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60 This synthesis documents current practices for the application of geophysical methods to transportation projects. Practices were gleaned from a review of literature, a survey of transpor- tation agencies, and case examples of select agencies. This chapter summarizes the most notable findings from each before presenting conclusions and suggestions for future research. Summary of Major Findings The review of the literature on near-surface geophysical methods found a wide range of surface and geophysical techniques available that are applicable to transportation-related problems. This synthesis focused on 15 surface techniques and nine borehole techniques. Brief summaries of each method were provided, as were select applications from the literature. The literature review was also used to assemble an updated matrix of surface and borehole methods and their applications, as well as a summary of relevant training resources on the application, implemen- tation, and fundamentals of geophysical methods. The synthesis survey revealed the vast majority of agencies use geophysical methods, with 43 of 48 respondents (90%) indicating their agency uses geophysical methods. This proportion is approximately the same as that found in the 2006 survey by Sirles. Agency motivations for using geophysical methods varied, with five reasons cited by at least half of the respondents; the most commonly cited was the ability to provide a subsurface image of a large mass of materials. Among the five agencies without geophysical experience, the most commonly cited reasons for not using geophysical methods were technical. All five indicated 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 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 respondent estimates provide some indication that agency spending on geophysical methods may have increased slightly. Eight geophysical methods are reported to have been used by at least one-third of respondents with knowledge of agency geophysical experience. The most common are ground-penetrating radar and seismic refraction, both of which were reported by nearly nine in 10 respondents. These same methods were also found to be the most commonly applied in the 2006 survey. However, active and passive surface wave methods, electrical tomography, microgravity, and optical and acoustic televiewer methods have all experienced a three- to four-fold increase in the C H A P T E R 5 Conclusions
Conclusions 61 proportion of agencies that report experience with those methods. Electrical resistivity imaging (ERI), in particular, was well represented in the case examples provided. Geologic objectives of the geophysical investigations are dominated by those pertaining to rock, especially determina- tion of the depth to bedrock, bedrock topography, and bedrock rippability. The majority of agencies with geophysics experience, 51%, use a mix of in-house capabilities and contractors for performance of geophysical test methods. Forty-four percent perform all geo- physical test methods using contractors. The most common source of funding for geophysical investigations is project design funds, which have been used by more than 90% of respondents. Responses to survey questions about potential new training resources were highly skewed toward a favorable view of such resources. At least 70% of respondents said new training resources would be somewhat likely or very likely to increase agency use of geophysical methods. That result held for three different training formats, with in-person training viewed as the most likely to increase agency use of geophysical methods. Another question asked about the per- ceived usefulness of five different training content areas. The majority of respondents indicated that all five content areas would be very useful, with (1) uses and applications of geophysical methods and (2) interpretation of engineering parameters from geophysical results viewed as the most useful. Five case examples were examined in this survey. The agencies interviewed were all identi- fied as prolific users of geophysical methods. The history of geophysics use among the agencies varied considerably, as did the means of implementation. Three of the agencies performed most geophysical measurements with in-house capabilities. These agencies considered in-house use to be cost saving, allowing them to use the methods for more projects. Two of the agencies per- formed the measurements almost exclusively by external contracting and reported few problems with the contracting process. The agencies provided interesting case examples where geophysical measurements had pro- vided a benefit through cost savings, were the only viable approach for the conditions encoun- tered, 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 investiga- tions 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 training for engineers on the capabilities, limitations, and typical applications of geophysical methods. Several commented that a National Highway Institute course on geophysics would be welcomed. Conclusions Comparison of survey results from this synthesis and the 2006 synthesis by Sirles provides a unique opportunity to evaluate how the state of practice for geophysical methods has changed among U.S. transportation agencies. The number of agencies that use geophysical methods and the frequency of geophysical applications do not appear to have changed significantly. Roughly nine of 10 agencies use geophysical methods. Most agencies are relatively casual users with five or fewer geophysical applications per year, though a small handful of agencies implement geo- physical methods routinely. Although the overall frequency of geophysical applications does not appear to have increased since 2006, the survey from this synthesis provides some evidence that agency spending on geophysical methods has increased slightly, and there is strong evidence in the survey results that a variety of methods from across the geophysical spectrum have been implemented by significantly more agencies since 2006.
62 Advancements in Use of Geophysical Methods for Transportation Projects The survey results from this synthesis indicate that agency use of geophysical methods is commonly motivated by a wide variety of factors. Among six potential reasons for using geophysical methods, five were selected by more than half of the agencies with geophysical experience as having motivated their use of geophysical methods. The case example agencies provide strong anecdotal support for these motivators. For example, MnDOT provided a case example where resistivity measurements were used exclusively for foundation condition assess- ment because of site access issues. NJDOT provided an interesting example of opting for a simple and quick passive geophysical method when drilling was too slow and difficult. Caltrans showed an excellent example of imaging a large volume of material between boreholes, and ODOT provided examples of imaging a shallow landslide with resistivity. Last, NJDOT provided an example from a time before it used geophysics, where major cost savings would have been real- ized if geophysical methods had been applied. As evidenced by survey results from this synthesis and the 2006 synthesis, the greatest deter- rent to greater application of geophysical methods is a lack of familiarity and understanding of the methods among agency engineers. Both the 2006 and the present survey indicate broad sup- port for development of new training resources related to geophysical methods. Suggestions for Future Research There are countless avenues for research into geophysical methods that could produce useful and effective results for transportation agencies. For example, there is a need to further develop correlations between geophysical measurements and geotechnical parameters. Toward this end, development of processing routines and interpretation methods that use several geophysical measurements from the same location could be beneficial. It is suggested that research into near-surface applications of cutting-edge methods such as FWI continue to be studied, and that continued documentation of the use of geophysical methods to infer geotechnical parameters and site conditions from case histories is encouraged. Transportation agencies that are able to do so might consider encouraging, supporting, and funding such research. The results of this and of the 2006 (Sirles) synthesis, however, point not to specific research needs but rather to a more basic undertaking that would improve agency use of geophysical methods: development of training resources. That lack of knowledge of geophysical methods among agency engineers impedes the use of geophysical methods, and that agency engineers are eager for new training resources were among the clearest of the survey results. Taken together, the survey findings suggest that development and implementation of training resources would effectively increase agency use of geophysical methods. The survey results also suggest two training content areas would be most useful: (1) appropriate selection of geophysical methods that consider investigation objectives and site characteristics and (2) interpretation of engineering parameters from geophysical results. The case example agency interviews support the survey findings. The most common comment on training was that there was a need to educate engi- neers on the value that geophysical methods can provide to their projects. The experience of case example agencies suggests that agencies could realize significant cost savings, project schedule accelerations, and reliability improvements if training resources were to success- fully move the state of practice toward more frequent and more thoughtful applications of geophysical methods.
