed drilled shafts. At the time of its publication, NCHRP Syn- thesis 42 was the most comprehensive study extant on the use of deep foundations for transportation structures. According to DiMaggio (2004), in 1980, driven piles accounted for more than 95% of transportation market share, based purely on repeating previous practice. Today, the practice is oriented toward matching the foundation type to project conditions. This has led to a wider variety of deep foundation types selected on the basis of subsur- face conditions, structural behavior, constructability, envi- ronmental constraints, and cost. A foundation type that has steadily increased in use over this time is the drilled shaft, a deep foundation constructed by placing fluid concrete in a drilled hole. A potentially effective way to use a drilled shaft is by bearing on, or extending into, rock. To achieve the perfor- mance and economy potentials of rock-socketed shafts, de- signers must be aware of the many issues that affect both cost and performance. Drilling and excavation in rock is generally more expensive and time consuming than in soil. Construction of a rock socket poses challenges and difficul- ties that are unique and may require specialized techniques, equipment, and experience. The first issue confronted by a foundation designer is to determine whether a rock-socketed foundation is necessary for bridge support. Factors to con- sider include the nature and magnitude of structural loads and factors related to rock mass characteristics, including depth to rock, rock type, rock mass engineering properties, and constructability. The additional costs and effort of con- struction in rock must be offset by its benefits. The princi- pal benefits normally are higher load-carrying capacity and the ability to limit deformations, compared with foundations not founded in rock. To make the appropriate cost compar- isons, rock-socket design must be based on rational models of behavior that reliably predict the capacity and load- deformation behavior. PROBLEM DEFINITION The engineering problem addressed by this synthesis is shown in Figure 1. A drilled shaft foundation is to be de- signed and constructed for support of a bridge structure. Sub- surface conditions may consist of soil underlain by rock. Upper portions of the rock may be partially to highly weath- ered, giving these materials engineering properties that are transitional between soil and rock, sometimes referred to as intermediate geomaterials, or IGM. Loads to be considered for design typically are determined by AASHTO Bridge Design Specifications, with proper consideration of load combinations and load factors. For foundation analysis, de- sign loads may be resolved into vertical (P), horizontal (H), and moment (M) components at the head of the shaft. A sub- surface investigation is required to provide information on all of the geomaterials through which the shaft must be con- structed and from which the foundation will derive its resis- tance to the design loads. The foundation designer then must determine the required dimensions (depth and diameter) and structural properties of drilled shafts that will provide adequate resistance and will limit vertical and horizontal deformations to a level that provides adequate service per- formance of the bridge. Trial designs are developed and evaluated with respect to: (1) cost, (2) performance, and (3) constructability. A major factor in all three criteria is whether the shaft needs to be extended into the rock or IGM layers. Rock sockets will generally increase costs, improve load-carrying and load-displacement perfor- mance, and make construction more challenging. SCOPE AND OBJECTIVES The overall objectives of this synthesis study are to • Collect and summarize information on current practices pertaining to each step of the process described pre- viously, along with their limitations and sources of uncertainty; • Identify emerging and promising technologies in each of these areas; • Identify the principal challenges in advancing the state of the practice; and • Provide suggestions for future developments and im- provements in the use and design of rock-socketed shafts. CHAPTER ONE INTRODUCTION

