A steep earthen bank slope adjacent to a tributary of Little Mill Creek had experienced significant erosion and sloughing endangering a commercial building at the top of the slope. Duffield Associates was retained to design a solution which would stabilize the slope and reduce the failure risk to the adjacent building.

bo1While conventional “hard” armoring such as rip-rap or stone filled wire gabion baskets could have been considered to rebuild the slope, Duffield Associates’ engineers developed a “green” solution which would provide long-term stability for the slope, protect the adjacent building and, over time, return the slope to an eco-friendly mature vegetated state. The selected design was a Mechanically Stabilized Earth (MSE) slope system consisting of structural fill reinforced with layers of polyester geogrid material and faced with “geobags” specially designed and fabricated for this project. While the design and construction of MSE slopes is becoming more commonplace, the use of vegetated “geobags” for the face of the slope is considered unique. In fact, this is only the second time such a system has been used in Delaware and only the first time this unique geobag application has been employed.

The geobags were fabricated utilizing high performance turf reinforcement matting (Pyramat®) to form approximately 1 cubic yard bags. This permanent erosion control material consists of 3D matrix of polypropylene yarns which are typically utilized in especially demanding erosion control applications. The bags were filled on-site with a mixture of granular soil and topsoil utilizing a time saving special template. The use of these flexible bags enabled the contractor to make adjustments in the overall shape of the slope and to accommodate the variable field conditions which would have been extremely difficult if a more rigid stabilization system, such as gabions, had been used.

Upon completion of the slope, stone rip-rap was placed along the slope’s toe for enhanced scour protection from the adjacent stream. A hydraulically applied mixture of seed and Flexterra® Flexible Growth Medium was applied over the geobags and the disturbed areas. In spite of near drought conditions, grass is growing on the slope. Over a relatively short time the small diameter trees and ground cover native to the stream banks in the area will erase all evidence of this construction. This unique eco-friendly engineering application has stabilized the eroding steep slope and prevented undermining of the foundation of the building at the top of the slope.