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APPENDIX C
Summary of Survey Results
Geotechnical/Materials Survey Results
The survey was transmitted to 50 state departments of transportation (DOTs), out of which a total of 41 responses were
received. Salient details from these surveys are listed in the following:
Q1. In how many pavement design projects per year (average) are resilient moduli properties of subgrades and unbound
bases determined/measured?
Twenty-two of the respondents reported that they do use resilient modulus tests in routine pavement design and 19 of
them noted that they do not measure the resilient moduli properties of subgrades and unbound bases. The responses
from state DOTs are presented in Figure 3 (see chapter two). Among the 22 respondents, half (11) stated that they
perform resilient modulus tests in more than 20 pavement projects per year. Table C1 provides further details of these
responses, in particular those who noted that they do not use resilient modulus tests.
Table C1
Reasons Provided by State DOTs for not Performing Tests for Resilient Moduli
Agency Comment
Arizona ADOT uses R values only in pavement design
Arkansas Test performed on subgrade soils only
California For routine designs, no testing is done for resilient modulus
Colorado Test for R value and correlate to MR
Illinois Mostly on nonstate, or local, routes
Indiana For all projects, MR values are required
Massachusetts High classification roadways only
Minnesota Current pavement design does not use MR
Mississippi Consultant is developing a materials library of MR values
Missouri Nearly all use soil correlations
Nevada NDOT measures R value to estimate resilient moduli
New Hampshire Use of resilient modulus is being reviewed at this time
New Mexico 1993 AASHTO R value correlation
North Carolina We have run tests for 20 years but do not have confidence
South Dakota Estimated based upon liquid limit and CBR
Utah We convert everything from a CBR value
Wisconsin Primarily through FWD testing and back calculation
Q2. Please indicate the types of soil that your agency has dealt with.
The different types of soils and unbounded bases that respondents dealt with are presented in Figure 4 (see chapter
two). The respondents were asked to choose more than one type of soil/unbound base materials, and hence the total
number of responses exceeds 41. The majority of state DOTs (28 of 41 respondents) mentioned that they encounter or
use silty clay soils, and 22 respondents reported that they use crushed stone aggregates in pavement layer systems. A
few other responses about different subgrades/bases are given in Table C2.
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Table C2
Other Soil/Unbound Base Types Mentioned by State DOTs in the Surveys
Comment
Agency Subgrade Soil Types Unbound Base Types
Delaware Hot-mix millings
Maine Peat
Maryland Chemically stabilized soils
Montana Pulverized RAP/gravel
New Hampshire Reclaimed asphalt/gravel
Pennsylvania Silts (ML)
South Dakota Quartzite
Washington, DC Uncontrolled fill
Q3. Does your agency use laboratory methods for the determination of resilient moduli properties? (Check all that apply.)
Twelve respondents noted that they do use laboratory methods for the determination of resilient moduli properties.
Q4. Who is responsible for performing resilient modulus (to be used for pavement design practice) laboratory tests?
Among those who responded positively, eight respondents noted that geotechnical/materials laboratories are the
responsible organization for performing resilient modulus tests. Three respondents noted that they use outside
laboratories for these tests. Another six respondents chose other methods, which are summarized in Table C3.
Table C3
Responses on Parties That Perform Resilient Modulus Tests for State DOTs
Agency Comment
Illinois Field Falling Weight Deflectometer (FWD)
Indiana In-house research lab plus outside INDOT
Maine University (WPI) testing on 6 soil types
Minnesota Research office
New York Highway Data Services Bureau
Washington, DC Geotechnical consultants
Q5. Do you follow specific guidelines regarding the number of tests to be performed per volume of the subgrade/unbound
base or length of the highway?
Seven and three respondents follow specific guidelines regarding the number of tests to be performed per volume of the
subgrade and unbound bases or length of the highway, respectively. Details of the guidelines for subgrades included one
test per mile of roadway, one test per project per new pavement, two to six projects per year, and tests on soil samples
when the soils vary.
Q6. What laboratory method is typically used?
Nine respondents reported that they use repeated load triaxial test to measure resilient moduli of soil samples. Five
respondents noted that they use correlations with California Bearing Ratio (CBR) and R values. The AASHTO T-307
guide was followed by four state DOT respondents for determining the resilient modulus properties.
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Q7. What procedure is followed in performing laboratory repeated load triaxial tests?
