Predicting Subgrade Moisture Content for Low-Volume Pavement Design Using In Situ Moisture Content Data

Subgrade soil strength and/or stiffness are major factors in the design and performance of pavements, particularly low-volume pavements. A practical method of realistically estimating in situ moisture content significantly improves the determination of the appropriate resilient modulus to be used for pavement design. Because of the variability in soil properties and soil behavior under repeated traffic loads, environmental factors, geometric factors, and site conditions, and because of the complexity of moisture movement in soils, the prediction of subgrade moisture content has been unreliable and complicated. In a study of subgrade moisture changes with time, temperature, precipitation, and depth at 18 sites across Arkansas, five general trends in subgrade moisture variation are identified. In addition, upper and lower equilibrium limits for subgrade moisture contents are estimated. These equilibrium values are independent of environmental factors and are solely dependent on soil properties and site conditions. Regression equations to predict upper and lower equilibrium values from soil properties are developed. It is shown that reasonable predictions of in situ moisture content may be developed, given the range of subgrade moisture content variation for a given soil type and the trends of moisture variation with temperature, precipitation, and depth. In addition, guidelines and issues to be considered when establishing a subgrade moisture content monitoring program are given. The information presented could provide agencies with responsibility for lowvolume roads valuable tools for obtaining reasonable estimates of subgrade moisture conditions without the need for extensive (and expensive) soil sampling and testing programs.