Advancements in Use of Geophysical Methods for Transportation Projects (2020)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

21 A survey was distributed electronically to 55 agencies, including state transportation agen- cies for all 50 states, Puerto Rico, and the District of Columbia as well as three offices of the Federal Lands Highway Division of FHWA. The complete survey questionnaire is provided in Appendix A, and complete survey results are presented in Appendix B. Forty-four of the 50 state DOTs responded to the survey, which represents a response rate of 88% for the state agencies. Responding agencies are shown in the map of Figure 13. In addition, the District of Columbia, Puerto Rico, Central Federal Lands Highway, and Western Federal Lands Highway responded to the survey. This chapter summarizes results of the survey, including general information on experience with geophysical methods; geophysical methods, objectives, and applications; and agency practices for application of geophysical methods. The first question of the survey asked if the respondent’s agency had experience with geo- physical 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 indicated their agency did have experience with geophysical methods were asked 14 follow-up questions. The follow-up ques- tions 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 geo- physical methods. Results from the survey are presented in this chapter, which is organized by survey topic. For most topics, results are compared with the results of the previous synthesis (Sirles 2006), which was summarized in Chapter 2. General Information on Use of Geophysical Methods Survey responses established which agencies have experience with geophysical methods, motivations for or against use of geophysical methods, frequency of use of geophysical methods, and agency spending on application of geophysical methods. Results for each topic are summarized in the following sections. Agencies with Geophysical Methods Experience For survey purposes, the term “geophysical methods” was defined as measurement techniques that apply physical principles to define geology and study earth materials. The survey defini- tion explicitly excluded methods used to characterize manufactured materials and structures. C H A P T E R 3 Survey Results

22 Advancements in Use of Geophysical Methods for Transportation Projects Forty-three of the 48 agencies (90%) that responded to the survey reported experience with geophysical methods as defined in the survey. Agencies with geophysical experience are shown in Figure 14. In addition, the Central and Western Federal Lands Highway divisions both indicated experience, while the District of Columbia indicated no experience. The 90% of agencies that use geophysical methods is little changed from the 86% reported in 2006 by Sirles. As in 2006, a small minority of agencies today do not use any geophysical methods. Although the proportion of agencies that use geophysical methods has remained essentially the same, the survey results shed light on how geophysical methods have changed among the agencies that use them. Motivations for and Reasons against Geophysical Methods The 43 respondents who indicated their agency had experience with geophysical methods were asked to select the factors that had motivated their agency to apply geophysical methods. The list of six possible factors appears in Figure 15. The survey software randomized the order in which the factors were presented. The reason selected most frequently was the capability of geophysical methods to produce subsurface imaging of a large mass of materials in two or three dimensions (selected by 33 of 43 respondents, or 77%). Four of the other five responses provided in the survey were also selected by a majority of respondents: site access by 29 respon- dents (67%), direct measurement of engineering parameters by 28 respondents (65%), cost- effectiveness by 27 respondents (63%), and quick implementation by 23 respondents (53%). The other response, environmental constraints, was selected by 15 respondents (35%). The survey also allowed respondents to list other motivations for use of geophysical methods. One agency used geophysical methods when conventional methods were unable to provide sufficient information to assess problematic areas, and another agency used geophysical methods to help geologists interpret site geology. CA AZ CO NM TX OK AR LA MO KY AL GA FL VA OH MI VTAK MT NV ME WA OR UT KS ID WY ND SD MN NE WI IA IL IN MS TN SC NC WV PA NY CT NJ DE MD MA NH PR HI RI LEGEND Response No Response Figure 13. Agency responses to survey. Transportation agency for the District of Columbia also responded, as did Central and Western Federal Lands Highway divisions.

Survey Results 23 CA AZ CO NM TX OK AR LA MO KY AL GA FL VA OH MI VTAK MT NV ME WA OR UT KS ID WY ND SD MN NE WI IA IL IN MS TN SC NC WV PA NY CT NJ DE MD MA NH PR HI RI LEGEND Geophysical Methods Experience No Geophysical Methods Experience No Response Figure 14. Agency experience with geophysical methods. In addition to agencies shown on map, Central Federal Lands Highway and Western Federal Lands Highway divisions indicated experience with geophysical methods, while District of Columbia indicated no experience. Subsurface imaging of a large mass of materials Site access (i.e. where conventional methods were not feasible) Direct measurement of engineering parameters (e.g. shear wave velocity) Cost-effective Quick implementation (from planning to reporting) Environmental constraints (i.e. where conventional methods not feasible) Other: Assess problematic sites Other: Assists geologist with site interpretation Number of Respondents 67% 65% 63% 53% 2% 35% 77% 2% 0 403530252015105 Figure 15. Agency motivations for use of geophysical methods (43 responses).

