Replacing compost with a hydraulic growth medium
The California Department of Transportation (Caltrans) recently completed interchange improvements on State Route 132 (SR 132) in the city of Tracy in San Joaquin County.
The affected area included the intersection of SR 132 and Bird Road. The existing interchange was an at-grade intersection, with stop signs on the Bird Road approaches. The Caltrans project included constructing an overpass interchange and making related highway improvements to enhance traffic flow and safety. Major construction on the overpass and on/off ramps was completed in early 2014.
The area has an average rainfall of 1.8 to 2.6 in. per month and temperatures ranging from 60°F to 70°F at the time of year the project was completed. The finished grades at the site ranged from relatively flat to more than 2H:1V around the bridge abutments. Soil varies throughout the region, but upper subsoil in this area typically is brown loam and lower subsoil is brown clay. Unamended subsoil in the area is not particularly well suited to grow grass. Caltrans’ BMP for this project called for grading and compacting the subsoil, spreading 4 in. of compost, seeding with a local grass mix and applying a tackified paper mulch to control erosion. Selby’s Erosion Control Co. Inc. of Newcastle, Calif., was contracted to perform the site remediation.
The typical application of compost, seed and mulch can be expensive and labor-intensive; generate significant dust; and increase site compaction. Large quantities of compost have to be trucked in, adding to transportation costs, and then typically are spread using heavy equipment that can further compact the soil. The seed and protective hydro-mulch layer are sprayed onto the compost using a hydroseeder. The hydromulch is designed to adhere to the compost surface and protect it from erosion, but the bond often is weak due to the loose consistency of the compost surface. This can lead to erosion problems and poor revegetation results and may require follow-up treatments.
Organic Earth Industries proposed testing an alternative solution utilizing its TerraVita hydraulic growth medium as a replacement for compost. The medium is designed to sustain vegetation through several growth cycles and accelerate natural topsoil building in the upper soil horizon. The product is composed of Seal of Testing Assurance-certified organic compost and an agronomically engineered organic fiber matrix with bonding agents. The fiber matrix is composed of heat-treated straw and other proprietary interlocking fibers. The matrix retains heat and moisture while allowing air and water to penetrate into the soil. The fibers decompose at different rates and release nitrogen and other micro-nutrients into the soil over time as plant growth begins to succeed in the treated area.
An onsite test was devised to compare the performance of TerraVita with compost as a control on similar site locations (slope and aspect).
A Two-in-One Approach
Organic Earth Industries also developed another hydraulic growth medium and mulch technology with the goal of delivering the benefits of both in one application. Earth Essence Omega replaces the application of both compost and mulch for erosion control and has a C-Factor of 0.003. Test areas on the bridge abutments were chosen to demonstrate the product.
Then Came the Rains
The first test of erosion control performance came with an unseasonably heavy rain event in late February. Approximately 3 in. of rain fell in the area over a 24-hour period, causing flash runoff in the construction area. With no diversions such as sand bags or wattles to direct water flow away from hillsides, the entire area was subjected to aggressive sheet flow from the road.
All of the test areas performed well compared with the control under these conditions. The Earth Essence Omega application on the bridge abutments also performed well. All soil remained intact.
The End Game
With all of the products performing well under aggressive erosion conditions, the focus turned to germination and plant growth. Emergence became evident within a few weeks. Although all of the treated areas showed acceptable levels of vegetation establishment after six weeks, the compost plus paper mulch (control) section had experienced severe wind shear and subsequent erosion. Wind was able to get between the mulch layer and the compost layer, separating the protective mulch layer from the loose compost.
The compost subsequently was exposed to further wind and water erosion, creating a mottled appearance in the control area.
Although the site is a long way from final disposition, all signs to date indicate that the solutions performed as well or better than the current practice (compost and mulch application) in controlling erosion and establishing vegetation.