Verification Infiltration Testing for the Stormwater Management Design for a Light Rail Extension Project in Washington State

Stormwater management was a critical element of the Downtown Redmond Link Extension (DRLE) project. This project exemplified the power of interdisciplinary collaboration, requiring close coordination among civil engineers, hydrogeologists, and construction teams to achieve success. Due to shallow groundwater and flat topography, managing stormwater required a complex network of collection, distribution, and infiltration facilities. The infiltration rate, a critical design parameter, largely controlled the footprint of the infiltration facilities. Ideally, an infiltration test would be conducted at the location of each infiltration facility. However, access restrictions made this impossible during the design phase, and pilot infiltration tests were conducted at 16 readily accessible and agreed-upon locations to assess the field infiltration rates for the design of the various facilities. In this regard, verification infiltration testing for each infiltration facility location was required, after construction, by regulatory and contractual obligations. Individual infiltration tests were performed at each of the 36 facilities. Location-specific target infiltration rates were calculated for each facility based upon the design infiltration rates with safety factors. The results of verification testing were compared to the target rate and used to determine the acceptability of the infiltration facilities as designed and constructed. Of the 36 facilities, only one required a minor redesign to meet the target infiltration rate.