SUBBASE PERMEABILITY AND PAVEMENT PERFORMANCE

Problems of premature pavement and shoulder distress in Pennsylvania have been attributed to excess water in the standard, dense-graded subbase. An experimental project was constructed to demonstrate the feasibility of providing good support and good internal drainage to the single layer of subbase with a two-layer system at a competitive cost. An additional long-term objective of the project was to determine the significance of the permeability of subbase layer materials on pavement performance. Five types of subbases, ranging from a very impermeable cement-stabilized material to a very permeable and uniformly graded crushed aggregate, were incorporated into the project. The study documented the manufacturing of the various materials and the associated unit costs. Base materials with permeabilities 3 orders of magnitude more than that of the standard subbase were placed for only a 5 percent increase in cost. The ability of the contractor to handle, place, and pave on the various subbases was evaluated. In-place permeabiliites were measured with the field permeability test device developed by L. K. Moulton of West Virginia University to statistically determine permeability by variation within a material section and between the five material sections. Initial pavement roughness measurements were made on the concrete pavement in each experimental section, and these measurements indicated no loss in construction quality with the use of the more permeable interlayers. Underdrain system outlets were instrumented so the rates at which the various subbase materials carry off infiltrated surface water could be determined throughout the long-term evaluation. (Author)