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6 CHAPTER TWO SUMMARY OF SURVEY RESPONSES Introduction surveys from various entities. The program then analyzes the gathered results in a professional format using bar charts and This chapter details the survey approach followed for this pie charts. The unique link for each state survey was created synthesis in gathering various details and specific infor- and e-mailed to materials/geotechnical engineers from all mation regarding the resilient properties of subgrades and 50 state DOTs. Each respondent received a special coding to unbound bases among state DOTs. The survey had two start providing responses to survey questions. components: one component was prepared for materials/ geotechnical engineers and the other one was prepared for Based on the responses, a few respondents were contacted pavement design engineers. for additional questions or a brief interview to review certain responses. These interviews were conducted to clarify any Test procedures followed, and test details (including the discrepancies found in the questionnaire or to obtain addi- number of samples per test and other information), con- tional information. Agencies that did not respond within stitutive model expression, empirical and semi-empirical four weeks were contacted by the software company. For expressions for resilient properties, and field test methods for pavement design surveys, another set of surveys was mailed determining resilient properties are queried in the surveys to pavement design engineers for each DOT. prepared for materials/geotechnical engineers. Pavement design details, structural support of unbound bases, and sub- In certain cases, the initial state contacts from a few state grades were included in the surveys prepared for pavement DOTs were not responsive to the survey; some were no lon- design engineers. All the survey results collected from the ger at their jobs owing to promotions or retirement, some left state DOTs are summarized in this chapter. the agency, and some failed to respond for other reasons. In those cases, alternate representatives were sought and sur- vey e-mails were sent. In all cases, another person from the Survey Questionnaire DOT was selected to complete the survey. Appendix B pres- ents the details of individuals from various DOTs reached One of the main objectives of Synthesis Topic 38-09 was to for these surveys. gather information about how various agencies determine resilient moduli of subgrades and unbound bases and how Data were either taken from the software program or these properties have been used in the design of pavement entered into Microsoft Excel for preparing illustrations of systems. To accomplish this objective, the survey question- the survey response results. The majority of these results naire was designed in three parts. Part I requests information are presented in Appendix C. A few salient questions are from the survey respondents about their affiliation agencies. included in this chapter for explanations. Part II of the survey was to be completed by the state materi- als/geotechnical engineer or agency official most knowledge- able about material testing practices, and Part III was to be Survey Results completed by the pavement design engineer or agency offi- cial most knowledgeable about pavement design practices. Geotechnical/Materials Survey Results The entire survey questionnaire is presented in Appendix A. The survey was transmitted to 50 state DOTs, and a total of 41 responses (82%) were received. In the survey analysis Survey Procedure Steps presented here, the total number of responses (N) used is 41. Salient details of the surveys are presented in this chapter; The following steps were followed for conducting the survey. A the rest of the details are provided in Appendix C. 40-question electronic survey containing primarily multiple-​ choice-type questions was used for Part II on a Web-based The responses from state DOTs with respect to design platform. The Web-based program is a professional software projects in which resilient moduli of subgrades and bases program that is used to host surveys and gather input for the were used are summarized in Figure 3. Twenty-two DOTs

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 7 FIGURE 3 Number of DOTs that responded favorably for measurement of resilient moduli properties of subgrade/unbound bases. responded that they do use resilient modulus tests in routine Seven respondents noted that they follow specific guide- pavement design and 19 noted that they do not measure the lines regarding the number of tests to be performed per resilient moduli properties of subgrades and unbound bases. volume of the subgrade or length of the highway. Three Among the 22 respondents, 11 stated that they perform respondents mentioned the same for unbound bases. Details resilient modulus tests in more than 20 pavement projects of these guidelines included one test per mile of roadway, annually. one test per project per new pavement, and tests on soil samples when the soil types varied along the length of the Figures 4a and 4b present the number of respondents pavement. that dealt with different types of soils and unbounded bases, respectively. The respondents are asked to choose more than Nine respondents have used RLT tests to measure resil- one type of soil/unbound base material. Hence, the overall ient moduli of soil samples. In the RLT tests, four followed numbers do not add up to the total number of respondents. AASHTO T-307 procedure, and two followed the NCHRP The majority of state DOTs mentioned that they encountered 1-28 A Harmonized procedure. The remaining respondents silty clay subgrade (28 of 41 respondents) in their pavement followed T-294, TP-46, or modified resilient modulus test projects and used crushed stone aggregates (22 of 41 respon- methods to determine the MR of subgrades and bases. dents) in pavement base layers. With respect to test procedure details, eight respondents Responses Related to Laboratory Measurements prepare laboratory-fabricated specimens for new pavement projects and four use similar specimens for rehabilitated With respect to MR measurements, 12 of the 41 respondents pavement design. Four respondents noted that they used noted that they do use laboratory methods to determine undisturbed field specimens for pavement rehabilitation resilient moduli properties. Among these 12, eight respon- work (see Figure 5). The total numbers of responses is dif- dents noted that geotechnical/materials laboratories were ferent, because a few of the respondents who did not respond responsible for performing resilient modulus tests. Four to the earlier question on laboratory measurements replied respondents noted that they use either outside laboratories or to this question and a few other respondents selected more a university laboratory to perform these tests. than one choice.

