Evaluation of Surface and Subsurface Processes in Permeable Pavement Infiltration Trenches

The hydrologic performance of permeable pavement systems can be affected by clogging at the pavement surface and/or clogging at the interface where the subsurface storage layer meets the underlying soil. The objective of this paper was to evaluate changes in infiltration and exfiltration using three pressure transducers installed in piezometers along the length of two, 2.47-m wide, permeable pavement strips. Each system was retrofitted in the parking lane of a curb-and-gutter system in Louisville, Kentucky. The strips received run-on from a drainage area about $20–25$ times larger than the paver area. Below each permeable pavement strip (16.8-m and 36.6-m long) was a deep (3.0 m) and narrow (0.6 m) infiltration trench. Piezometers were installed at the bottom of the trench about 1 m from the uphill edge, and at roughly one-third and two-thirds along the permeable paver strips, to measure the rise and fall of water for 13 months. Initially, infiltrating run-on was localized near the uphill edge. During periods with intense runoff, the localized inflow accumulated in the trench faster than it could move laterally creating a measureable subsurface gradient between piezometers. Runoff transported solids to the uphill edge where a portion was filtered and accumulated between the paver blocks. With time, surface clogging progressed along the paver strip past the next piezometer. The localized infiltration caused the subsurface water level gradient between piezometers to reverse. After each event, the exfiltration rate per wetted surface area was calculated for fixed 0.15-m intervals. The exfiltration rate decreased drastically after the first few events. The primary cause of the initial exfiltration rate decline was attributed to infiltrating water rinsing the fine solids attached to the washed aggregate and depositing them at the bottom of the trench. Silt and clay-sized particles (particle size smaller than 75 μm) accounted for about 1.7–1.8% of the washed aggregate mass. This large source of fine-grained particles from construction materials can be eliminated if cleaner aggregate is available and used. A continued and significant decrease in exfiltration rate with age was measured during year 1 of use. To optimize the design of these systems, a lifecycle analysis incorporating exfiltration rate decline with age should be included. Another item to consider during the design process is the variability of urban soils. Although these sites are across the street from one another, exfiltration rates were markedly different. Installation should be targeted in soils with larger hydraulic conductivities to improve hydrologic performance, so more preconstruction soil borings and soil tests are necessary to characterize the in situ soils.