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Soil amendments help DOT stabilize steep slopes
The South Carolina Department of Transportation (SCDOT) is a designated Municipal Separate Storm Sewer System responsible for minimizing sediment-laden runoff and protecting water quality. To achieve these goals, SCDOT requires projects to be stabilized with vegetative cover prior to completion. The department has developed specifications for seeding and erosion control products to facilitate growth of vegetation that provides immediate erosion control as well as sustainable, long-term erosion prevention. Unfortunately, SCDOT frequently encounters soils with low levels of organic matter (less than 1%), which typically are considered deficient and detrimental to plant growth.
To address poor soils low in organic matter, SCDOT developed specifications for two types of soil amendments: compost and hydraulic biotic soil amendments (HBSAs). Compost is defined as a product resulting from the controlled biological decomposition of organic material under aerobic conditions, and must be US Composting Council Seal of Testing Assurance-certified. Compost typically is applied pneumatically and provides some erosion prevention on mild slopes.
HBSAs are an emerging category of engineered soil amendments that consist of organic matter and nutrient sources combined with soil-building and biostimulant components. HBSAs typically are applied hydraulically using hydroseeding equipment and may include erosion control properties (one-step application HBSAs) or require a separate erosion control product (two-step application HBSAs).
In 2014, SCDOT performed an emergency bridge repair on Cypress Gardens Road in Moncks Corner, S.C. The bridge spanned a railroad track and was flat, with no rise above the grade of the road. After an oversized train caused serious damage, SCDOT constructed an emergency replacement bridge that was much taller. The additional bridge clearance resulted in steep fill slopes where the road was built up to the height of the new bridge. The total slope area was 1.9 acres, with an average slope of 2H:1V and some steeper areas.
SCDOT performed seeding per specifications three times in 2014 and 2015 but could not establish permanent vegetation, leading to severe rill erosion of the steep slopes. As a result, an organic matter test was conducted, indicating the soil contained only 0.7% organic matter. The unsuccessful seeding attempts, slope erosion and low organic matter made Cypress Gardens Road a prime candidate for demonstrating the use of soil amendments.
The site was divided into plots to demonstrate three soil amendment treatments: compost, a one-step HBSA and a two-step HBSA. Product applications, along with seeding per SCDOT specifications, occurred on Jan. 21, 2016. Regular site visits were performed every two to four weeks for the next 18 months.
The compost plot showed minor rill erosion shortly after application. The HBSA products did not noticeably erode. On all plots, temporary cover species (ryegrass and rye grain) emerged in February and provided temporary cover through March and April. In May, the cool season species began to give way to permanent species in the seed mixture (Bermuda grass and weeping lovegrass) as well as volunteer grass and weed species. The HBSA plots achieved a sufficient stand of permanent vegetation by late July. The compost plot took slightly longer, achieving stabilization by late September.
On Oct. 8, 2016, Hurricane Matthew resulted in a 100-year event with more than 10 in. of rain in 24 hours, and no erosion or damage to slopes was observed afterwards. In June 2017, organic matter tests were performed for each plot. The results ranged between 1.3% and 1.9% organic matter, an 86% to 171% increase from the initial test.