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Use of Geophysics for Transportation Projects (2006)

Chapter: Chapter Four - Agency Practice Methods and Applications

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Suggested Citation:"Chapter Four - Agency Practice Methods and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Use of Geophysics for Transportation Projects. Washington, DC: The National Academies Press. doi: 10.17226/13941.
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Suggested Citation:"Chapter Four - Agency Practice Methods and Applications." National Academies of Sciences, Engineering, and Medicine. 2006. Use of Geophysics for Transportation Projects. Washington, DC: The National Academies Press. doi: 10.17226/13941.
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Page 16

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16 This chapter focuses on the compiled results of Part 2 (Meth- ods and Applications) of the questionnaire. Part 2 contains data gathered through the responses and interviews, and cov- ers the main issues related to what geophysics is being used, how it is being applied, and who approves its use. GEOPHYSICAL METHODS AND TECHNIQUES Results from Questions 21 through 27 are presented graphi- cally in Appendix C, Part 2—Methods and Applications. Figure 9 displays the geophysical methods most commonly used by the respondent agencies based on results from Ques- tion 22. It is apparent from this chart that the use of NDT is incorporated with geophysical methods, likely as a result of the overlap between the technologies (e.g., crosshole seismic for shear wave velocity versus crosshole sonic logging for drilled shaft integrity). The questionnaire allowed for signif- icant flexibility in completing Question 22; therefore, re- sponses varied considerably. The responses to “. . . the three most commonly used methods . . .” are categorized by the 10 surface geophysical methods defined in Question 21 (a through j). This simplified the answers and allowed for a graphical representation of the data shown in (Figure 9). Seismic and GPR methods make up greater than 50% of the overall usage of geophysics among transportation agen- cies. Significant results are that (1) vibration monitoring represents a high percentage of use (22%), (2) electrical re- sistivity ranks fourth at approximately 10%, and (3) there is an obvious lack of EM methods used. A number of “other” methods were designated by respondents that do not fit the primary methods and they are listed along with all the re- sponses in Table C1 in Appendix C. Results from Questions 21a through 21l also indicated that seismic, GPR, and vibration monitoring are the most com- monly used methods. Electrical, borehole logging, and a myr- iad of other methods are actively used. Magnetic methods have been used by 12 agencies and microgravity by 5. Refraction and borehole seismic techniques (crosshole and downhole) are the most common seismic techniques. Two-dimensional pro- filing is well ahead of any other electrical technique commonly used. Time-domain and frequency-domain electromagnetic techniques are applied about equally, although very infre- quently. Marine and airborne geophysical investigations appear to be very rarely conducted. Vibration monitoring is equally split by technique for construction monitoring (e.g., pile driving and dynamic compaction) and blast monitoring (e.g., rock mass excavation and quarry operations). APPLICATIONS FOR GEOPHYSICAL INVESTIGATIONS Results from Questions 23 and 24 deal specifically with the application of geophysics on projects. Figure 10 displays the most common applications for which geophysics is used by the respondent agencies. A footnote to this figure is required because almost 25% of the applications described in the re- sponses to this set of questions fall into the NDT category. It was established that NDT was not a focus of this synthesis; however, this figure shows that the difference between the application of NDT or geophysics continues to be confusing. Similar to the results from Question 22 (Figure 9), Question 24 was worded in a way that allowed substantial freedom for the responses. As with Question 22, these responses were categorized based on the applications defined in Question 23. This allowed the variety of responses to Question 24 to be limited to the graphical presentation shown in Figure 10. The responses were categorized on a general basis deter- mined by the variety and different descriptions of applications. For example, “mapping depth to rock,” “mapping topography of rock,” or “mapping bedrock strength” were all placed in the bedrock mapping category. The categorization permitted a better illustration of the responses to this question. All the re- sponses are listed in Table C2. Actual values for each particu- lar application are shown in separate charts in Appendix C (Questions 23a through 23h). Questions 23 and 24 all indicate that one-third of the geotechnical applications involved the use CHAPTER FOUR AGENCY PRACTICE—METHODS AND APPLICATIONS 5% 9% 10% 22% 6% 22% 26% Seismic GPRVibrationMonitoring Resistivity NDT Borehole Logging Others N=130 FIGURE 9 Most commonly used geophysical methods (51 agencies completed Question 22).

17 of geophysics to map bedrock characteristics such as depth, topography, or rippability. Numerous other applications, including a large representation of NDT applications, were provided by the respondents. As might be expected, roadway subsidence issues and soil mapping are dominant applications as well. SELECTION CRITERIA AND APPROVAL FOR GEOPHYSICAL INVESTIGATIONS Often the application of geophysics is warranted for a project; however, the particular method or technique may or may not be immediately obvious or a combination of methods might need to be determined to best meet the objective(s). Trans- portation agencies were queried about who helps select the ap- propriate method when geophysics is being proposed on a project. Figure 11 identifies the distribution for this responsi- bility selected. The data show that prior experience by the agency or the individual plays the key role in a selection process; however, it is quite evenly distributed between in- house geophysicists, engineers, and contractors. All the pro- fessional experience and other factors should assist in the method selection process. There appears to be a reasonable amount of “selection-by-committee” (i.e., no formal approach) as well. The authorization process, and who is responsible for the authorization, is equally important for geophysical investiga- tions. Figure 12 identifies who is typically responsible for ap- proving the “appropriate” use of geophysics on a particular project. As expected, either the project or program manager or the highway engineer typically approves the selection. Based on these results, it is likely that an in-house or a con- tract geophysicist who selects the method(s) does not have the authority to approve its use. Table C3 lists other comments regarding experiences with the methods used, the applications, and the selection of ap- propriate geophysics for projects that are not covered in this section. 1%10% 11% 22% 24% 32% Others Bedrock Mapping NDT Subsidence Investigations Mapping Soil Mapping Man-Made Features N=113 FIGURE 10 Most common applications of geophysics (50 agencies completed Question 24). 14 22 21 29 5 7 12 5 2 0 10 20 30 40 50 60 R es po nd en ts Contractor Specifications In-house Geophysicist Highway Engineer Experience Only Known Method ASTM / AASHTO No Formal Approach Preferences No Response N=117 3 18 3 2 6 24 13 18 1 0 10 20 30 40 50 60 R es po nd en ts Contractor Highway Engineer Program Manager Contracting / Procurement In-house Geophysicist Project Manager Division/Branch Manager Other No Response N=88 FIGURE 11 Individual who recommends the overall approach to the selection of the appropriate geophysical method. FIGURE 12 Individual who approves the appropriate geophysical method.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 357: Use of Geophysics for Transportation Projects examines the state of the practice regarding the use of geophysics for transportation projects. The report focuses on who is using geophysics and why, which methods and applications are the most commonly used, the use of in-house expertise compared with contracting private consultants, and how geophysical service contracts are procured and implemented.

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