One respondent mentioned AASHTO T-294, four respondents noted AASHTO T-307, one follows TP-46 procedure,
another two follow the NCHRP 1-28 A harmonized procedure, and one respondent follows modified resilient modulus
test methods.
Q8. Provide more details about the test procedure followed by your agency.
No specific details are given by the respondents, and those who responded noted that they follow the test procedure
according to the standard method.
Q9. Provide details on the laboratory specimens used in the resilient moduli tests. (Check all that apply.)
Eight and four state DOT respondents use laboratory-fabricated specimens for new and rehabilitated pavement design,
respectively. Please see Figure 5 in chapter two for these details.
Q10. Describe the laboratory procedures followed for specimen preparation. (Check all that apply.)
Among the respondents, they follow impact compaction (four), static compaction (three), and vibratory compaction
methods (two) to prepare laboratory specimens for MR testing. Please see Figure 6 in chapter two for more details.
Q11. Is any moisture conditioning of specimens performed prior to resilient modulus testing?
Four respondents noted that they consider moisture conditioning of specimens before resilient modulus testing. No
moisture conditioning is mentioned in the standard test procedures for resilient properties.
Q12. How many laboratory tests are performed for each subgrade and base location to establish average moduli
properties?
Two respondents noted that they perform three tests for subgrade. Other agencies reported that the number of tests
varies and depends on their engineering judgment. Arkansas State Highway and Transportation Department typically
uses one test per one mile of roadway.
Q13. Which regression model form is used to analyze the laboratory resilient moduli results?
Among those that responded, Washington State uses a theta model (MR = k1 × θk2), Kansas DOT uses a deviatoric
stress model (MR = k 3 × σd k4), and Maryland State Highway Administration uses a three-parameter regression model
(MR = k1 × (Sc) k2 × (S3) k3).
Q14. How do you determine the field resilient moduli (for pavement design) from the laboratory test results?
Only one respondent noted using a regression model with field-confining and deviatoric stresses to determine resilient
moduli. Another respondent applied field-confining and deviatoric stresses in the laboratory. Other responses are
presented in Table C4.
Table C4
Details of Other Methods to Determine Field Resilient Moduli from Laboratory Test Results
Agency Comment
Arkansas Design Value is lowest MR lab value.
Indiana With representative confining stress and deviator stress
Maryland Apply 85th percentile of saturated results (on the low end)
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Q15. What are the problems of the laboratory resilient modulus tests? (Check all that apply and rate the top three of them
by indicating 1, 2, and 3 in the comment box.)
The majority of state DOTs (6 of 41) responded that they are unsure whether this method provides true modulus of
subgrade in the field. The overall responses for this question are presented in Figure 7 in chapter two.
Q16. What are the strong points of the laboratory resilient modulus tests?
Four respondents indicated that the laboratory resilient modulus tests are better test methods, and two respondents
reported that these tests are a better indicator of field performance.
Q17. Does your agency use field methods for the determination of resilient moduli properties? (Check all that apply.)
Sixteen of 41 respondents stated that their agency performs field tests to determine the resilient moduli properties of
soils.
Q18. Check the methods used by your agency to determine the field subgrade and unbound base moduli. (Check all that
apply.)
Twenty-four respondents noted that they use Falling Weight Deflectometer (FWD) tests to determine resilient modulus
of subgrade/unbound bases; three respondents use Dynaflect method; and one respondent uses GeoGauge methods. The
respondent from Maine noted using the pavement seismic pavement analyzer (PSPA) method for research projects.
Q19. Explain the main intent in performing field FWD tests.
Twenty state DOTs noted that the main intent in performing field FWD tests is to determine subgrade moduli for
pavement rehabilitation. Twelve state agencies indicated that FWD tests are useful in the determination of structural
coefficients of pavement layers. Only three respondents reported that the FWD test is conducted to ensure that laboratory
moduli represent field moduli.
Q20. Do you follow any specific guidelines regarding the number of FWD tests to be performed per length of the highway?
Eleven state DOTs responded that they follow specific guidelines regarding the number of FWD tests. Some respondents
provided details, and these are presented in Table C5.
Table C5
Specific Guidelines Followed by State DOTs Regarding the Number of FWD Tests
Agency Comment
Indiana 1993 AASHTO, ELMOD 5.0 software
Kansas ASTM D4695
Maryland Typically multiple weight drops at ~ 9000 lb MDSHA 2006 Pavement Design Guide
Massachusetts Project and condition dependent
Mississippi 200-ft test interval
Missouri Performed more for research
Montana Network Level-250m, Project-100m
New York Mostly follow LTPP Guide.
