We use a drainage layer under in our road structure to allow for water infiltration to freely drain in order to prevent a build up of pore pressures, both within the road structure itself and/or on top of the underlying subgrade.
In my particular situation (mine haul roads) our pavement is gravel. Although our climate is generally on the dry side, in spring and late summer we can have a lot of moisture. Hauling in wet conditions saturates the surface gravel and the constant traffic of the heavy trucks pounds the moisture down into the granular road structure. Our granular material is fairly clean (i.e. low fines - <6% passing the #200 sieve) which allows the water to freely drain and prevents a build up of pore pressures from weakening the road. Even though our surface gets pretty sloppy the structural integrity of the road structure remains.
Below the granular we have a buffer zone of sand above the clay subgrade. This sand layer allows any water that has migrated down to the top of the clay to run off through the sand or, if the top of the clay gets slightly moist, it will prevent high pore pressures from building up at the top of the subgrade by giving the water a drainage path.
In the 7 or so years that we’ve been using this design we have not had a major road failure in which we had to rip up the road for repairs. Our trucks are quite large (240-ton to 400-ton load capacity) so pore pressure generation within the road structure and subgrade can be wicked!!
I know this explanation is a bit “windy”……but I get asked about this all the time from our operations group who always ask why we build with the different materials.
