Effect of subsurface drainage on the structural capacity of flexible pavement.

Following the recommendation of the Virginia Transportation Research Council's Pavement Research Advisory Committee, this project was initiated to determine the effectiveness of including subsurface drainage systems in pavements in Virginia. The researchers sought to determine the effectiveness of these systems by conducting a literature review and by comparing the strengths of pavement sections with and without a subsurface drainage layer in a limited field investigation involving two pavement structures in Virginia. The strength of the pavement structure was analyzed using the falling weight deflectometer. The researchers concluded that the drainage layer appears to affect positively the in-situ subgrade resilient modulus and the in-situ structure number. Further, inclusion of a properly constructed drainage layer does not adversely affect the deflection of a pavement and thus does not introduce a weakness into the pavement structure. However, the condition of the outlet pipes appears to be of high importance. The researchers recommend that tests with additional sites be conducted in the spring when the subgrade moisture is expected to be highest; that the Virginia Department of Transportation (VDOT) develop a maintenance program to maintain functioning drainage outlet pipes; and that VDOT continue the practice of constructing subsurface drainage features on high-priority pavements. In 2005, VDOT anticipates spending approximately $45 million on resurfacing interstate and primary roadways. According to the literature review, the average service life of flexible pavements (time between successive rehabilitation efforts) is approximately 9 years. Including subsurface drainage features offers a 4-year extension of service life (a 44% extension). Thus it can be approximated that the current practice of including subsurface drainage features is saving VDOT approximately $20 million per year. However, the amount of this cost savings may not be fully realized if drainage outlet pipes are blocked or partially blocked. As reported in the literature review, nonfunctioning drains accelerate pavement deterioration and thus may actually shorten the service life of pavement structures