Just about everybody acknowledges that mass grading and construction equipment will compact urban soils and reduce their capacity to infiltrate runoff, but it takes a lot of digging to find research that actually quantifies how construction activities actually change the soil infiltration rate or runoff coefficient. Several papers have documented the impact of mass grading, clearing and the passage of construction equipment on the compaction of soils, as measured by increase in bulk density.

Several new papers go further and relate urban soil compaction to changes in the infiltration rate and/or runoff coefficient, which is more useful number for stormwater managers. Two of the best are by Justin Gregory who looked at soil compaction in North Central Florida, and Bob Pitt who examined a wide range of urban soils in Alabama. The effect of typical compaction on infiltration rates and runoff coefficients was about an order of magnitude. These data were used to develop the new Bay-wide disturbed soil runoff coefficients to protect native soils and forest cover, which are outlined in the soil restoration design specification.

The next question is whether soil compaction can be alleviated through some combination of tilling, subsoiling and compost amendments. For an early review, click here. However, recent work present fairly encouraging results on how hydrological properties of soils can be effectively restored. As a result, new soil restoration specifications have been crafted in Seattle and elsewhere that helped inspire the refined Baywide soil restoration specification presented in this issue.

Native Soil/forest, illustrated by King Co. DNRP[/caption]

Disturbed soil after construction, illustrated by King Co. DNRP
After construction, compacted soils make root growth and tunneling difficult and have fewer air and water pockets.