24 Advancements in Use of Geophysical Methods for Transportation Projects The 2006 survey by Sirles inquired about motivations for geophysical methods in a different manner, by asking respondents to select the greatest value provided by geophysical methods. The speed of data acquisition, cost benefits, better subsurface characterization, and ability to perform two- and three-dimensional assessments were all selected by between 15% and 21% of respondents. Those top reasons are consistent with the findings presented in Figure 15. The five respondents who indicated their agency had no experience with geophysical methods were asked to select the reasons that their agency had not used geophysical methods. The list of possible reasons appears in Figure 16. The survey software randomized the order in which the reasons were presented. All five respondents (100%) indicated they had not used geophysical methods because agency engineers were unfamiliar with the methods. Four respon- dents (80%) selected a similar response: agency engineers did not know how to interpret results from geophysical methods. Three respondents (60%) selected contracting difficulties, and two respondents (40%) selected a lack of local contractors or in-house expertise for performing geophysical methods. Three reasons had one respondent (20%) each: high cost, reluctance to apply new methods for geotechnical site characterization, and uncertainty of results. One agency selected “other” and indicated that incorrect information from a geophysical method had led to a construction claim. Three of the potential reasons for not using geophysical methods were not selected by any respondents: unreliability of results, time required for implementation, and site access. Although the 2006 survey did not ask respondents without geophysical experience why their agencies had not used geophysical methods, it did ask respondents with geophysical experience to identify the biggest deterrents to use. The most frequent responses were lack of understanding of geophysical results, nonuniqueness of geophysical results, lack of con- fidence in geophysical results, and results that created more questions than they answered. These deterrents are generally consistent with the unfamiliarity and interpretation challenges identified in Figure 16. Agency engineers are unfamiliar with methods Agency engineers do not know how to interpret results Contracting difficulties No local contractors or in-house expertise Cost is too great Reluctance to apply new methods for site characterization Too uncertain (i.e. imprecise) Too unreliable (i.e. inaccurate) Take too long (from planning to reporting) Site access prevents effective implementation Other: Bad experience leading to claim Number of Respondents 60% 20% 20% 20% 20% 0 100% 0 0 40% 80% 0 1 2 3 4 5 6 Figure 16. Agency reasons for not using geophysical methods (5 responses).

Survey Results 25 Frequency of Application of Geophysical Methods and Agency Spending on Geophysical Methods The 43 respondents who indicated agency use of geophysical methods were asked how frequently their agency had used geophysical methods over the past 5 years. Respondents were asked to select among the ranges shown in Figure 17. The results are somewhat bell-shaped, with the largest number of respondents, 12 (28%), selecting 3 to 5 agency applications of geophysical methods per year. Eighteen respondents indicated less frequent use (on average, 1 application or less per year or 2 applications per year), while 12 respondents indicated more frequent use (6 to 10 appli- cations per year or more than 10 applications per year). Notably, seven agencies indicated in their responses that they use geophysical methods more than 10 times per year: Caltrans, Florida DOT, Kansas DOT, Minnesota DOT, South Carolina DOT, Virginia DOT, and Wisconsin DOT. The frequency of geophysical method applications reported in Figure 17 is strikingly consistent with the results of the 2006 survey. As reported in Chapter 2 (Figure 1), the 2006 survey found that 56% of agencies used geophysical methods 1 to 5 times per year, while 9% used them less than 1 time per year on average. The corresponding categories in Figure 17 (n ≤ 1, n = 2, 3 ≤ n ≤ 5) sum to 70%. The 2006 survey found that 32% of agencies used geophysical methods more than 5 times per year; 28% of respondents from the more recent agency survey indicated more than 5 applications per year. The 43 respondents who indicated agency use of geophysical methods were also asked how the frequency of use has changed compared with 5 years ago. The results, shown in Figure 18, indicate that agency use of geophysical methods has increased for about half of the agencies surveyed (21 agencies, or 49%). Eighteen respondents (42%) indicated that use of geophysical methods has stayed about the same over 5 years, and four respondents (9%) indicated that agency use has decreased. Interestingly, 60% of respondents to the 2006 survey by Sirles also indicated that agency use of geophysical methods was increasing. The results presented in Figure 17 do not indicate that any significant increase has actually occurred. The 43 respondents that indicated agency experience with geophysical methods were also asked to estimate average annual agency spending on geophysical methods over the 0 2 4 6 8 10 12 14 n ≤ 1 n = 2 3 ≤ n ≤ 5 6 ≤ n ≤ 10 I don't know. N um be r o f R es po nd en ts Avg. No. of Applications of Geophysical Methods per Year from 2014-2018, n 10 < n Figure 17. Average annual number of applications of geophysical methods over 5 years (43 responses).

