Impacts of Construction Activity on Bioretention Performance

Bioretention cells are incorporated as part of low impact development LID because of their ability to release influent runoff as exfiltration to the soil or evapotranspiration to the atmosphere. However, little care is taken as to the techniques used to excavate bioretention cells, and there is little concern as to the soil-moisture condition during excavation. Certain excavation techniques and soil-moisture conditions create higher levels of compaction which consequently reduce infiltration capacity. Two excavation techniques, the conventional “scoop” method which purposefully smears the underlying soil surface and the “rake” method which uses the teeth of an excavator’s bucket to scarify the underlying soil surface, were tested. Field tests were conducted on three soil types sand, loamy sand, and clay under a variety of antecedent soil-moisture conditions. Multiple hydraulic conductivity, surface infiltration, and soil compaction measurements were taken for each excavated condition. In all cases, the rake method of excavation tended to yield more permeable, less compacted soils than the scoop method. The difference of infiltration and hydraulic conductivity between the two excavation techniques was statistically significant p 0.05 when tests were conducted in wet soil conditions. Also, the infiltration rate at the clay site was significantly lower p 0.05 , and the hydraulic conductivity at the sandy site was significantly lower p 0.05 when the scoop methodology was used. Based on results of the experiment and because essentially no extra cost is associated with the rake method of excavation, it is recommended over the conventional scoop method. Another recommendation is to excavate under relatively dry soil conditions. The use of the rake method under dry soil conditions is expected to increase long-term exfiltration from bioretention cells. DOI: 10.1061/ ASCE HE.1943-5584.0000165 CE Database subject headings: Biological processes; Sustainable development; Construction; Compaction; Storm water management; Runoff. Author keywords: Bioretention; Low impact development; LID; Exfiltration; Construction impacts; Compaction; Storm water runoff.