Measurement and Optimization of Permeability in Bioretention Soil Media

Between October 2013 and September 2014, DC Water completed construction of 15 bioretention facilities as part of two projects in Washington, DC, stemming from the utility's long-term control plan (LTCP) for combined sewer overflows (CSOs). Many of the facilities, which were designed to drain within a day after rainfall events, exhibited surface ponding several days after rainfall had stopped. The design team conducted a detailed investigation of possible causes and identified the bioretention soil mixture (BSM) as the culprit. DC Water's investigation into BSM permeability included a review of regional bioretention experience (which revealed a lack of pre- and postconstruction permeability testing to validate BSM and facility performance) and more than 400 mesocosm and field permeability tests performed on 25 unique BSM mix designs. The investigation also included particle size analyses, chemical testing, compaction testing, review of the supplier's soil mixing and distribution processes, and installation procedures. This investigation yielded a better understanding of BSM permeability and development of a well draining BSM mix design that will be applied in the utility's proposed large-scale green infrastructure (GI) program.