Date of Award

Fall 2023

Document Type

Open Access Dissertation


Civil and Environmental Engineering

First Advisor

Sarah Gassman


The new Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the subgrade resilient modulus (MR) as the key input parameter to represent the subgrade soil behavior for pavement design. This research focused on field and laboratory characterization of subgrade resilient modulus assessing the effects of predicted distresses and IRI on the flexible pavement. The results of this study are presented in the form of three chapters and peer-reviewed journal manuscripts. Firstly (Chapter 3), MR was obtained indirectly from Falling Weight Deflectometer (FWD) data using a back-calculation tool (i.e., SCDOT, BAKFAA, and AASHTOWare 2017) or from empirical correlations with soil index properties. MR can also be obtained directly using repeated load triaxial tests (AASHTO T-307-99). The field test program included (FWD) tests and soil sampling in this study. These field tests were performed on 15 South Carolina pavement sections (11 AC and 4 PCC pavement sections) to estimate the subgrade soil's MR. This study involved extensive laboratory characterization of subgrade soils collected underneath the pavement sections. At each pavement site, samples of subgrade soil were collected from beneath the pavement, and their geotechnical properties were evaluated in the Geotechnical Laboratory at the University of South Carolina. Tests performed include Atterberg Limits (ASTM D 4318/AASHTO T 89 and T 90), grain size analysis (ASTM D 421 and D 422/AASHTO T 87 and T 88), soil classification (ASTM D 2487/AASHTO M 145), specific gravity (ASTM D 854/AASHTO T 100), moisture content (ASTM D 2216-90/AASHTO T 265), standard Proctor compaction (ASTM D 698-78/AASHTO T 99-90), California Bearing Ratio (ASTM D 1883-87/AASHTO T 193-81), and repeated load triaxial tests (AASHTO T 307). Results show that the MR values found from the FWD data have similar trends to the laboratory-measured MR values. However, results from lab testing were 33-75% lower than the back-calculated MR. Laboratory-measured MR and back-calculated MR was used to develop a model and to determine a factor (c-factor) of 0.30, 0.35, and 0.32 for coarse-grained, fine-grained, and all types of soils, respectively. These c-factors can be used in South Carolina and other geologically similar regions as a Level 2 input in the AASHTOWare Pavement ME Design based on soil classification. The local calibration and implementation of the MEPDG will facilitate the research studies. Secondly (Chapter 4), the effect of subgrade MR on the predicted distresses and IRI for AC design per AASHTOWare Pavement Mechanistic-Empirical Design (PMED) was studied. In this study, 18 thin-walled Shelby tube samples of fine-grained subgrade soils were available from two sites. The samples were tested per AASHTO T307-99 to obtain the laboratory-measured MR. In addition, non-destructive tests (e.g., Falling Weight Deflectometer) were performed on the same sections to obtain the back-calculated MR(FWD) using the back-calculation tool (AASHTOWare 2017). A subgrade MR catalog was established to select the Input Level 2 for the AASHTOWare PMED analysis (v 2.6.1). The PMED analysis was run for 20 years. The Mechanistic-Empirical Pavement Design Guide (MEPDG) and global calibration coefficient values were used to predict asphalt concrete pavement distresses and International Roughness Index (IRI) for each pavement section. MEPDG-predicted values were compared with field-measured values to assess bias and standard error. Finally (Chapter 5), the stress conditions' effect in calculating MR for subgrade soil was studied. The resilient modulus increases with an increase in confining pressure, whereas, for an increase in deviatoric stress, it increases for granular soils and decreases for fine-grained soils. The value of MR is highly stress dependent, with the stress state (i.e., bulk stress) a function of the position of the materials in the pavement structure and applied traffic loading. Applying excessive vertical stress at the top of the subgrade without knowing the appropriate stress state can result in permanent deformation. In situ, stress must be calculated to determine the correct resilient modulus. To facilitate the implementation of MEPDG, this study develops a methodology to select the appropriate subgrade resilient modulus for predicting rutting and IRI. Results show that MR obtained from in situ stress is approximately 1.4 times higher than the MR estimate from NCHRP-285. Thus, the in situ stress significantly affects the calculation of subgrade MR and, subsequently, the use of MR in the predicted rutting, with IRI using the AASHTOWare pavement mechanistic-empirical design. Results also show that the pavement sections were classified as in “Good” and “Fair” conditions for rutting and IRI, respectively, considering in situ MR.


© 2024, Kazi Moninul Islam