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8 FIGURE 4 Types of subgrades encountered and unbound base material used in pavement systems by different state DOTs: (a) subgrade; (b) unbound bases.

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 9 FIGURE 5  Details on the laboratory specimens used in resilient moduli tests. Figure 6 presents survey responses related to specimen Figure 7 summarizes the responses with respect to prob- preparation procedures followed for laboratory testing for lems experienced in the laboratory MR testing. Most of the both subgrades and bases. For subgrades, the numbers respondents (six) noted that they are unsure whether the test responded favorably for impact compaction, static compac- method provides true modulus of subgrades for pavement tion, kneading compaction, and vibratory compaction meth- design and rehabilitation. With respect to advantages of lab- ods (4, 3, 2, and 2, respectively) and favorably as well for oratory testing, four respondents indicated that the labora- bases (1, 1, 0, and 4, respectively). tory resilient modulus tests are better test methods, whereas two respondents reported that the laboratory tests are better With respect to moisture conditioning of the soil speci- indicators of field performance. mens in resilient modulus testing, four respondents noted that they consider moisture conditioning of specimens Field MR Measurements and FWD Studies before resilient modulus testing. Moisture conditioning was not specified in the standard test procedures for the measure- Twenty-five of the 41 respondents stated that their agency ment of resilient properties. With respect to the number of performs field tests to determine the resilient moduli prop- tests per soil type, two respondents noted that they perform erties of soils. Twenty-four respondents noted that they use three tests for each subgrade type. Four others reported that FWD tests to determine resilient modulus of subgrade and the number of tests per soil type varies and depends on their unbound bases, whereas three respondents mentioned the engineering judgment. Dynaflect method and one respondent noted the GeoGauge method. In the case of other responses (three), a respondent Regarding the selection of laboratory moduli for pave- from Maine noted the use of a Portable Seismic Pavement ment design, one respondent noted using a regression model Analyzer method for their projects. with field confining pressure and deviatoric stresses to determine the design moduli for both bases and subgrades. Twenty respondents noted that the main intent in per- Another respondent applied confining and deviatoric forming field FWD tests is to determine subgrade moduli stresses simulating field conditions in laboratory testing. for pavement rehabilitation. Twelve respondents indicated Five respondents include the lowest moduli measured in the that FWD tests are useful in determining structural coef- laboratory to taking an 85th percentile value from the satu- ficients of pavement layers. Three respondents reported that rated specimen test results. the FWD test is used to ensure that laboratory moduli are

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10 FIGURE 6 Laboratory compaction procedures followed in soil specimen preparation. representative of field moduli. Eleven respondents noted 200 ft (60 m) intervals for rural roads, and 810 ft (250 m) that they follow specific guidelines regarding the number of intervals for the network level to 310 ft (100 m) for the proj- FWD tests. Some of the guidelines are reported as FWD ect level. Others mentioned the use of 9,000 lb of load to drop tests conducted at 50 ft (15 m) intervals for urban roads and on a pavement section several times. FIGURE 7  Problems related to laboratory resilient modulus tests.

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 11 Figures 8 and 9 present survey responses regarding With respect to advantages, the majority of the respon- the limitations of FWD methods for subgrades and bases, dents (18) noted that the FWD tests are faster test methods. respectively. Respondents were asked to list their top three Fourteen respondents indicated that this method is inexpen- responses to this question. Most respondents (five) men- sive (see Figure 10). Because the respondents were asked to tioned that there is no correlation among the laboratory-​ select more than one choice, the total number of responses determined moduli and that different backcalculation is higher than 41. The Maryland State Highway Adminis- software provided different moduli. tration stated that the FWD test was typically performed at FIGURE 8  Responses on limitations of FWD tests for MR backcalculation of subgrades. FIGURE 9  Responses on limitations of FWD tests for MR backcalculation of unbound bases.