North Dakota Urban every 50'; Rural every 200'
Ohio DOT ODOT Pavement Design and Rehabilitation Manual
Washington See e-mail
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Q21a. Please provide the following information related to FWD tests and analysis.
The information provided by state DOTs relating to FWD tests (manufacturer of FWD) and analysis (interpretation of
results) are summarized in Table C6.
Table C6
Details of FWD Tests and Analysis
Comments
Responsible division for Responsible personnel/division
Agency Backcalculation software performing FWD tests for analyzing FWD tests
Colorado — Materials Region pavement designer
DC — Consultant Consultant
Illinois Illi-Pave Pavement Tech Unit Same as above
Kansas In-house spreadsheets Pavement Evaluation Pavement Design
Maine DARWin 3.1 Planning Planning
Pavement & Geotechnical
Maryland Deflexus Field Explorations Division
Division
Massachusetts Proprietary or AASHTO Pavement Pavement
Mississippi ELMOD Research Division MS DOT Research Division
Missouri Evercalc Construction and Materials Construction and Materials
Materials/Pavement Analysis/NDT
Montana Modulus Materials/Pavement
Testing
Nevada Modulus Materials Materials
New Mexico Jils Materials Pavements
North Dakota ELMOD Materials and Research Materials and Research Division
New York DELMAT— NYSDOT Procedure Pavement Management Section Pavement Management Section
Ohio In-house program Pavement Engineering Pavement Engineering
Washington WSDOT EVERCALC Pavements division Pavement division
In-house program based on 1993 Pavement Design Engineer/
Wyoming Field Services/Materials
Guide Materials
Q21b. Please check mark the following that can be described as limitations of FWD methods. (Check all that apply and rate
the top three of them by indicating 1, 2, and 3 in the comment box next to each response.)
Please see Figures 8 and 9 (in chapter two) for the responses on the limitations of FWD methods for both subgrade and
unbound bases by responding state DOTs.
Q22. What are the strong points of the FWD tests? (Check all that apply.)
The majority of respondents (18 of 41) noted that the FWD tests are faster test methods. Fourteen respondents indicated
this method as inexpensive (see Figure 10 in chapter two). Again, the respondents are asked to select more than one
choice. As a result, the total number of the responses will exceed 41. Maryland State Highway Administration stated
that this test can be performed in many test locations (150 per project). Ohio DOT pointed out the repeatability of these
test results. Montana DOT states that this method would be good when used in conjunction with ground-penetrating
radar (GPR).
Q23. Do you recommend the use of your FWD backcalculation software for other DOTs for pavement layer moduli
determination?
Fourteen respondents agreed to share their FWD backcalculation software with other DOTs.
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Q24. In your agency, are there any specific guidelines on how to interpret the FWD moduli? If the answer is yes, please
provide the reference details and how we can get that information.
The information from a few state DOTs is summarized in Table C7.
Table C7
Specific Guidelines for Interpreting FWD Moduli
Agency Comment
Colorado Yes—use DARWIN
Indiana 1993 AASHTO Guide
Maryland MDSHA 2006 Pavement Design Guide
New Mexico R-value shift relative to lab results
New York Yes. The method is under review
Questions Related to Other Nondestructive Tests (Dynaflect/seismic methods/GeoGauge/others):
Q25. Explain the main intent in performing these tests.
Only one respondent indicated using other nondestructive tests for quality control/quality assessment (QC/QA) studies
and for indirect compaction quality evaluation for new pavement construction. Another respondent noted that they
use these tests for the determination of subgrade moduli for pavement rehabilitation. Two respondents stated that they
perform these tests to determine the structural coefficients of pavement layers.
Q26. Do you follow specific guidelines regarding the number of tests to be performed per volume of the subgrade/unbound
base or length of the highway?
One respondent mentioned that they follow specific guidelines regarding the number of tests to be performed. The
respondent from North Dakota reported that they perform these tests at 50-ft intervals in urban areas and in 200-ft
intervals in rural areas.
Q27. Do you recommend this method for other DOTs for determination of layer moduli?
Out of three responses received for this question, one respondent agreed to recommend this method for other DOTs,
and two respondents did not agree to recommend this method for other DOTs.
