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3 Effective and economical characterization of subsurface site conditions is a critical component of transportation projects. Uncertainties associated with insufficient or poor site characteriza- tion can lead to overly conservative designs, increased risk of poor performance, cost increases attributable to changed conditions, and project delays (Boeckmann and Loehr 2016). Geophysical methods provide a means to rapidly and economically characterize subsurface conditions and infer soil properties over a spatial extent that is not possible with conventional methods. Despite these benefits, challenges and obstacles remain that must be overcome if routine implementa- tion of geophysical methods for transportation projects is to be realized. In 2006, the National Academies published NCHRP Synthesis 357: Use of Geophysics for Transportation Projects (Sirles), which provided an eye-opening look at the way transportation agencies were using geophysical methods and the obstacles faced in implementing those methods. Among the many issues identified in that survey was the need for improved education, better training resources, and consistent standards. Since publication of that report, much has changed in the development and application of geophysical methods, including new and improved methods, expanded applications for trans- portation projects, and more user-friendly software. These changes in the state of practice since 2006 are the motivation for this synthesis, which documents current practices and challenges in the use of geophysical methods for transportation projects. Current practices for using geophysical methods on transportation projects are investigated through a literature review of geophysical methods and applications, a survey of transportation agencies, and case examples of agencies with particularly valuable experiences using geophysical methods. Particular focus is given to identifying obstacles to use and how they can be overcome. Objectives The primary objective of this synthesis is to evaluate the current state of practice in the use of geophysical methods by transportation agencies. The focus is on assessing changes in the state of practice since publication of NCHRP Synthesis 357 in 2006, including the reasons for changes in practice and the improvements achieved in practice. Through use of a comprehensive survey sent to state transportation agencies, this study aimed to quantify â¢ Frequency of use of geophysical methods, â¢ Changes in use over the past 5 years, â¢ Reasons for using geophysical methods, â¢ Common methods and applications used, â¢ Policies and guidelines for implementing geophysics, C H A P T E R 1 Introduction
4 Advancements in Use of Geophysical Methods for Transportation Projects â¢ Budgeting and contracting practices for geophysical measurements, and â¢ Familiarity with educational and training resources. The survey included several questions that were similar to those asked of the agencies in 2006 to allow for comparison of the 2006 state of practice and the 2019 state of practice. A second objective of this synthesis was to develop an updated matrix relating the geophysical methods to appropriate objectives. This updated matrix includes the addition of several methods and applications that were not shown in the 2006 synthesis. Methodology and Outline The required information was gathered through three activities: a literature review, a survey of transportation agencies, and interviews with five agencies selected for case examples on the basis of their high frequency of geophysics use. These activities are summarized below, and results of each activity are presented in Chapters 2, 3, and 4, respectively. Conclusions are presented in Chapter 5. Literature Review The literature review is separated into three sections. The first section presents a concise summary of the important findings from NCHRP Synthesis 357: Use of Geophysics for Transpor- tation Projects (Sirles 2006). Select figures from that document that are relevant to the findings of the current survey are reproduced here to aid in comparisons of the 2006 and 2019 state of practice in the use of geophysics on transportation projects. The second section presents a brief overview of each of the geophysical methods mentioned in the survey. The purpose of this review is not to provide a comprehensive background on each method, but to familiarize the reader with the basic principles, implementation, and applica- tions of each method referenced in the survey questions and synthesis document. Sources in the literature that present the specific methods in more detail are also included in this section, as are select examples of recent and noteworthy applications of some of the methods. The third section provides a review of educational and training resources on the use of geophysics that are available to transportation agencies. The section is divided into the fol- lowing three areas of education and training: (1) education resources for understanding the appropriate applications, capabilities, and limitations of geophysical methods for potential users; (2) educational resources for a more in-depth understanding of the principles behind geophysical methods and the interpretation of the results; and (3) training on how to perform specific geophysical measurements. Survey A survey was distributed electronically to 55 agencies, including state transportation agen- cies for all 50 U.S. states, Puerto Rico, and the District of Columbia as well as three offices of the Federal Lands Highway Division of FHWA. Forty-four of the 50 state DOTs responded to the survey, which corresponds to a response rate of 88% for the state agencies. In addition, responses were received from Puerto Rico, the District of Columbia, Central Federal Lands Highway, and Western Federal Lands Highway. The survey was distributed to agency geotech- nical engineers, but the survey instructions encouraged the geotechnical engineers to share the survey with any colleagues who might be better equipped to answer questions regarding agency geophysical practices. The first question of the survey asked if the respondentâs agency
Introduction 5 had experience with geophysical methods for geotechnical site investigation. Respondents who indicated their agency did not have experience with geophysical methods were asked to indicate the reasons their agency had not used geophysical methods. Respondents who indi- cated their agency did have experience with geophysical methods were asked 14 follow-up questions. The follow-up questions inquired about motivations for application of geophysical methods, frequency of use of geophysical methods, types of methods used, design applications for geophysical methods, and contractual issues, among other topics. Finally, all respondents (including those without geophysical experience) were asked three questions about training resources related to geophysical methods. Results of the survey are presented in Chapter 3. Case Examples Survey responses were reviewed to identify five agencies that apply geophysical methods frequently and have experience with noteworthy or challenging applications of geophysical methods. Additional investigation into these agenciesâ histories of developing geophysical capabilities, common practices and applications, and specific case details was conducted through interviews with agency personnel and reviews of available agency documents. Chapter 4 includes the results of each case example and a summary of lessons learned from the five case example agencies. Synthesis Definition of Geophysical Methods For the purposes of this synthesis and its survey, the term âgeophysical methodsâ is defined as measurement techniques that apply physical principles to define geology and study earth materials. This definition does not include nondestructive evaluation (NDE) methods that are used to characterize the condition or properties of man-made materials and structures. While some methods can be applied to both geological and man-made materials (e.g., surface wave methods), this survey considers only the application of these methods for characterization of geotechnical materials.