Pavement sections constructed over expansive soil deposits often exhibit excessive distresses due to volume changes in the underlying soil strata. Differential movements within these deposits resulting from fluctuations in the moisture content manifest themselves in the form of localized heaves on the pavement surface. A pavement section near the western border of Idaho has experienced recurrent damage due to volume changes in the underlying expansive soil layer; traditional treatment methods such as lime stabilization and moisture barrier installation have provided partial relief over the years. A recently concluded forensic research study at Boise State University investigated the causes for failure of earlier treatment methods. This study involved extensive laboratory characterization of expansive soil samples collected from underneath this pavement section to identify location of the problematic soil strata, and to propose suitable rehabilitation measures. Laboratory characterization included tests such as moisture content, Atterberg limits and One-Dimensional swell test to determine the potential vertical rise (PVR) and establish approximate active zone. Laboratory test results indicated that the most expansive soil deposits were at a depth of at least 1.83 m from the pavement surface. PVR values calculated closely matched with the surface profile trends observed in the field. In addition, the soluble sulfate tests performed on various soil samples indicated that sulfate heaving could be a problem for calcium-based stabilizer. Based on the findings, the research team proposed that the pavement section be reinforced using a flexible mechanical system that dissipates the swell pressures originating from the underlying clay layers.
The resilient modulus (MR) of subgrade material is an important parameter in pavement design using the Mechanistic-Empirical Pavement Design Guide (MEPDG) and has a significant influence on pavement performance. MR can be obtained indirectly from falling weight deflectometer (FWD) data using a back-calculation tool (i.e., 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, 2017). In this study, the field test program included FWD tests and soil sampling. These field tests were performed on six asphalt pavement sections in South Carolina, U.S., to estimate the MR of the subgrade soil. This study involved extensive laboratory characterization of subgrade soils collected from underneath the pavement sections. Laboratory characterization included index tests (sieve analysis, Atterberg limits, specific gravity, moisture content, and standard Proctor density tests) on bulk samples and repeated load triaxial tests on thin-walled tube samples to obtain a direct measure of MR. 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 were used to determine a C-factor of 0.33, 0.25, and 0.29 for coarse-grained, fine-grained, and all types of soils, respectively. This parameter can be used to estimate resilient modulus for MEPDG Level 2 design inputs across South Carolina and similar geologic regions. The research studies will be facilitated by the local calibration and implementation of the MEPDG.
The subgrade soil stiffness, which depends on the in-situ moisture content and soil index characteristics, is a key factor in pavement rutting. Due to variations in the compaction process used during construction and seasonal changes, the subgrade soil moisture content may deviate from the desired condition. The resilient modulus (MR), an important parameter of the Mechanistic-Empirical (M-E) pavement design process, is used to specify the subgrade soil stiffness. Repeated load triaxial tests, which can be challenging and time-consuming to execute, are often used to determine MR. As a result, correlations between MR and more accessible stiffness metrics and index qualities are frequently used. California bearing ratio (CBR) and repeated load triaxial tests were carried out in this investigation. Soil specimens were fabricated at moisture levels that were both above and below the optimum moisture content (wopt). The results of the two tests were correlated, and statistical models were created to correlate the parameters of the generalized constitutive resilient modulus model with the characteristics of the soil index. Additionally, utilizing the MR found for subgrade soils compacted at wopt and ±2%wopt, pavement rutting was analyzed for three base layer types. The results demonstrated that a laboratory-measured MR (MR(Lab)) decreases as the moisture content increases. Specimens compacted at −2%wopt showed higher MR(Lab) than specimens compacted at wopt. Specimens compacted at +2%wopt showed lower MR(Lab) than specimens compacted at wopt. Results also indicated that the MR(Lab) predicted higher pavement rutting compared to field measured MR (MR(Lab)). If a stabilized aggregate foundation layer was employed instead of an untreated granular base, subgrade soil moisture condition showed a significant impact on rutting.
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