Q28. Please check mark the following that can be described as limitations of these methods. (Check all that apply and rate
the top three of them by indicating 1, 2, and 3.)
The limitations identified by respondents for subgrades and unbound bases are presented in Figures C1a and C1b,
respectively. The majority of state DOTs that responded opined that their major limitation would be the requirement of
skilled personnel to analyze the test results. Respondents from Ohio mentioned that both nonlinearity of soil modulus
and load magnitude applied in the test as limitations of these methods.
Q29. What are the strong points of these nondestructive tests? (Check all that apply.)
According to four respondents, nondestructive tests are observed to be faster test methods. Two respondents noted
that these methods are inexpensive, while another two noted that these methods provide results that are not affected
by boundary conditions. Another two respondents reveal that these methods provide reliable structural numbers for
pavement design.
Q30. Overall, how do you assess the nondestructive field tests for moduli determination?
Three respondents noted that these test procedures are well established, whereas another three respondents pointed out
poor reproducibility problems and the need for research on analysis routines.
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FIGURE C1. Limitations of other nondestructive tests responses by state DOTs.
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Q31. Does your agency use empirical or semiempirical correlations for the determination of resilient moduli properties?
Fourteen respondents stated that they use empirical or semiempirical correlations.
Q32. What types of correlations are used to determine the resilient moduli?
Direct correlations between resilient modulus and other soil properties are used for both subgrades and unbound bases
by eight and six respondents, respectively (see Figure 11 in chapter two). Other correlations used by different DOTs
are presented in Tables C8a and C8b.
Table C8
Other Correlations Used for Determining Resilient Moduli by State DOTs
(a) Subgrades (b) Unbound Bases
Agency Correlation Type Agency Correlation Type
Colorado R-value correlate to MR Colorado R-value correlate to MR
Montana MDT uses R-Value testing Louisiana Not at this time
Nevada MR = f(R-Value) Montana MDT uses R-Value testing
New Mexico MR = 1,000 + (555)(R-value) New Mexico MR = 1,000 + (555)(R-value)
North Dakota FWD North Dakota FWD
Pennsylvania CBR correlations Ohio Estimated from research results
Utah CBR Testing is used
Q33. How were these correlations developed? Please provide additional details including the reference details, if
available.
State DOTs (seven responded) reported that they use correlations recommended by the AASHTO design guide, and
six state DOTs developed their own correlations locally using the database collected over several years. Only two
respondents use the correlations drawn from the literature. Additional information provided by different agencies
related to these correlations is summarized in Table C9.
Table C9
Additional Information Related to Empirical Correlations
Not local, from the Not local, from the Local, developed by
papers published in recommended the database collected
Agency the literature AASHTO design guide over several years
Colorado 1972 AASHTO Guide
Indiana 1993 AASHTO Guide
Yes, correlation with
Kansas
KS Triaxial Method
Correlations between Soil
Louisiana
Support Value and MR
Massachusetts 1993 Guide and MEPDG
Mississippi Materials Library
Missouri MEPDG
Montana NCHRP No.128
Nevada In house MR – R correlations
New Mexico 1993 AASHTO Guide
New York NYSDOT & CORNELL
Combination of ’93
Utah
guide and MEPDG
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Q34. How do you characterize the level of reliability of the correlations used by your DOT?
The level of reliability of the correlations by state DOTs can be found in Figure 12 (in chapter two). The majority of
agencies characterized the level of reliability of these correlations as fair for both subgrades (seven) and unbound bases
(eight). Few agencies noted these methods as very good to good.
Q35. Do you check the correlation predictions by performing additional tests (laboratory or field nondestructive tests). If
the answer is yes, please list type(s) of tests conducted.
Additional tests for evaluation are performed by five respondents for subgrades and by three respondents for unbound
bases. Kansas stated that they verify the correlation predictions if they have field FWD data. Colorado specified that
they perform plasticity index, gradation, density, and moisture tests to cross-check the correlation predictions.
Q36. Are the local correlations updated frequently? If the answer is yes, please specify the frequency time period for the
updates?
Only one respondent noted updating their local correlations. The majority of respondents (13 respondents for subgrade
and nine respondents for unbound bases) stated that they do not update their correlations.
Q37. Overall, please check mark the following that can be described as limitations of these methods. (Check all that apply
and rate the top three of them by indicating 1, 2, and 3):
Figures 13a and 13b of chapter two present the limitations as identified by the DOT for subgrades and bases, respectively.
