Numerical Simulation of Moisture Fluctuations in Unsaturated Expansive Clay, Heave/Settlement Predictions, and Validation with Field Measurements

Expansive soil problem is one of the major problems in the field of geotechnical engineering. The change in water content in unsaturated expansive soil is the major reason of heave or shrinkage of the soil. Numerical simulation is a valuable tool for analysis of water content or suction variation in unsaturated soil. Numerically solving unsaturated soil problems needs the assessment of moisture variation in time and space as a response to the variation of environmental factors such as rainfall and evaporation rate. This study involves a comparison of measured water contents of an expansive clay subgrade beneath a flexible pavement in the country road FM 2 project by TXDOT with numerical simulations conducted using VADOSE/W (Geo-Slope 2012). With the input of soil properties, climate data, and proper initial and boundary conditions, the numerical simulation results gave a good approximation of the water content variations as compared with the field measurements. Both numerical results and field measurements show that significant moisture fluctuations occurred in the clay subgrade under the unpaved shoulder and in the soil below the ditch, while moisture fluctuation was negligible in the subgrade below the pavement. This difference in water content change indicates the probable cause of the volume change as well as the longitudinal crack. After the model calibration and validation on the water content change, the heave/shrinkage of the subgrade soil was evaluated following the empirical equations based on the one-dimensional deformation assumption. For heave or shrinkage prediction the extreme weather (extreme dry and extreme wet period) conditions were considered. This study is of value in the sense that, the numerical results reached a good agreement with the measured moisture fluctuations. The research outcome indicates that VADOSE/W is a robust tool, capable of predicting the moisture content fluctuations below pavement with proper soil properties and initial and boundary conditions specified. This study highlights the convincing capacity of numerical simulations for predicting the moisture fluctuations in expansive soils. The analysis is simple but efficient for use in conventional engineering practice for designs of pavement and shallow foundation on expansive soils.