26 Advancements in Use of Geophysical Methods for Transportation Projects previous 5 years. Respondents were asked to select among the ranges shown in Figure 19, which presents responses to the question. The most common response was $50,000 or less, with 16 respondents (37%). Seven respondents (16%) indicated the next smallest range, $50,001 to $100,000, and eight respondents (19%) selected $100,001 to $150,000. Five respondents selected higher ranges, and seven respondents selected “I don’t know.” The results from Figure 19 may indicate that agency spending on geophysical applications has increased somewhat since the 2006 survey. Though the most common response was $50,001 or less per year for both surveys, nearly half of the 2006 respondents selected that range, compared with about one-third of respondents in Figure 19. Methods, Applications, and Objectives of Geophysical Investigations The 43 respondents who indicated agency experience with geophysical methods were asked to select the methods their agency had used from among the methods listed in Table 2, which also presents results. Four respondents were unsure of which specific geophysical methods had been used at their agency; percentages shown in Table 2 are therefore based on 39 responses. 4 9% 18 42% 21 49% Less frequent About the same frequency More frequent Figure 18. Frequency of agency use of geophysical methods in 2019 versus 2014 (43 responses). 0 2 4 6 8 10 12 14 16 18 $50,001 to $100,000 $100,001 to $150,000 $150,001 to $250,000 $250,001 to $500,000 More than $500,000 I don’t know. N um be r o f R es po nd en ts Annual Agency Spending on Geophysical Methods, 2014-2018 0 to $50,000 Figure 19. Average annual agency spending on application of geophysical methods over 5 years (43 responses).

Survey Results 27 Geophysical Method Number Percent Seismic methods: Seismic refraction 33 85 Seismic reflection 10 26 Seismic tomography 9 23 H/V spectral ratio 2 5 Full waveform inversion 3 8 Active source surface wave techniques (e.g., SASW, MASW) 19 49 Passive surface wave techniques (e.g., ReMi) 15 38 Seismic methods, but I don’t know which ones specifically. 0 0 Electrical methods: 1D resistivity soundings (e.g., VES) 4 10 2D resistivity profiling (e.g., Dipole/Dipole, Wenner, etc.) 16 41 2D resistivity imaging (e.g., pole-Dipole, electrical resistivity tomography [ERT], etc.) 13 33 Induced polarization (IP) 5 13 Self-potential (SP) 2 5 Electrical methods, but I don’t know which ones specifically. 3 8 Electromagnetic (EM) methods: Ground-penetrating radar (GPR) 34 87 Time-domain EM 7 18 Frequency-domain EM (terrain conductivity) 4 10 Very low frequency (VLF) 0 0 Seismoelectric 1 3 Electromagnetic methods, but I don’t know which ones specifically. 1 3 Magnetic methods: Total-field 2 5 Gradiometer 1 3 Magnetic methods, but I don’t know which ones specifically. 2 5 Gravity methods: Microgravity 8 21 Standard gravity 0 0 Gravity methods, but I don’t know which ones specifically. 3 8 Borehole logging methods: Downhole seismic 17 44 Crosshole seismic 15 38 Electrical (e.g., SP, resistivity, e-logs) 3 8 Electromagnetic induction 1 3 Nuclear (e.g., gamma-gamma, natural gamma, neutron, etc.) 4 10 Optical televiewer 18 46 Acoustic televiewer 9 23 Suspension logging (e.g., PS logger) 2 5 Hydrophysical 0 0 Borehole deviation 7 18 Other responses: Other: Full waveform sonic 1 3 Other: Cement bond logging (CBL) 1 3 Other: Full waveform borehole sonic 1 3 Other: Single station passive seismic stratigraphy 1 3 Other: Capacitively coupled resistivity 1 3 Note: VES = vertical electrical sounding. Table 2. Geophysical methods used by respondent agencies (39 responses).

