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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Advancements in Use of Geophysical Methods for Transportation Projects. Washington, DC: The National Academies Press. doi: 10.17226/25809.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2020. Advancements in Use of Geophysical Methods for Transportation Projects. Washington, DC: The National Academies Press. doi: 10.17226/25809.
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1 Geophysical methods provide an effective and economical means to characterize sub- surface conditions for transportation projects. Despite these benefits, challenges and obstacles remain that must be overcome if routine implementation of geophysical methods for transportation projects is to be realized. In 2006, the Transportation Research Board pub- lished NCHRP Synthesis 357: Use of Geophysics for Transportation Projects (Sirles 2006). Since publication of the report, much has changed in the development and application of geophysical methods. These changes in the state of practice since 2006 are the motivation for this study. This synthesis documents current practices for the application of geophysi- cal methods in transportation projects. Practices were gleaned from a review of literature, a survey of transportation agencies, and case examples of select agencies. A review of current literature of near-surface geophysical methods found that numerous surface and borehole geophysical methods are available and relevant to transportation infrastructure problems. This document provides a brief overview of 15 surface geo- physical methods and nine borehole methods. Compared with 2006, the ability to rapidly generate two-dimensional and three-dimensional subsurface images has improved with developments in tomographic and full waveform inversion techniques. Also, the capa- bilities of active and passive surface wave methods have progressed significantly since 2006. In addition, the capabilities of borehole imaging and logging tools have continued to improve. An updated matrix of geophysical methods and their application is provided in this document. A survey was distributed electronically to 55 agencies, including state transportation agencies for all 50 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 represents a response rate of 88% for the state agencies. In addition, the District of Columbia, Puerto Rico, Central Federal Lands Highway, and Western Federal Lands Highway responded to the survey. Results of the survey indicate the vast majority of agencies use geophysical methods, with 43 of 48 respondents (90%) indicating their agency uses geophysical methods. That propor- tion is little changed from the results of a 2006 NCHRP synthesis survey (Sirles), in which 86% of respondents indicated use of geophysical methods by their agency. A wide variety of motivations for using geophysical methods were selected by survey respondents, but most commonly cited was the ability to provide a subsurface image of a large mass of materials. All five respondents who indicated their agency did not have experience with geophysical methods reported that agency engineers were unfamiliar with geophysical methods. According to the survey results, the most common estimate for how frequently agencies use geophysical methods is three to five times per year. Seven respondents indicated their S U M M A R Y Advancements in Use of Geophysical Methods for Transportation Projects

2 Advancements in Use of Geophysical Methods for Transportation Projects agency uses geophysical methods more than 10 times per year. For half of the agencies with geophysical experience, respondents indicated the frequency of use of geophysical methods is increasing. There is not much evidence that the frequency of application has increased since the 2006 survey by Sirles. The estimated frequency of application of geophysical methods was largely the same as reported by Sirles for the 2006 survey, but survey respon- dent estimates provide some indication that agency spending on geophysical methods may have increased slightly. Ground-penetrating radar and seismic refraction are the most commonly applied geo- physical methods, with nearly 90% of survey respondents indicating use of each method. The 2006 survey by Sirles also found general seismic and ground-penetrating radar to be the most common methods. This survey, however, indicated a significant increase in the proportion of agencies with experience in several geophysical methods—notably seismic refraction, surface wave methods (active and passive), electrical tomography, microgravity, and both optical and acoustic televiewer methods. Survey results indicate that geologic objectives pertaining to rock, specifically determining the depth to bedrock, bedrock topography, and bedrock rippability, are the most common. Responses to survey questions about potential new training resources were highly skewed toward a favorable view of new training resources. At least 70% of respondents said new training resources would be somewhat likely or very likely to increase agency use of geo- physical methods. That result held for three different training formats, with in-person train- ing viewed as most likely to increase use of geophysical methods. Another question asked about the usefulness of five different training content areas. The majority of respondents indicated all five content areas would be “very useful,” with uses and applications of geo- physical methods and interpretation of engineering parameters from geophysical results viewed as most useful. Case examples were provided by five agencies identified as high-frequency users of geo- physical methods. The history of use of geophysical methods among the agencies varied considerably, as did the means of implementation. Three of the agencies performed most of their geophysical measurements in-house. These agencies considered such methods to save costs and used them on more projects. Two of the agencies performed measurements almost exclusively through external contracting and reported few problems with the con- tracting process. The agencies provided several interesting case examples in which geophysical measure- ments provided a benefit through cost savings, were the only viable approach for the condi- tions encountered, allowed for imaging of a large subsurface volume, or allowed for rapid collection of subsurface information. Examples where geophysical results were confirmed by ground truth were also provided. All agencies commented on the need to supplement geophysical investigations with a drilling and sampling program whenever possible. The agencies interviewed also indicated a need for training resources. The primary need identified by the agencies was for training for engineers on the capabilities, limitations, and typical applications of geophysical methods.

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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.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 547: Advancements in Use of Geophysical Methods for Transportation Projects evaluates the current state of practice in the use of geophysical methods by state transportation agencies.

Challenges and obstacles remain that must be overcome if routine implementation of geophysical methods for transportation projects is to be realized. Uncertainties associated with insufficient or poor site characterization can lead to overly conservative designs, increased risk of poor performance, cost increases attributable to changed conditions, and project delays.

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