The majority of state DOTs opined that the correlations were developed from a limited database.
Final Summary Questions for Geotechnical/Materials Engineer
Q38. Overall, please rate your satisfaction (could be your pavement design group) with respect to the methods followed to
determine resilient properties of soils/unbound bases:
Twelve and 10 respondents are satisfied with the existing methods to determine resilient moduli properties of
subgrades and unbound bases, respectively (see Figure 14 in Chapter 2). They also noted that these methods could still
be improved.
Q39. If the answer is not satisfied or satisfied but methods could still be improved, please cite the reasons for your responses.
Figure 15 in chapter two presents a summary of various levels of satisfaction for the methods used for determining MR.
Table C10 lists a few of the reasons for unsatisfactory responses.
Table C10
Other Reasons for No Satisfaction in Current Methods
Agency Comments
Kansas Correlation between lab and FWD
Minnesota Data quality check procedures
New Mexico Empirical
South Dakota Interested in looking at new technology
Q40. Please identify any other issues pertaining to resilient properties of subgrades/unbound bases that you feel should be
addressed in this synthesis.
Table C11 presents a few of the issues pertaining to resilient modulus of subgrades/unbound bases expressed by the
state DOTs.
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Table C11
Other Issues Pertaining to Resilient Modulus of Subgrade/Unbound Bases
Agency Comments
Arkansas Procedures should be developed to determine a recommended DESIGN MR Value from the laboratory test
results.
California We do not test for MR for routine designs and that is why most questions were left unanswered.
Colorado FWD obtains MR values for each drop; MR values as input to rigid pavement design & do away with sub-
grade modulus k-value.
Connecticut A research study was recently completed to address resilient properties. We are evaluating whether these
range values/correlations need to be implemented.
Delaware We have just started resilient testing.
Indiana Most engineers do not care about the stress level. This should be pointed out in the synthesis study. Local
data should be collected.
Maryland A new pavement design methodology was recently introduced (MDSHA 2006 Pavement Design Guide).
Minnesota 1. How many LVDTs are needed to get accurate results;
2. Need to develop a set of data quality procedure to ensure high quality modulus;
3. How to relate lab MR to the field MR.
Missouri Although we have not directly measured subgrade or unbound granular base resilient modulus with triaxial
testing (T 307) for any previous projects, the U. of Missouri is in the midst of performing this test (at
optimum and wet of optimum) on 30 subgrade soils and 5 granular base materials that are representative of
Missouri sources. These results will be used to create a materials database library in the MEPDG for use in
project designs.
Montana MDT is considering using resilient modulus testing in the future. It is believed that the MR testing will pro-
vide better inputs for the M-E Pavement Design Guide. The current R-Value testing program is outdated, but
currently MDT is still using it because of our familiarity with it and a multitude of past data and experience.
North Carolina Test repeatability has been our greatest concern. Cost of the test is also prohibitive for small projects.
Washington We have no real issues with the FWD or backcalculation process; however, improvements are still needed to
refine the procedure and to improve the accuracy and repeatability of the results.
Washington, DC No specific issues at this time
Wyoming The 1993 AASHTO Design Guide method predicts Pavement Modulus and Subgrade Modulus; however,
there are never only two layers. Perhaps it is time for a new standard to be adopted. Also, the AASHTO
T-307 procedure could use some attention. We have a test device used in a research capacity only and have
found that the equipment specified in the procedure is very difficult to acquire; in fact, we have resorted to
custom fabrication. Furthermore, detailed guidelines on interpreting the data and a precision and bias study
would be helpful.
Pavement Group Survey Results
The survey was transmitted to 50 state DOTs, and a total 40 responses were received. Salient details from these survey analy-
ses are presented in the following:
Q1. When designing pavements, which method is used primarily?
Most of the state DOTs (24 of the total DOTs contacted) mentioned that they use the 1993 AASHTO design guide
to design pavements. This was followed by seven respondents who mentioned that they use 1972 design guide. A
summary of the responses is presented in Figure 16 (in chapter two). Apart from the standard design guides, a few state
agencies, including Illinois, Washington, New York, Alaska, and Texas, mentioned that they use agency-developed
procedures. Only one agency reported using MEPDG.
Q2. What moduli property is considered true moduli for pavement design?