The major challenges faced by U.S. transportation agencies in the use of rock-socketed drilled shafts for highway bridges were identified by NCHRP Topic Panel 36-12 as follows: • The first challenge is characterizing the nature of the rock or IGM. By its nature, rock and IGM are highly variable and difficult to characterize for engineering purposes. To effectively design drilled shafts in rock and IGM, engineers must accommodate high levels of un- certainty. Issues to be addressed include quantifying material characteristics, rock mass behavior, and appro- priate application of laboratory and field test methods. • The second challenge is determination of the axial load capacity of rock-socketed shafts. Rock-socketed shafts resist axial load in both side shear and end bearing. De- signers need well-documented methods for assessing side shear and end bearing. Different methods are ap- propriate for different types of geology. There are many issues related to characterizing the rock and construc- tion that affect design for axial loading. • The third challenge is analysis and design of rock- socketed shafts under lateral loading. It has been a cus- tomary practice to adopt the techniques developed for laterally loaded piles in soil to solve the problem of rock-socketed shafts under lateral loading. There exist several analysis and design methods specifically for rock-socketed shafts under lateral loading; their appli- cation in practice remains limited. METHODOLOGY A literature review was conducted on all topics related to drilled shafts in rock or IGM. To assess current practice, the primary manuals used by transportation engineers for drilled 4 shaft design were consulted. These include Drilled Shafts: Construction Procedures and Design Methods by O’Neill and Reese (1999) and the AASHTO LRFD Bridge Design Specifications (3rd ed. 2004). In addition, a draft version of Section 10, “Foundations,” of the 2006 Interim AASHTO LRFD Bridge Design Specifications was reviewed. A questionnaire was developed and sent to the principal geotechnical and structural engineers of 52 U.S. transportation agencies (including Puerto Rico and the District of Columbia) and the Canadian provinces. The primary purpose of the survey was to define the current state of practice for rock-socketed shafts. Questions were grouped into the following categories: • Use of rock-socketed shafts by the agency, • Evaluation of rock and IGM properties, • Design methods for axial loading, • Design methods for lateral loading, • Structural design, • Construction, and • Field load and integrity testing. Thirty-two U.S. and one Canadian provincial transporta- tion agencies responded to the questionnaire, completely or in part. A list of responding agencies and a summary of re- sponses to the questions are given in Appendix A. The ques- tionnaire was also sent to several consulting firms and drilled shaft contractors. Two contractors responded to the survey. Based on responses to the questionnaire, selected state agency personnel and contractors were interviewed. ORGANIZATION OF SYNTHESIS The synthesis is presented in six chapters and two appendixes. Chapter one defines the problem, objectives, scope, and methodology of the study. This chapter also provides an overview of the foundation design process used by state de- partment of transportation (DOT) agencies. This overview provides a framework for understanding the interrelationships between site characterization, material property evaluation, geotechnical and structural design, load testing, and con- struction of rock-socketed shafts. Each of these topics is con- sidered in subsequent chapters. Chapter two reviews methods of site characterization and material property evaluation that are applicable to rock-socketed shafts. Chapter three is a com- pilation and critical review of methods used for analysis and design of rock sockets for axial loading. Chapter four reviews and summarizes analysis methods for rock sockets under lat- eral and moment loading and discusses structural design is- sues relevant to rock sockets. Chapter five provides an overview of current technologies for rock-socket construction and considers some of the construction issues identified by the survey. This chapter also covers field load testing of rock- socketed shafts and the role of load testing within the context of state DOT foundation engineering programs. Chapter six M P WEATHERED ROCK OR IGM ROCK H SOIL FIGURE 1 Rock-socketed shaft designed for highway bridge structure.

5is a summary of the principal findings of this study and pre- sents steps that can lead to more effective use, design, and construction of rock sockets for bridge foundations. In each chapter, significant findings derived from the survey are iden- tified and discussed. Appendix A provides a list of survey re- spondents and Appendix B presents the questionnaire and a compilation of the responses to each question. DESIGN PROCESS Structural foundation design within state DOTs is typically a joint effort between the structural and geotechnical divisions. The geotechnical group may include engineering geologists and both groups may operate under the supervision of a chief bridge engineer. As a starting point, consider Figure 2, which shows a flow chart of the overall foundation design process. The chart is from the Washington State DOT Geotechnical Design Manual (2005). Based on responses to Question 4 of the survey questionnaire (Appendix B) and interviews with DOT personnel, Figure 2 typifies the process followed by many states. A summary of each step, also based on the Washington State DOT manual (2005), is as follows. Conceptual Bridge Foundation Design An informal communication/report is produced by the Geo- technical Division (GD) at the request of the Bridge and Structures Office (Bridge). Information provided includes a brief description of the anticipated site conditions, conceptual foundation types considered to be feasible, and conceptual evaluation of potential geotechnical hazards such as lique- faction. The purpose of these recommendations is to provide sufficient geotechnical information to allow a bridge prelim- inary plan to be produced. Develop Site Data and Preliminary Plan Bridge obtains site data from the regional office and devel- ops a preliminary bridge plan (or other structure) adequate for GD to locate borings in preparation for final design of the structure. Bridge would also provide the following informa- tion to GD to support development of the preliminary foun- dation design: • Anticipated structure type and magnitudes of tolerable settlement (total and differential). • At abutments, the approximate maximum elevation fea- sible for the top of the foundation. • For interior piers, the number of columns anticipated and, if there will be single foundation elements for each column or if one foundation element will support mul- tiple columns. • At stream crossings, the depth of scour anticipated, if known. Typically, GD will pursue this issue with the Hydraulics Office. • Known constraints that would affect the foundations in terms of type, location, or size, or any known con- straints that would affect the assumptions made to de- termine the nominal resistance of the foundation (e.g., utilities that must remain, construction staging needs, excavation, shoring and falsework needs, and other constructability issues). Preliminary Foundation Design A memorandum is produced by GD at the request of Bridge that provides geotechnical data adequate to conduct struc- tural analysis and modeling for all load groups to be consid- ered. The geotechnical data are preliminary and not in final form for publication and transmittal to potential bidders. At this stage, foundation recommendations are subject to change, depending on the results of structural analysis and modeling and the effect that modeling and analysis has on foundation types, locations, sizes, and depths, as well as any design assumptions made by the geotechnical designer. Pre- liminary foundation recommendations may also be subject to change based on construction staging needs and other constructability issues discovered during this phase. Geotech- nical work conducted during this stage typically includes com- pletion of the field exploration program to the final PS&E level (Plans, Specifications, & Estimates), development of foundation types and feasible capacities, foundation depths iterate Bridge and Structures Office (Bridge) requests conceptual foundation recommendations from Geotechnical Division (GD) GD provides conceptual foundation recommendations to Bridge Bridge obtains site data, develops draft preliminary plan, and provides initial foundation needs input to GD GD provides preliminary foundation design recommendations GD performs final geotechnical design and provides final geotechnical report for the structure Bridge performs final structural modeling and develops final PS&E for structure Bridge performs structural analysis and modeling and provides feedback to GD regarding foundation loads, type, size, depth, and configuration needed for structural purposes FIGURE 2 Design process for Load and Resistance Factor Design (Washington State DOT 2005). PS&E = Plans, Specifications, & Estimates.

