United States Department of Transportation
i
Development of a subgrade drainage model for unpaved roads.
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2015-10-01
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[PDF-2.06 MB]
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Abstract:With over 68 thousand miles of gravel roads in Iowa and the importance of these roads within the farm-to-market
transportation system, proper water management becomes critical for maintaining the integrity of the roadway
materials. However, the build-up of water within the aggregate subbase can lead to frost boils and ultimately
potholes forming at the road surface. The aggregate subbase and subgrade soils under these gravel roads are
produced with material opportunistically chosen from local sources near the site and, many times, the
compositions of these sublayers are far from ideal in terms of proper water drainage with the full effects of this
shortcut not being fully understood.
The primary objective of this project was to provide a physically-based model for evaluating the drainability of
potential subbase and subgrade materials for gravel roads in Iowa. The Richards equation provided the
appropriate framework to study the transient unsaturated flow that usually occurs through the subbase and
subgrade of a gravel road. From which, we identified that the saturated hydraulic conductivity, Ks, was a key
parameter driving the time to drain of subgrade soils found in Iowa, thus being a good proxy variable for accessing
roadway drainability. Using Ks, derived from soil texture, we were able to identify potential problem areas in
terms of roadway drainage .
It was found that there is a threshold for Ks of 15 cm/day that determines if the roadway will drain efficiently,
based on the requirement that the time to drain, Td, the surface roadway layer does not exceed a 2-hr limit. Two
of the three highest abundant textures (loam and silty clay loam), which cover nearly 60% of the state of Iowa,
were found to have average Td values greater than the 2-hr limit.
With such a large percentage of the state at risk for the formation of boils due to the soil with relatively low
saturated hydraulic conductivity values, it seems pertinent that we propose alternative design and/or maintenance
practices to limit the expensive repair work in Iowa. The addition of drain tiles or French mattresses my help
address drainage problems. However, before pursuing this recommendation, a comprehensive cost-benefit
analysis is needed.
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