Use of Geophysics for Transportation Projects (2006)

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27 The results of this synthesis are summarized in this chapter. The chapter also includes recommendations for future re- search regarding geophysical investigations, as defined by the respondent agencies. The implementation of geophysical investigation tech- niques is increasing among transportation agencies, with project-specific applications continuing to diversify. How- ever, there remains some skepticism among those engineers and geoscientists who are attempting to implement the tech- nology on their projects and within their agencies. Through the use of the questionnaire, comments, interviews, and dis- cussion of case histories, it appears that in some instances implementation of geophysics is being undertaken • Without proper selection of the technique for the spe- cific application(s), • With an inadequately defined scope of work, • With inadequate means to acquire and objectively interpret data, and • By individuals with inadequate education or experience in the field. These issues could be because transportation agency fund- ing is insufficient for implementing geophysics as a reason- able alternative on field programs prior to conventional geo- technical investigation procedures (i.e., drill and sample). The synthesis indicates that typical geophysical field investigation expenditures are small (generally less than $10,000) and at this level contracting out the service can be burdensome un- less previously experienced or known contractors are avail- able or larger Infinite Delivery/Infinite Quantity contracts can be used to award a task order with notice-to-proceed. The synthesis identified the most common geophysical methods used by the 58 respondent agencies as (1) seismic, (2) ground penetrating radar, and (3) vibration monitoring. Generally, these geophysical methods are the most fre- quently used because they are best suited to resolve the ma- jority of geotechnical engineering problems and applications for subsurface characterization. The most common geotech- nical applications identified by this synthesis relate to (1) bedrock mapping, (2) roadway subsidence problems, and (3) mapping (characterizing) soil deposits. It became clear through analysis of the literature that the distinction between nondestructive testing (NDT) and geo- physical methods is not clear. Many times respondents in- cluded NDT technologies in their comments, responses, and examples although geophysics is the topic for this synthesis. Although significant overlap exists between the physics of the two technologies, it is the application that distinguishes be- tween the two. As defined in this synthesis, geophysical tech- niques are applied to earth (geo-) materials, whereas NDT is applied to man-made structures. Results indicated that the need for improved education and training is a primary concern, as well as the need for better equipment and software. Also, the development of standards could help engineers increase their level of com- fort with geophysics. However, because geophysics is such a specialized field, and because engineering problems have risks associated with them, it appears that it might be some time before geophysics will be used as routinely as it is in the exploration for natural resources. That is, geophysics is the standard by which oil and gas and other natural re- sources (e.g., metals and coal) are located in the subsur- face. It took nearly 50 years for the natural resources exploration geophysics industry to develop the tools and expertise necessary to make geophysics the primary tool that used ahead of or in lieu of conventional exploration methods. Results from the survey questionnaire indicated that al- most 50% of transportation agencies and the engineers (i.e., the end-users) have been applying geophysics technology for less than 10 years. Therefore, it should not be a surprise that obstacles such as understanding, cost, and skepticism continue to restrict its use. It is apparent, however, that the geoscientists and engineers associated with transportation programs believe that geophysics should be a user-friendly technology. Their request is for inexpensive approaches to acquire data and simple methods to objectively interpret the data. The paradox seems to be that geophysics is, by virtue, a technical and complex science. When applying theory to earth materials such as rocks and soils, which are inherently heterogeneous, the complexities compound. Engineers need to quantify material properties and site (subsurface) charac- teristics owing to the risk factors and safety needs associated with planning and constructing facilities used by the public. Utilizing the best available technology, or set of technolo- gies, is appropriate when applied correctly, even if the results are qualitative and dependent on qualified individuals to pro- duce subjective interpretations. CHAPTER SEVEN CONCLUSIONS AND FUTURE RESEARCH NEEDS

As shown by agencies responding to this synthesis, geo- physics can aid in transportation planning and construction programs. Incorporating geophysics into geotechnical pro- jects will simply take time, which will ultimately build a more solid base of experience. A few transportation agencies have already demonstrated the value of using this technology regularly and over extended periods of time, whereas most reported only “...occasional use.” The agencies that use it regularly understand the science and can promote the bene- fits within their agency. The values of geophysics as defined by this synthesis include: • Cost-effectiveness, • High density of measurements, • Quick acquisition over large areas, • The combination of two- and three-dimensional assess- ment, and • Visualization of subsurface features. As noted in the survey responses, comments, and discus- sions, these values and benefits will only come through edu- cation, training, and experience. Finally, transportation agencies do not implement geo- physics the same across the country and in Canada. Geologic settings, materials, and tests necessary to plan or construct transportation projects can vary greatly from state to state, from province to province, and from country to country. This is primarily the result of a lack of experience and education about the application of the technology, not the availability or cost. As costs for traditional geotechnical sampling and testing increase and the geophysical community continues to educate the end-users as well as advance its methods (hardware and software) there can be a much wider acceptance of the tech- nology among transportation agencies. Geophysics can be used in a variety of settings, and when applied correctly can provide savings and permit quicker site assessment for geo- technical projects. Using multiple geophysical methods and integrating the data with standard geologic and geotechnical site-specific data may ultimately lead to more consistent use of geophysics. Table C9 presents final comments from respon- dents concerning other items not covered by the questionnaire and general comments about the synthesis. The final survey question (see Appendix B, Question 63) sought open comments on the future needs for geophysics technology. Comments were provided by 35% of the re- sponding agencies and are presented in Table C8 (Appendix C). A brief review is provided here. As anticipated, responses varied considerably for this topic; however, a definite call for standards and more educational op- portunities were predominate. The Tennessee Department of Transportation respondent may have stated it best: “training, training, training, and ...training.” Development of a National Highway Institute course on the application of geophysics to geophysical problems was also a common theme. Simply put, it is through good, constructive, case-history education (i.e., experience-driven) that transportation agency engineers throughout the United States and Canada will be able to fully make use of existing state-of-the-practice technologies and appropriate applications of geophysical methods to help solve transportation-related problems. In addition, the development of “off-the-shelf” methods and applications appears to be a need, as well as easy-to-use (inexpensive) field instrumenta- tion and software tools for interpretation. The most frequent comment regarding a particular geophysical technique in- volved more research into surface wave methodologies and applications (i.e., Spectral Analysis of Surface Waves, Multi- Channel Analysis of Surface Waves, and microtremor meth- ods such as ReMi). Within the geophysical industry, these are among the most significant emerging technologies and their applications are just being realized. In addition, it was evident that an understanding of the difference between geophysics and NDT, the similarity of the physics applied, and the simi- lar nature of applications needed by engineers will overlap until additional educational opportunities exist for the two technologies. 28