needed, p-y curve data and soil spring data for seismic mod- eling, seismic site characterization and estimated ground acceleration, and recommendations to address known con- structability issues. A description of subsurface conditions and a preliminary subsurface profile would also be provided at this stage; however, detailed boring logs and laboratory test data would usually not be provided. Structural Analysis and Modeling Bridge uses the preliminary foundation design recommenda- tions provided by GD to perform structural modeling of the foundation system and superstructure. Through this model- ing, Bridge determines and distributes the loads within the structure for all appropriate load cases, factors the loads as appropriate, and sizes the foundations using foundation nom- inal resistances and resistance factors provided by GD. Con- structability and construction staging needs continue to be investigated during this phase. Bridge provides the following feedback to GD to allow them to check their preliminary foundation design and produce the Final Geotechnical Re- port for the structure: 6 • Anticipated foundation loads (including load factors and load groups used), • Foundation size/diameter and depth required to meet structural needs, • Foundation details that could affect the geotechnical design of the foundations, and • Size and configuration of deep foundation groups. Final Foundation Design This design step results in a formal report produced by GD that provides final geotechnical recommendations for the subject structure. This report includes all geotechnical data obtained at the site, including final boring logs, subsurface profiles, laboratory test data, all final foundation recommen- dations, and final constructability recommendations for the structure. At this time, GD checks the preliminary foundation design in consideration of the structural foundation design results determined by Bridge, and makes modifications as needed to accommodate the structural design needs provided by Bridge. Some state DOTs may also make this report avail- able to potential bidders. FIGURE 3 Design and construction process for drilled shaft foundations (adapted from Paikowsky et al. 2004a). QA/QC = quality assurance/quality control; NDT/NDE = nondestructive testing/evaluations.

7Final Structural Modeling and Development of Plans, Specifications, and Estimates Bridge makes the required adjustments to the structural model to accommodate changes in the geotechnical foundation recommendations as transmitted in the final geotechnical report. From this, the bridge design and final PS&E are completed. A similar design process is recommended if a con- sultant or design–builder is performing one or both design functions. Design Process in Relation to the Synthesis Based on the process described previously and followed by most state DOTs, Figure 3 is a flowchart of the design and construction process for drilled shaft foundations that pro- vides a framework for the topics addressed by this synthesis. In each subsequent chapter, the topics being covered are con- sidered within the context of the overall process as shown in Figure 3. This includes site investigation, geomaterial prop- erty evaluation, and design for axial and lateral loading.

Next: Chapter Two - Site and Geomaterial Characterization »
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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 360: Rock-Socketed Shafts for Highway Structure Foundations explores current practices pertaining to each step of the design process, along with the limitations; identifies emerging and promising technologies; examines the principal challenges in advancing the state of the practice; and investigates future developments and potential improvements in the use and design of rock-socketed shafts.

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