Lane/shoulder drop-off or heave due to expansive soils beneath is a common distress in both concrete and asphalt pavements, resulting in substantial discomfort, safety hazard and vehicle damage. The situation becomes worse when exposed to moisture changes, particularly in Texas, Colorado,
Arizona, California, etc. where many soils are expansive soils. Therefore, expansive soils pose great challenges for the design of pavements and foundations. One of the most difficult issues for designing pavements and foundations on expansive soils is to predict the volume change of the soils.
In this study, current methods for movement predictions are summarised and their relationships and shortcomings are discussed. It is found that all these methods have the same theoretical basis. Based on the theory of unsaturated soil mechanics, it is found that the water content and the mechanical
stress can be used to determine the soil status. Hence, a void ratio versus mechanical stress and water content surface is constructed, which coupled both mechanical stress and suction's influences on volume change of expansive soils. The new surface is developed into a coupled water content
method that can be used to predict the potential vertical swell and the potential vertical shrink simultaneously, while all the existing methods can only predict the potential vertical rise. The method is used to analyse the data collected from a construction site at Arlington, Texas. The
predicted movements match the measured data reasonably well. The method is simple and overcomes the shortcomings existing in the current movement prediction methods for expansive soils.
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