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12 FIGURE 10  Responses on advantages of FWD tests. many test locations in each project (150 per project). Mon- MR Correlations tana DOT stated that the FWD method would be good when used in conjunction with ground-penetrating radar (GPR). Fourteen respondents stated that they use empirical or semi- With respect to the use of FWD moduli for pavement design, empirical correlations. Figure 11 presents various responses a few respondents noted that they use 1993 AASHTO design with respect to the use of correlations. Direct correlations guide recommendations for reducing FWD moduli to deter- between MR and other soil properties were used for both mine the resilient modulus value. subgrades and unbound bases by eight and six of the respon- dents, respectively (Figure 11). Seven respondents reported Field Tests and Nondestructive Tests Other Than FWD that they used correlations that were recommended by the AASHTO design guide and six reported that they use local One respondent stated that the agency uses other nondestruc- or their own correlations. Only two respondents used the tive tests for quality control and quality assurance (QC/QA) correlations drawn from the literature. studies for indirect compaction quality evaluation for new pavement construction. Another respondent noted that the Figure 12 shows the level of reliability of the correla- agency uses these tests to determine subgrade moduli for tions per the respondent DOTs. The majority of the agencies pavement rehabilitation. Two respondents stated that their (seven for subgrades and eight for unbound bases) charac- agency usually performs or conducts these tests to deter- terized the level of reliability of these correlations as fair. mine the structural coefficients of pavement layers. Three noted these methods as “very good” to “good.” Addi- tional tests for evaluation are performed by five respondents For the overall assessments of other nondestructive meth- for subgrades and three respondents for unbound bases. A ods for subgrades, three respondents noted that these test Kansas DOT respondent stated that they verify the corre- procedures are well established, whereas one respondent lation predictions if they have FWD field data. Colorado pointed out poor reproducibility problems and the need specified that they perform plasticity index (PI), gradation, for more research on analysis routines. From the responses density, and moisture tests to cross-check the correlation received on other nondestructive methods, more studies are predictions. The majority did not respond to this ques- still needed to better understand and validate the methods tion, implying that they do not use correlations for moduli for MR estimation. predictions.

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 13 FIGURE 11 Types of correlations used to determine the resilient moduli by state DOTs. FIGURE 12  Reliability of correlation methods characterized by state DOTs.

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14 Figures 13a and 13b present the limitations of the (10 for subgrades and seven for bases) opined that correlations as identified by the DOT for subgrades the correlations were developed from a limited and bases, respectively. The majority of the respondents database. FIGURE 13 Limitations of correlation methods to determine resilient modulus: (a) subgrades; (b) unbound bases.

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 15 Final Summary Questions for Geotechnical/Materials and unbound bases, respectively (see Figure 14). Figure 15 Engineer presents reasons for dissatisfaction with the current methods. Overall, the survey response is limited because of the lack of Sixteen and 11 of the respondents are satisfied with the existing responses. Reasons for no responses are attributed to a lack of methods to determine resilient moduli properties of subgrades familiarity with the MR, changes in test procedures, length of FIGURE 14  Responses related to satisfaction of methods to determine resilient moduli properties by state DOTs. FIGURE 15  Reasons for being not satisfied with current methods.

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16 the survey, uncertainty with new design guide, and other rea- and R value for subgrades have been used in correlations sons. Nevertheless, the available responses show the need for to estimate moduli of both subgrade and unbound bases. certain actions items such as better technology transfer, more Figure 17 shows respondent responses regarding the use of reliable test procedures without constantly changing them, methods to determine resilient moduli for pavement design. developing correlations with a stronger database, defining the design moduli parameters more accurately, and providing The majority of pavement group respondents (five) noted simple yet practical procedures for implementation. that they use a correction factor on FWD moduli to deter- mine the MR design value. Other respondents (six) noted that Pavement Group Survey Results they provide input in various forms, including laboratory test-related procedural steps, which can be seen in Figure The survey was transmitted to 50 state DOTs, and a total of 18. Table 2 summarizes survey responses from pavement 40 responses were received (80%). The number 40 is used and geotechnical engineers. here as the total respondent number in the following analy- sis. A comprehensive summary of the survey results is pro- Figure 19 shows the number of respondents that use vided in Appendix C. Important details from these survey various computer methods and programs to design flexible analyses are presented in the following discussion. pavements. A total of 20 respondents noted that they use the DARWIN program to design pavements. Sixteen respon- Most of the respondents (24) mentioned that they use the dents use other methods, such as spreadsheets and other “1993 AASHTO design guide to design pavements.” This design guides, whose details are presented in Table 3. was followed by seven respondents who mentioned that they used the 1972 design guide. Figure 16 summarizes the Figure 20 depicts the number of respondents who consider response results. Apart from the standard design guides, seasonal variations in determining the resilient modulus. For a few state agencies, including Illinois, Washington, New determining the effective roadbed resilient modulus, three York, Alaska, and Texas, mentioned that they use agency- respondents use laboratory tests, four use FWD field tests, developed procedures. One agency notably responded that it and 15 use other methods, which are summarized in Table started using MEPDG. 4. The total number of responses is higher than 40 because some respondents chose more than one option. The majority of respondents (18 for subgrades and 19 for bases) use resilient modulus obtained from different methods Figure 21 shows the number of respondents who use dif- other than direct laboratory and field measurement. Indirect ferent methods for characterizing the structural coefficients methods using CBR values, grain size and soil classifications, and structural support of bases and subgrades. The major- FIGURE 16  Methods used for designing pavements by state DOTs.