28 Advancements in Use of Geophysical Methods for Transportation Projects The most commonly used methods are ground-penetrating radar and seismic refraction, with 34 and 33 respondents, respectively (87% and 85%). Among other seismic methods, active source surface wave techniques had been used by about half of respondents (19, or 49%), and seismic reflection and seismic tomography had each been used by about a quarter of respon- dents. Other than ground-penetrating radar, electromagnetic methods were not used com- monly. Several borehole logging methods were used by about half of responding agencies, including optical televiewer, with 18 respondents (46%), and downhole seismic and crosshole seismic, with 17 and 15 respondents (44% and 38%), respectively. About half of agencies had used at least one electrical method, most commonly 2D resistivity profiling, with 16 respon- dents (41%). Magnetic and gravity methods were not used commonly, although one-fifth of agencies had used the microgravity method (8 respondents, or 21%). Compared with the 2006 survey, the results in Table 2 indicate little change in the most com- monly applied geophysical methods, but they show a significant increase in the proportion of agencies that have applied some of the methods. Sirles (2006) reported that seismic methods were the most commonly used among geophysical methods, followed by GPR, resistivity, and borehole logging. (Two categories excluded from this survey, vibration monitoring and non- destructive testing, were also included in Sirles’s ranking of common methods.) The 2006 list is consistent with the results from Table 2. However, some of the percentages in Table 2 indicate a significant increase in agency experience with particular methods: • Seismic refraction, from 59% in 2006 to 85% in 2018 • Active source surface wave techniques such as SASW/MASW, from 17% to 49% • Passive surface wave techniques such as ReMi, from 17% to 38% • 2D resistivity imaging such as electrical tomography, from 9% to 33% • Microgravity, from 5% to 21% • Optical televiewer, from 12% to 46% • Acoustic televiewer, from 9% to 23% The survey also requested that respondents with agency geophysical experience select the design applications for which their agency had undertaken geophysical investigations from among the options listed in Table 3. Table 3 also shows the responses to the survey question. Results indicate Geophysical Method Number Percent Routine design of bridge foundations 21 49 Routine design of embankments or cut slopes 20 47 Routine design of retaining walls 11 26 Seismic site effects 17 40 Liquefaction 12 28 Landslide evaluation 12 28 Utility location 18 42 Evaluation of roadway subsidence 28 65 Evaluation of scour (extent of existing scour, potential for future scour) 3 7 Evaluation and QC of construction (e.g., fill placement, excavation of unsuitable material) 6 14 Forensic investigation of failed infrastructure 9 21 Other: Rock mechanics, i.e., rock stability analysis, rippability, etc. 2 5 Other: Roadway cut evaluation 2 5 Other: Excavation characteristics 1 2 Other: Postconstruction monitoring 1 2 Other: Soundwall design 1 2 Other: Archeological investigation 1 2 Note: QC = quality control. Table 3. Applications of agency geophysical investigations (43 responses).

