STATE-OF-THE-PRACTICE IN MARYLAND--PART II

Based on the present methods of determining the permeability of bituminous concrete surface and base courses, the Maryland State Highway Administration (MSHA) considers that its bituminous mixes provide courses that are reasonably impermeable. As outlined in Part I of this paper, four types of subsurface drainage systems are used by MSHA: extended granular subgrades or subbases, continuous longitudinal pipe underdrain, subgrade drains, and conventional pipe underdrain. The drainage components that comprise MSHA drainage systems are influenced by material availability and the economics associated in the material selection. Although the material components appear in line, the means whereby the surface water can get to the drainage medium is still questionable, as is the location and geometrics of the system. Problems with any subsurface waters have generally been minimized by MSHA design practices. The primary concern is with surface runoff on open section facilities getting to the drainage medium. MSHA is hoping that problem conditions can be minimized by concentrating on material specifications and geometrics to limit the entrance of water, and by emphasizing material quality control to effect the desired rapid drainage through the system.