Subgrade Moisture under Oklahoma Highways

Data and conclusions from a 6-yr field study of subgrade moisture under existing Oklahoma highways are presented to show the effects of soil, climate, and highway design on subgrade moisture conditions. Effects of subgrade moisture conditions on expansive subgrade volume change and overall highway performance are also described. Subgrade moisture contents were found to increase, over a 2-yr. interval, from original conditions to an equilibrium value of 1.1 times to 1.3 times the subgrade plastic limit. This moisture increase caused lateral and vertical subgrade expansion, reduced subgrade soil support, and rendered the pavement system pervious to rainfall infiltration and evaporation, with resulting rapid pavement deterioration. Suggestions for design of highways on expansive Oklahoma subgrades are summarized.