Effect of Subgrade Conditions on Pavement Analysis and Performance Prediction: A Study for Idaho Conditions
Date of Final Oral Examination (Defense)
Type of Culminating Activity
Master of Science in Civil Engineering
Debakanta Mishra, Ph.D.
Bhaskar Chittoori, Ph.D.
Emad Kassem, Ph.D.
The Mechanistic-Empirical (M-E) pavement design approach detailed in the Mechanistic-Empirical Pavement Design Guide (MEPDG), and subsequently implemented through AASHTOWare® Pavement ME Design relies extensively on detailed material properties that ultimately govern the analysis and performance prediction results. For unbound materials like soils and aggregates, Resilient Modulus (MR) is the most critical input parameter affecting layer response under vehicular and environmental loading. Representing a material’s ability to ‘recover’ after loading, resilient modulus is determined in the laboratory through repeated load triaxial testing. Although the original test protocol to measure the resilient modulus value of a soil or aggregate was developed back in the 1980’s, this test is still not widely used by state highway agencies because it is cumbersome, and requires significant investments towards equipment and personnel training. Accordingly, most agencies rely on correlation equations to predict the resilient modulus values for soils and aggregates from other easy-to-determine material properties. However, these correlation equations are mostly region specific, and therefore, do not produce adequate results across different geographic regions. This has led several state highway agencies to undertake local calibration efforts for improved prediction of material properties.
Over the past decade, the Idaho Transportation Department (ITD) has invested significant resources to facilitate state-wide implementation of mechanistic-empirical pavement design practices. A research study was recently undertaken by ITD to develop a database of resilient modulus properties for different soils and aggregates commonly used in the state of Idaho for pavement applications. Another objective of the study was to assess the adequacy of different correlation equations currently available to predict soil and aggregate resilient modulus from easy-to-determine material (strength and index) properties. This Master’s thesis is based on tasks carried out under the scope of the above-mentioned project, and focuses on laboratory characterization and analysis of representative subgrade soil types collected from across Idaho. An extensive laboratory test matrix was developed involving commonly used mechanical and index tests, repeated load triaxial tests for resilient modulus determination, as well as tests to study the soil permanent deformation (plastic strain) behavior. Effect of moisture variation on soil strength, modulus, and permanent deformation properties was also studied by testing soil specimens at three different moisture contents. The test results were thoroughly analyzed to evaluate the feasibility of predicting resilient modulus from other material properties.
Findings from this research effort have been documented in the form of two journal manuscripts. The first manuscript highlights the importance of using adequate subgrade resilient modulus values during pavement design. Eight different soil types were randomly selected from a total of sixteen soil types, and the corresponding laboratory test results were used to highlight the limitations of ITD’s current approach with assumed resilient modulus values. The second manuscript focuses on highlighting the importance of unbound material permanent deformation characterization during pavement design, and how small changes in moisture content can lead to significant differences in the rutting behavior of subgrade soils. First, a new permanent deformation testing protocol was developed to simulate typical stress states experienced by subgrade layers under vehicular loading. Subsequently, permanent deformation tests were carried out on subgrade soil types collected from two distinctly different regions in Idaho as far as annual precipitation is concerned. Tests were conducted at three different moisture contents to highlight how the rutting potential of the subgrade may change significantly based on site precipitation and drainage characteristics. Finally, recommendations were made regarding how state highway agencies can accurately represent resilient modulus properties of soils during pavement analysis and performance prediction using AASHTOWare® Pavement ME Design.
Jibon, Md, "Effect of Subgrade Conditions on Pavement Analysis and Performance Prediction: A Study for Idaho Conditions" (2019). Boise State University Theses and Dissertations. 1603.