Survey Results 29 a relatively wide variety of applications, with some applications more common than others but none dominating. The most common applications were evaluation of roadway subsidence (28 respondents, 65%), routine design of bridge foundations (21 respondents, 49%), and routine design of embankments or cut slopes (20 respondents, 47%). Among the eight other applications presented to respondents (i.e., not including the “other” responses), all but one were selected by at least six respondents (14%). Respondents with agency geophysical experience were also asked to select the geological investi- gation objectives for which geophysical methods had been performed. The objectives presented to respondents and their responses to the question are shown in Table 4. The most common objective was to determine the depth to bedrock, which had been attempted by all but four respondents (38 of 42, or 90%). Determining the topography of bedrock was also a common objective, with 32 respondents (76%). The third most common objectives also concerned rock evaluation: specifically, mapping bedrock strength (i.e., rippability), with 23 respondents (55%). The next most common objective was related to identifying subsurface voids, most commonly karst, with 20 respondents (48%). Mapping soil overburden lithology (18 respondents, 43%) and mapping the groundwater table and evaluating subsurface voids (specifically failed culverts/sewers) (both 17 respondents, 40%) were the other geologic objectives with more than a third of respondents. Geological Investigation Objective Number Percent Evaluation of rock: Depth to bedrock 38 90 Topography of bedrock 32 76 Faulting in bedrock 7 17 Fractures in bedrock 12 29 Mapping bedrock strength (i.e., rippability) 23 55 Mapping weak zones in bedrock (e.g., shear zones or weathered areas) 12 29 Mapping lithology in bedrock 5 12 Estimating rock mass stiffness (e.g., elastic modulus, shear modulus, etc.) 7 17 Estimating rock mass density 5 12 Evaluation of soil: Mapping lithology in overburden soils 18 43 Mapping sand and/or gravel deposits (i.e., borrow investigations) 7 17 Mapping clay (i.e., excavation issues for expansive or swelling clays) 1 2 Mapping unsuitable materials (e.g., rubble, organics, etc.) 11 26 Estimating soil stiffness (e.g., elastic modulus, shear modulus, etc.) 11 26 Estimating soil density 5 12 Estimating clay content 2 5 Evaluation of groundwater: Mapping groundwater table 17 40 Mapping groundwater flow 3 7 Mapping groundwater salinity 2 5 Landslide evaluation: Mapping landslide extents (laterally) 5 12 Slip surface identification and definition 10 24 Evaluation of deformations 3 7 Evaluation of sinkholes, voids, or erosion features: Karst or other dissolution features 20 48 Failed culverts/sewers 17 40 Abandoned mines 16 38 Scour features 2 5 Other: Mapping animal burrows in and under foundations 1 2 Other: ARD characterization 1 2 Other: Low-density sands above karstic bedrock 1 2 Other: Location of manmade features, piping/sinkholes 2 5 Note: ARD = acid rock drainage. Table 4. Geologic investigation objectives of agency geophysical investigations (42 responses).

30 Advancements in Use of Geophysical Methods for Transportation Projects The geologic investigation objective results presented in Table 4 indicate the objectives are largely the same as indicated by the 2006 survey by Sirles. Both surveys indicated evaluating the depth to bedrock is most common, while mapping soil lithology and potential voids were also common objectives. The 2006 survey question also found subsidence investigations to be a common application, which is consistent with the results from Table 3. Agency Practices for Application of Geophysical Methods The 43 respondents who indicated agency experience with geophysical methods were asked a series of questions about agency administrative practices related to geophysical methods. Responses to those questions are presented in this section, which covers policy and procedure documents related to geophysical methods, use of contractors for performance of geophysical methods, and agency funding mechanisms for geophysical methods. Agency Policy and Procedure Documents Related to Geophysical Methods As shown in Figure 20, of the 43 respondents who indicated their agency has experience with geophysical methods, nine (21%) indicated their agency has policies, guidelines, or procedures for application of geophysical methods. Seven of those nine agencies shared documentation of such policies, guidelines, or procedures, which are summarized in Table 5. The documents vary in their level of detail, with some specifying when specific methods are to be used and others pointing readers to other resources. In addition to the document described in Table 5, Caltrans developed a more extensive research report (Coe et al. 2018) that summarized the use of geo- physical methods for geotechnical engineering as discussed in Chapters 2 and 4. In-house versus Contractor Performance of Geophysical Methods The 43 respondents with agency geophysical experience were asked whether geophysical methods were performed in-house, by contractors, or by a mix of both in-house staff and contractors. The majority of respondents, 22 (51%), indicated a mix of both in-house staff and contractors. As shown in Figure 21, 19 respondents (44%) indicated that all geophysical test methods were performed by contractors. Only two agencies, Kansas DOT and Mississippi DOT, indicated that all geophysical test methods were performed in-house. Those methods performed by a mix of both in-house staff and contractors are listed in Appendix B3. Agencies that use contractors to perform at least some geophysical test methods rely on a variety of contract types. The distribution of contract types is shown in Figure 22, with 37 respondents. 9 21% 33 77% 1 2% Yes No I don't know. Has your agency established specific policies, guidelines, and/or procedures for application of geophysical methods? Figure 20. Agencies with policies, guidelines, or procedures related to geophysical methods (43 responses).