The majority of state DOTs use resilient modulus obtained from different methods other than direct laboratory and
field measurement. Indirect methods using CBR values, grain size/soil classifications, and R value for subgrades have
been used in correlations to estimate moduli of both subgrade and unbound bases. Figure 17 (in chapter two) shows
the number of responding state DOTs that use different methods to determined resilient moduli for pavement design.
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Table C12a and C12b provide further details of the soil properties through which the moduli of subgrades and bases
are determined.
Table C12
Moduli Determination Procedures Followed by the DOTs
(a) Subgrade (b) Unbound Bases
State DOT Agency Properties to Estimate MR Agency Comment
California (Caltrans) R-value Alabama Assigned a layer coefficient
Georgia Soil Support is correlated to California (Caltrans) R-value and gravel factor for base
soaked CBR Delaware Default
Hawaii R-value Florida Standard values for approved base
Illinois Grain size analysis. types
Maine Correlated by soil classifica- Georgia Structural Coefficient
tion/soils Hawaii R-value
Minnesota R-value Illinois Historical typical values from the
Nevada R-value design manual
New Hampshire Soil Support value Minnesota Granular Equivalent (GE)
Ohio CBR Montana We assume the base modulus is
30,000
Pennsylvania CBR
Nevada R-value
Puerto Rico Soil classification
New Hampshire Soil Support value
South Carolina Laboratory CBR
Ohio Default value
South Dakota Liquid Limit & CBR
Puerto Rico Soil class correlations with Mr
Virginia CBR values
South Carolina Structural coefficients
South Dakota Typical values from the 1993 Guide
Virginia From CBR values
Q3. Do you provide any input to Materials/Geotechnical engineers with regards to resilient modulus testing? If the answer
is no, please skip this question. If the answer is yes, please provide the following details:
The majority of respondents (33 of 41) noted that they do not provide any input to Materials/Geotechnical engineers
with regard to resilient modulus property. Other respondents (11) noted that they provide input in various forms, which
can be seen in Figure 18 (in chapter two). The total responses exceed 41 because respondents can choose more than
one answer for their response. Table 2 (in chapter two) summarizes more details about the responses of the interactions
between Pavement engineers and Geotechnical engineers.
Q4. If the stress analysis underneath a pavement shows negative (tensile) stresses either in the base or subgrade layer, what
type of recommendations do you provide to Geotechnical/Materials engineer?
Four respondents are aware of having negative (tensile) stresses either in the base or subgrade layers. The few DOTs
that responded noted that they either consider geosynthetics or use thicker pavements.
Q5. What, if any, computer programs are used by your agency for design of pavements?
Figure 19 (in chapter two) shows the number of respondents that use computer methods to design pavements. Twenty of
the 40 responding state DOTs noted that they follow the DARWIN program to design pavements. Another 16 use other
methods, such as spreadsheets and other guides, whose details are presented in Table 3 (in chapter two).
Q6. For determining the effective roadbed resilient modulus, how do you consider seasonal variations?
Figure 20 (in chapter two) depicts the total number of respondents who consider seasonal variations in determining the
resilient modulus. For determining the effective roadbed resilient modulus, three respondents use laboratory tests, four
use field FWD tests, and 15 use other methods, which are summarized in Table 4 (chapter two).
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Q7. For characterizing the structural coefficients of unbound base, which procedures do you follow? and
Q8. For characterizing the structural support of subgrades (either in the form of a SN or others), which procedures do you
follow?
The percentage of respondents for both bases and subgrades are shown in Figure 21 (see chapter two). The majority
of state DOTs (17 respondents) use local correlations; six DOTs use the 1993 AASHTO design guide for determining
structural coefficients. Tables C13 and C14 summarize the written responses received from state DOTs with respect to
characterization of structural coefficients of unbound bases and subgrades, respectively.
Table C13
Responses on Characterization of Structural Coefficients of Unbound Bases
Agency Response
Alabama Coefficients were established soon after AASHO Road Test and have not been varied. They were based
on CBR and material characteristics.
Alaska Reduction Factors are applied.
California (Caltrans) We use known gravel factor for bases (like layer coefficient in AASHTO). This is a limitation of course
because it restrict us from using new materials. Only traditional materials that we know their gravel
factors are used.
Colorado CDOT has a list of coefficients based on R-Value.
Florida Use standard base material type values that were developed through lab tests, plate load pit tests, and
then field test sections to develop structural coefficients.
Georgia Historically established structural coefficients
Illinois Do not use structural coefficients
Indiana For unbound base, we use layer coefficient as 0.14.
