The impact of different construction practices and techniques were examined to find ways to improve bioretention performance. Scroll down to learn more about this study.

Soil Type

Clay Site - Raleigh, NC (Piedmont)

Sand Site - Nashville, NC (Upper Coastal Plain)

Excavation Technique

Rake Method (teeth of bucket to scarify the surface)

Scoop Method (back of bucket compacts and smears surface)

Antecedent Moisture Condition

Relatively Dry Soil (10-14 warm dry days)

Relatively Wet Soil (irrigated the day before excavation to simulate rainfall)

Soil Testing Methods

Infiltration (double ring infiltrometer)

Saturated Hydraulic Conductivity

Soil Properties (soil cores)

Soil Compaction (cone penetrometer)

Particle Analysis (hydrometer method)

Recommendations

Based on the results from this research, it is recommended to use the "rake" method of excavation over "scoop" method to increase potential for exfiltration from bottom of bioretention cell. It is recommended to use the "rake" method for excavating the final 9-12 inches. This will help to scarify and bottom layer and prevent it from becoming as compacted as with the "scoop" method.

It is also recommended to excavate when the soil is relatively dry. The impact on infiltration rate and saturated hydaulic conductivity was more pronounced in wet conditions. A few recommendations to follow are: (1) avoid excavation immediately following a storm, (2) avoid having water ponded in the bottom of the cut, and (3) avoid excavation if a storm will hit before the cell can be refilled.