Agency Description of Agency Documentation of Geophysical Methods Caltrans Caltrans’s Geophysics and Geology Branch of the Division of Engineering Services has a quality management plan that documents agency objectives and practices for quality assurance related to geophysical method testing and data. In addition to administrative information, the document lists and briefly describes the ASTM standards used by the branch and provides a bibliography of useful resources. Among the appendices to the document, one contains one- page summaries of 14 methods performed by the branch and another documents equipment used by the branch. In addition, Caltrans (2019) has published a more general geotechnical manual online. The manual includes a paragraph on potential uses and benefits of geophysical methods for small and large projects. Colorado DOT A 2006 internal agency document, Geophysical Investigations for Subsurface Characterization, includes a table for selecting geophysical methods that is based on ASTM D6429-99. The rest of the document focuses on procedures and highway applications of seismic refraction. Maine DOT Maine DOT’s (2003) Bridge Design Guide references the use of geophysical methods (ground-penetrating radar and “various seismic methods”) in investigations of existing foundations under consideration for reuse. Maryland DOT The Maryland DOT ’s (2018) Pavement and Geotechnical Design Guide includes a section on geophysical investigations (Section 3.07). The section includes a table with commonly used geophysical methods for various investigation applications. The section also includes summaries of the principles, advantages, and limitations of five commonly used geophysical methods. South Carolina DOT South Carolina DOT’s (2010) Geotechnical Design Manual includes Section 5.3.10: Geophysical Testing Methods. The section includes information about eight methods: surface, downhole, and crosshole shear wave velocity methods; suspension logging; acoustic televiewer; seismic refraction; seismic reflection; and resistivity. For each method, the manual explains general principles, notes potential applications, and typically references external documents for test procedures. Virginia DOT Chapter 3 of the agency’s Manual of Instructions for the Materials Division (VDOT 2019) covers geotechnical engineering, and Section 303.02 pertains to subsurface exploration methods. The section references the U.S. Army Corps of Engineers (2001) manual EM 1110- 1-1804, Geotechnical Investigations—specifically Table 4-2, which identifies potential uses for various geophysical methods. The manual states that methods scoring 3 or 4 are “appropriate for use on VDOT projects” (p. III-23). The manual stipulates that the use of geophysical methods must be related to project requirements, must consider existing boring data, must be performed under direct supervision of a licensed professional engineer, and must be approved by the district materials engineer. The manual also lists methods useful for characterizing intermediate geomaterials. Wyoming DOT Chapter 12 of the agency’s Geology Manual focuses on geophysical methods. The chapter includes descriptions of methods for electrical resistivity, seismic refraction, and ground-penetrating radar. The chapter emphasizes rippability evaluations via seismic refraction. Table 5. Description of agency documentation of policies, guidelines, or procedures related to geophysical methods. 2 5% 19 44% 22 51% All in-house All by contractors Some in-house, some contractor Figure 21. Performance of geophysical methods by in-house staff or contractors (43 responses).

32 Advancements in Use of Geophysical Methods for Transportation Projects Just more than half of the respondents, 19 (51%), have used lump sum or firm-fixed-price contracts. The four other types of contracts listed as potential responses (unit price; indefinite delivery, indefinite quantity; cost-plus; and time and materials) had all been used by approxi- mately one-third of respondents. Funding Mechanisms for Application of Geophysical Methods Several survey questions were asked of the respondents with agency geophysical experience in an effort to determine how applications of geophysical methods are funded. First, respon- dents were asked how subsurface investigations in general are funded; this question addressed all methods of subsurface investigation, not just geophysical methods. Results are shown in Figure 23. Only five of 42 respondents (12%) indicated that subsurface investigations are funded through annual agency budget allocations. For 33 respondents (79%), other agency funds are used to fund subsurface investigations, while four respondents (9%) indicated that both annual budget allocations and other agency funds are used. Among the five agencies that fund subsurface investigations through annual agency budget allocations, two include specific allocations for geophysical investigations; the other three do Lump sum / firm fixed price Unit price (i.e. cost per subsurface profile, etc.) Indefinite delivery, indefinite quantity (IDIQ) or similar On-Call service agreement Cost-plus (a.k.a. cost reimbursement Time and materials Number of Respondents 51% 38% 30% 27% 32% 0 403530252015105 Figure 22. Contract types used for performance of geophysical test methods (37 responses). 5 12% 33 79% 4 9% Annual agency budget allocations Other agency funds Combination of agency budget allocations and other agency funds Figure 23. Agency funding sources for subsurface investigations (42 responses).