Maine AASHTO 1993
Michigan We use the structural coefficients recommended by the AASHO Road Test.
Minnesota Granular Equivalent chart
Mississippi Use a value of 0.09
Montana We give new unbound base a SC of 0.14 per in. We reduce the SC of existing base using the gradation
(i.e., P200) and adjust the SC based on the infiltration of fines.
Nevada 1993 AASHTO
New Hampshire Layer coefficients were determined by research.
New York We use NYSDOT Thickness Table for both PCC and HMA pavement.
North Carolina Unbound base coefficient is 0.13
North Dakota Selected from range given in 1993 AASHTO Design Guide.
Ohio Unbound base is assigned a structural coefficient of 0.14.
Pennsylvania Structural coefficients of unbound bases are set for all dense bases and open graded bases.
Puerto Rico 1993 AASHTO Design Guide
South Carolina Our structural coefficients were developed in the late 1960s based on full-scale static deflection testing
of test pavements compared to reference pavements constructed with AASHO Road Test material.
South Dakota Suggested values from the 1993 guide
Texas Backcalculation of in situ modulus using deflection data.
Utah Defaults in 1993 guide
Vermont We have several FWD test results for pavement materials. If we try something new, we attempt to esti-
mate it using our collective engineering judgment.
Washington For rehabilitation—backcalculation for determining in-situ stiffness & LF;
For new design—established values based on experience.
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Table C14
Responses on Characterization of Structural Coefficients/Numbers of Subgrades
Agency Response
Alabama AASHTO T 307-99, Resilient Modulus of Subgrade Soils and Untreated Base/Subbase Materials
Alaska FWD Stiffness Values
California (Caltrans) We use the California test method for R-value.
Colorado CDOT has a list of coefficients based on R-Value.
Florida Lab MR testing for new construction of existing alignment and potential borrow pits. FWD backcalculation
of MR for existing alignments.
Georgia Soil Support is correlated to soaked CBR.
Indiana CBR or Resilient modulus from lab test
Maine AASHTO 1993
Michigan We backcalculate a resilient modulus from FWD using the MICHBACK program or use testing done in
the 1970s/1980s on typical soil types.
Minnesota Granular Equivalent chart
Mississippi Use double correlation:& LF;
1. Use soil classification to get soaked CBR&LF;
2. Use CBR to get soil support value for flexible and “k” value for rigid.
Montana R-Value testing correlations for resilient modulus. We will use FWD modulus as a supplement to the
R-Value modulus.
Nevada 1993 AASHTO
New Hampshire Currently, we do not characterize subgrades on a project specific basis. We assume a Soil Support Value
of 4.5 for all cases. Based on the types of soil that we have this is conservative but it’s our current practice.
New York NYSDOT procedure which was developed based on AASHTO 1993 Guide.
North Dakota Selected from range given in 1993 AASHTO Design Guide
Ohio Group index and atteberg limits are used to estimate CBR. CBR is used to estimate MR.
Oklahoma Resilient Modulus as determined by AASHTO T-307
Pennsylvania CBR
PR A&TA 1993 AASHTO Design Guide
South Carolina The design SSV is based on laboratory CBR testing of the predominant soil types expected on the project and
adjusted by the Geotechnical Materials Engineer to account for the potential variability on a given project.
South Dakota Suggested values from the 1993 guide
Texas Backcalculation of in situ modulus using deflection data.
Utah Defaults in 1993 guide
Vermont Subgrade, SN = 0
Virginia AASHTO
Q9. Overall, please rate your satisfaction (response should be in terms of design group, not in terms of an individual) with
respect to the use of resilient properties of soils used in the pavement design:
Figure 22 (see chapter two) presents the number of responses for various satisfaction levels derived from the use of
resilient properties in the pavement design. Half of the respondents are satisfied with the use of resilient properties
of subgrades in designing pavements. Twelve respondents expressed satisfaction with respect to the use of resilient
properties of bases for the pavement design.
Q10. If the answer is not satisfied or satisfied but could be improved, please mark the reasons for your responses. Also,
please rank the top three of your choices.
The reasons for being not satisfied with using resilient modulus properties in pavement design are further explored.
These responses are summarized in Figures 23 and 24 (in chapter two) for subgrades and bases, respectively. The
majority of respondents attributed reasons for their dissatisfaction to the complicated laboratory or field test procedures
and complicated correlations required to determine the moduli of both subgrades and unbound bases.
