United States Department of Transportation
i
Design highwater clearances for highway pavements : volume I, final report, August 2008.
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2008-08-01
[PDF-2.81 MB]
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Edition:Final report; Oct. 2005-Apr. 2008.
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Abstract:High groundwater table exerts detrimental effects on the roadway base and the whole pavement. Base
clearance guidelines have been developed to prevent water from entering the pavement system in order to
reduce its detrimental effects. In these guidelines a minimum height, the clearance, between a groundwater
level and a particular elevation within the pavement system is specified. This report presents an
experimental study to evaluate the effects of high groundwater and moisture on determining pavement base
clearance for granular subgrades. Full-scale in-lab test-pit tests were conducted to simulate pavement
profile and vehicle dynamic impact on the pavement. Eleven types of subgrade were tested for this study. From
the test, using layer theory, the results of the resilient modulus for each layer (layer resilient modulus)
can be compared with the resilient modulus results from laboratory test. The dominant factor or factors of
the effect of moisture to resilient modulus will be discussed.
The results showed that a 36-in. base clearance was considered adequate for the base protection of
most of the A-3 and A-2-4 subgrades against high groundwater tables. The lab resilient modulus and layer
resilient modulus had the same trend for each soil according to the moisture content change. The percent of
fines or the percent of clays of subgrade soil was not a good indicator to measure the influence of moisture
effect on the resilient modulus. The coefficient of uniformity and coefficient of curvature of the subgrade
gradations, which better represent the whole shape of the gradation curve, are better indicators of the
effect of moisture on modulus. The SR70 A-2-4 (14% fines), A-2-4 (30% fines), Oolite A-1, Branch A-2-4 (23%
fines) soils were extremely susceptible to the change of high groundwater table; the equivalent modulus
reduction rates were more than 50% for lowering the base clearance from 2 ft. to 0 ft. For the Levy A-3 (4%),
SR70 A-3 (8%), A-2-4 (12%), A-2-4 (24%), and Spring Cemetery A-2-4 (15%) soils, the reduction rates were also
very significant for the base clearance from 2 ft. to 0 ft. with the equivalent modulus reduction rates in
the range of 21% to 45%.
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