Survey Results 33 not. For the 33 agencies that fund subsurface investigations through other sources, those sources are summarized in Figure 24. Nearly every responding agency (30 of 33, or 91%) indicated that project design funds have been used to fund geophysical investigations. Project construc- tion funds have also been used commonly, by 17 of 33 respondents (52%). About one-third of respondents have used agency maintenance funds and agency emergency response funds. Three respondents indicated “other” sources of funding, with two citing a state- or district- wide contract for geophysical services and one citing research funds. Training Resources Related to Geophysical Methods All respondents, including those without agency geophysical experience, were asked a series of questions on the use of existing training resources for geophysical methods, perceived utility of resource format, and potential content for future resources. The first question asked about the use of five existing resources. The resources and responses are shown in Figure 25. The most commonly used resource is FHWA’s Application of Geophysical Methods to Highway Related Prob- lems (Wightman et al. 2004), with 26 respondents (54%) having used the resource. About half of respondents (22, or 46%) had used ASTM or AASHTO standards related to geophysical methods. These 22 respondents were asked to list the specific ASTM and AASHTO standards they had used; results are presented in Appendix B3. Nineteen respondents (40%) had used FHWA’s Every Day Counts (EDC)-5 webinar, which was introduced shortly before the survey was adminis- tered. About one-third of respondents had used the other two resources presented in the survey question, NCHRP Synthesis 357 (Sirles 2006) and Transportation Research Circular No. E-C130 (Anderson et al. 2008). For four of the five resources, the majority of respondents who had not actually used the resource were familiar with the resource. The exception was Transportation Research Circular No. E-C130, with which 20 respondents (42%) were not familiar. The respondents were also asked how likely it would be that each of three new training resources would increase their agency’s use of geophysical methods. The three potential resources included in-person training (e.g., a National Highway Institute course), an online webinar, and a Number of Respondents 0 403530252015105 Project design funds Project construction funds Agency maintenance funds Agency emergency response funds Other: State-or district-wide contract for geophysical services Other: Research funds 91% 33% 36% 6% 3% 52% Figure 24. Funding sources for geophysical methods among agencies that do not include annual agency budget allocations for subsurface investigations (33 responses).

34 Advancements in Use of Geophysical Methods for Transportation Projects Figure 25. Agency use of various training resources related to geophysical methods (48 responses). Figure 26. Respondent perception of how likely new resources would be to increase agency use of geophysical methods (NHI = National Highway Institute) (48 responses). guidance manual (e.g., a new FHWA Geotechnical Engineering Circular). Results are presented in Figure 26. The respondents generally viewed all three potential resources as likely to increase their agency’s use of geophysical methods. For each resource, at least 70% of respondents indicated that the resource would be somewhat likely or very likely to increase agency use of geophysical methods. In-person training was viewed most favorably, with 27 respondents (56%) indicating that in-person training would be very likely to increase agency use of geophysical methods and 15 respondents (31%) indicating that in-person training would be somewhat likely to increase use. An online webinar was viewed least favorably, though 35 respondents (73%) indicated that an online webinar would be somewhat likely or very likely to increase agency use of geophysical methods.

Survey Results 35 The final survey question asked respondents to indicate the perceived usefulness of five content areas for new training resources on geophysical methods. The five content areas and the per- ceived usefulness of each are presented in Figure 27. The respondents viewed all five content areas favorably, with the majority of respondents indicating that each content area would be very useful and at most one respondent (2%) indicating that any of the content areas would not be useful. The content areas viewed most favorably were (1) use and applications of geophysical methods and (2) interpretation of engineering parameters from results of geophysical methods. For both con- tent areas, 39 respondents (81%) indicated that training on the topic would be very useful. Noteworthy and Challenging Applications of Geophysical Methods Respondents with agency geophysical experience were asked to describe any noteworthy or challenging applications of geophysical methods. Results are presented in Appendix B3. The results were used to aid in the selection of agencies for the case example portion of the synthesis, which is presented in Chapter 4. Summary of Significant Findings • Forty-three of 48 responding agencies (90%) have experience with geophysical methods. This is approximately the same proportion found in the 2006 survey by Sirles. • Agency use of geophysical methods is commonly motivated by a variety of factors. Among six potential reasons listed in the survey, five were selected by more than half of the agencies with geophysical experience as having motivated them to use geophysical methods. With 77% of respondents, the most commonly selected reason was that geophysical methods can provide a subsurface image of a large mass of materials. Site access, direct measurement of engineer- ing parameters, cost-effectiveness, and quick implementation were also cited by at least half of the respondents. • Among the five agencies without geophysical experience, the most commonly cited reasons for an agency’s not using geophysical methods were technical. All five agencies indicated that Figure 27. Respondent perception of how useful various content areas would be for new training or guidance resources (48 responses).

