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Rainfall induced distress in low volume pavements
Annual expenses of distresses caused by volume change of expansive soils induce significant financial burden on maintenance authorities. The problems occur as the expansive soils within the unsaturated zone of the soil experience variation in moisture content. Swelling and shrinkage of the soils causes structural cracks, malfunction of piping and irrigation systems and loss of support for ground slabs. Due to their low stiffness, light loading and extensive construction, pavements are particularly vulnerable to the effects of expansive soils. Accurate understanding of behavior of expansive subgrade, interaction of subgrade pavement structure and incorporation of the results in design processes can improve the pavement service life significantly. The objective of this research was to; 1) study the behavior of expansive soils and their constitutive performance; 2) determine the real time pattern of subgrade moisture and temperature variation in the entire depth of active zone; 3) determine the pattern of real time pavement deformation in response to the site's moisture balance; 4) provide a comprehensive data analysis on the collected data; 5) determine the vertical deformation of the site using soil's constitutive relations; 6) model the real time moisture variation and volume change of the expansive soil; 7) determine the interaction of subgrade volume change and hot mix asphalt pavement distress; 8) determine the possible service life of the pavement under normal loading and mix design and 9) provide conclusions and recommendations for future study. A selected farm to market pavement site was selected for monitoring and field instrumentation and investigated through resistivity imaging and in-situ modulus measurement. Samples were collected and tested in the laboratory to determine the soil's properties. The site was instrumented with moisture/temperature sensors, rain gauge and inclinometers casings. The monitoring activities included regular data acquisition, horizontal profiling and topographical surveying. A comparison was drawn between the observed field volume change, current Texas Department of Transportation (TxDOT) method and soil's constitutive relations. The collected data were later analyzed to determine the possible trend of moisture and temperature variation and pavement deformation. Finite element was used to model transient moisture variation and volumetric deformation of the soil. In addition, the direct effect of volume change on pavement distress development was studied under three different surface temperatures. Results indicated that the in-situ soil is classified as CH and possesses high swelling potential. Both moisture variations extend to 4.5m depth, which exceed the current estimations of active zone. Trend of moisture variation was best described as a combination of seasonal and temporary variation. The seasonal trend consisted of sinusoidal fluctuation as a function of site's moisture balance, while the temporary increase happened in response to rainfall and dissipated rapidly. The temperature was recorded as a first degree Fourier series at all depths. Although the average temperature was roughly constant, the domain of temperature variation and lag time varied with soil depth. The pavement deformation was a mixed function of seasonal variation and excessive swelling and shrinkage in response to the available moisture infiltration. Comparing the observed variation with predictive methods, it was determined the current TxDOT method provides inaccurate measurements and can be considered effective only for under special conditions. However, estimation of swell and shrink of the soil can be acceptably done through soil constitutive modeling. The transient analysis of site's moisture and deformation analysis indicated that excessive deformation occur in the site that may not be captured by surveying, mainly because surveying is a discrete data acquisition method which misses data records between survey intervals. Excessive movement can accelerate pavement deterioration, which was confirmed with direct analysis of coupled swell/distress analysis. Provided the stress strain behavior of asphalt and volumetric deformation of the subgrade soil, it was determined that the initial cracks are most possible to occur within two years of pavement construction. Increase of asphalt stiffness did not improve pavement's lifetime; therefore, controlling the subgrade swelling potential may be more effective in pavement distress prevention.