| Prediction of equilibrium moisture beneath covered areas is an important aspect in highway, building, and airfield designs, especially when developing arid or semi-arid regions that involve unsaturated soils. The complex non-linear nature of unsaturated soil properties and the difficulty in measuring soil suction have limited the practice in this area in the past. The purpose of this research was to investigate the unsaturated soil moisture behavior beneath pavements and search for a reliable moisture prediction method that could be effectively used by practicing engineers.; The findings of the project titled “Environmental Effects in Pavement Mix and Structural Design,” sponsored by the National Cooperative Highway Research Program (NCHRP), provided the basis for this study. Soil samples collected from beneath the pavement of 29 Long-Term Pavement Performance (LTPP) sites located throughout the United States were subjected to suction measurements, determination of moisture content, dry density, index properties, specific gravity, and saturated hydraulic conductivity. The existing database containing soil-water characteristic curves (SWCCs) was enhanced with over 90 experimentally determined curves. As part of the research, a pressure plate device capable of simulating the overburden pressure and tracking the moisture and density changes with change in suction was developed and calibrated.; Based on the experimental data, two models were developed to predict equilibrium matric suction beneath paved areas by relating percentage of fines in unbound materials and a climatic index called Thornthwaite Moisture Index (TMI). Also, modifications were made to an existing family of SWCCs by introducing new correlation parameters based on soil index properties.; Measured moisture contents of the 29 sites visited were compared with the values predicted from the new models as part of the study. The predicted values agreed fairly well with the directly measured field values. In addition, field measured Time Domain Reflectometry moisture contents were compared to the model predictions. This agreement was somewhat of lower quality.; As part of the NCHRP 9-23 project, the new moisture prediction models and the improved SWCCs are to be incorporated into the Integrated Climatic Model (ICM). ICM is a one-dimensional coupled heat and moisture flow program developed for the Federal Highway Administration that is incorporated into the 2002 Guide for Pavement Analysis and Design. |