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 17 FIGURE 17  Procedures followed to determine design moduli by state DOTs for subgrades and unbound bases. Table 2 Details Provided by Pavement Engineers Related to Resilient Modulus Property Determination Comments Applied stresses Moisture conditioning Correction factor for Correction factor for Agency in the laboratory in the laboratory MR from the laboratory MR from FWD Axial stresses: 2, 4, 6, 8, A-6 & A-7 soils on wet Alabama — 0.33 and 10 psi side of optimum Currently use 1.0 but we Florida 2 psi confining pressure Optimum No correction are researching Kansas — — — 0.33 Maine — — — 0.29 (for Spring thaw) AASHTO Specimens at opt. mois- Oklahoma — — T-307 ture and at opt + 2% New York — — — Yes , no specifics ity of the respondents (17 for subgrades and 18 for bases) The reasons for being not satisfied with using resilient use local correlations, followed by those who used the 1993 modulus properties in pavement design are further explored AASHTO design guide to determine structural coefficients and these responses are summarized in Figures 23 and 24 for (six for subgrades and five for bases). More details on the subgrades and bases, respectively. The majority of respon- methods followed are included in Appendix C. dents attributed reasons for their dissatisfaction to the com- plicated laboratory or field test procedures and complicated Figure 22 presents the numbers of responses for various correlations required to determine the moduli of both sub- satisfaction levels derived from the use of resilient proper- grades and unbound bases. These responses are in agree- ties in the pavement design. The majority of the respondents ment with those expressed by materials engineers. Action (20 for subgrades and 12 for bases) are satisfied with the use items mentioned at the end of the geotechnical survey are of resilient properties in designing pavements. In compari- also valid in this case and are recommended for future son, these numbers are higher than those from the geotech- implementation. nical survey.

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18 FIGURE 18  Responses on ‘Type of Input’ provided to materials/geotechnical engineers. FIGURE 19 Number of respondents using various pavement design-related computer programs. FIGURE 20  Percent respondents on how they considered seasonal variations in determining effective resilient modulus.

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 19 Table 3 Table 4 Responses Related to Computer Methods Used in Responses on How Seasonal Variations Were Designing Pavements Considered in Determining Effective Resilient Modulus Agency Comments Agency Comment Alaska ELMOD, AK Flexible Pavement Design Guide (2004) Lowest value for material in saturated Arkansas conditions California Caltrans programs NewCon90 and CalAC Reduction factors are applied on the inter- Florida 1998 Supplement to AASHTO rigid Alaska preted moduli spreadsheet California No consideration by our empirical method Hawaii Excel spreadsheet Colorado Engineering judgment Illinois ILLI-PAVE Illinois Typical values Minnesota MnPAVE (for comparison to MnDOT method) Kansas Correlation with soil properties Mississippi In-house for flexible, 1998 Supplement By using a correction factor (0.29) on the Maine New Hampshire Excel spreadsheet interpreted moduli New York NYSDOT Procedure Michigan Spring time value is used North Carolina In-house spreadsheet Minnesota Design chart based on wet conditions Ohio ACPA’s AASHTO design software New Assume a ‘Soil Support Value’ of 4.5 Hampshire Puerto Rica ELMOD 4.5 FROM DYNATEST New York Seasonal subgrade & subbase moduli South Carolina Spreadsheet based on ‘72 Guide North Dakota FWD tests conducted in summer/fall Texas FPS-19W South Carolina Minimal seasonal effects in our climate Washington WSDOT developed M-E software (Everpave) Utah Use worst case Wisconsin WisPave Washington Developed seasonal effects in-house FIGURE 21  Percentage of respondents on how they characterize structural coefficient of unbound bases and structural support or number of subgrades.

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20 FIGURE 22  Responses related to satisfaction in using MR properties to design pavements FIGURE 23  Reasons for not being satisfied (subgrade).