36 Advancements in Use of Geophysical Methods for Transportation Projects agency engineers were unfamiliar with geophysical methods, and four of five (80%) indicated that agency engineers did not know how to interpret geophysical results. Multiple respondents also selected practical challenges—contracting difficulties and a lack of local contractors or in-house expertise—as reasons for their agency’s not using geophysical methods. • The most common estimate for how frequently agencies use geophysical methods is 3 to 5 times per year, with 28% of respondents with geophysical experience selecting frequencies within this range. Among the other respondents with geophysical experience, slightly more than half indicated less frequent application of geophysical methods, whereas slightly less than half indicated more frequent use. The latter group included seven respondents who indicated that their agency uses geophysical methods more than 10 times per year. According to responses to a subsequent question, the frequency of use of geophysical methods is increasing for about half the agencies that use geophysical methods. The estimated frequency of geophysical appli- cations based on this survey is largely unchanged from the results of the 2006 survey by Sirles. • Respondents most commonly estimated average annual agency spending on geophysical methods to be less than $50,000, although about 12% of respondents estimated average annual agency geophysics spending to exceed $150,000. Although $50,000 or less was the most com- mon response, the percentage of respondents who selected $50,000 or less decreased signifi- cantly compared with the 2006 survey by Sirles, which could indicate that agency spending is increasing. • Nine geophysical methods were reported to have been used by at least one-third of respondents with knowledge of agency geophysical experience: ground-penetrating radar (34 respondents, 87%), seismic refraction (33 respondents, 85%), active source surface wave techniques (e.g., SASW, MASW; 19 respondents, 49%), optical televiewer (18 respondents, 46%), downhole seismic (17 respondents, 44%), 2D resistivity profiling (16 respondents, 41%), passive surface wave techniques (e.g., ReMi; 15 respondents, 38%), crosshole seismic (15 respondents, 38%), and 2D resistivity imaging (13 respondents, 33%). • The most commonly used geophysical methods from the 2006 survey by Sirles were seismic, GPR, resistivity, and borehole logging—a list that is consistent with this survey. However, this survey found a considerably greater proportion of agencies with experience using seismic refraction, surface wave methods (active and passive), electrical tomography, microgravity, and both optical and acoustic televiewer methods. • Survey results indicate agencies implement geophysical methods for a relatively wide variety of applications, with some more common than others but none dominating. The five applications reported by at least 40% of respondents are evaluation of roadway subsidence, routine design of bridge foundations, routine design of embankments or cut slopes, evaluation of seismic side effects, and utility location. • Geologic investigation objectives pertaining to rock—especially to determine the depth to bedrock, bedrock topography, and bedrock rippability—dominated the use of geophysical methods. Objectives pertaining to evaluation of sinkholes, voids, or erosion features; mapping overburden lithology; and mapping the groundwater table were also reported by at least a third of respondents with agency geophysical experience. • Nine agencies have established specific policies, guidelines, or procedures for application of geophysical methods. Seven agencies shared documentation of such policies, guidelines, or procedures for review. Most of the documents included general information on geophysical methods and commonly referenced external documents (e.g., the ASTM standards, FHWA’s Application of Geophysical Methods to Highway Related Problems [Wightman et al. 2004], or the U.S. Army Corps of Engineers’ Geotechnical Investigations manual [2001]) for more detailed information. • 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 geophysical test methods using contractors.

Survey Results 37 • The most common source of funding for geophysical investigations is project design funds, with 30 of 33 respondents indicating such funds had been used. Project construction funds were also used by more than half the respondents. Five agencies have annual agency budget allocations specifically for geophysical investigations. • About half of respondents had used three resources related to geophysical methods: FHWA’s Application of Geophysical Methods to Highway Related Problems (Wightman et al. 2004), ASTM or AASHTO standards pertaining geophysical methods, and the EDC-5 webinar on geophysical methods. Most of the respondents who had not used these resources were familiar with them. • Responses to two questions about potential new training resources were highly skewed toward a favorable view of new training resources. One question asked how likely it would be that each of three resources would increase agency use of geophysics: in-person training, an online webinar, and a guidance manual. For all three potential formats, at least 70% of respondents said the new resource would be somewhat likely or very likely to increase agency use of geo- physical methods, with in-person training viewed as the most likely. The second question asked about the perceived usefulness of five different training content areas. The majority of respon- dents indicated that all five content areas would be very useful, with (1) use and applications of geophysical methods and (2) interpretation of engineering parameters from geophysical results viewed as the most useful.