Cruise ships have christenings. Skyscrapers have topping-out parties and freeways have ribbon-cutting ceremonies. But landfill caps just happen. No party. No fanfare. Just the quiet completion of a project that's equally complex.
Seeding and erosion control - the final tasks on landfill closure projects - are perhaps the most overlooked, yet vital, of all construction activities. They assure long-term compliance with RCRA Subtitle D requirements and minimize future maintenance expenses.
Due to a sizable exposed area and often steep slope conditions, landfill caps are ripe for attack from the unrelenting forces of wind and water. Once revegetated, however, the risk of soil movement is minimized greatly. Therefore, temporary protection to establish vegetation is a must. Fortunately, the recipe for successful revegetation is as basic as earth science gets: 1) keep seeds and soil intact; 2) expedite germination and; 3) enhance plant growth and establishment.
The ingredients are equally simple:
* One seedbed - preferably topsoil, prepared and ready for seeding;
* Fertilizer and amendments - asnecessary, to improve growing conditions;
* Seeds - typically, grass species, in generous proportions, and;
* erosion control blankets (ECB) spread liberally over cap.
ECB is a biocomposite material manufactured from organic and synthetic elements. The organic portion generally involves wood, straw or coconut fibers which are combined with synthetic nettings, typically made of polypropylene. These nettings also can be manufactured from twisted paper or cotton string. In either case, ECBs are furnished in rolls between four and eight feet in width and typically cover 50 to 80 square yards (sy).
"We view ECBs as a critical element in successful landfill closure projects. They are the final step in completing the project and are the first thing everybody sees," said Paul Costello, project manager for SEA Consultants, Cambridge, Mass. "They're literally the frosting on our cake."
Rainfall simulation studies in controlled conditions have demonstrated that ECBs can reduce soil loss on unvegetated slopes by as much as 95 percent. With approximately 15 acres (75,000 sy) of highly exposed, 3:1 slope area, a landfill closure project in Dennis, Mass., designed by SEA, used a Curlex erosion control blankets, manufactured by American Excelsior Company (AEC), Arlington, Texas.
Rather than using a wide spatula, knife or spray nozzle, ECBs are rolled out like carpet. In Dennis, steel staples (6'l x 1'w) made from 11-gauge wire were installed liberally along the blanket's edge at the top of the slope. Thus, the typical small termination trench was not required.
To assure anchorage at the joints, the ECB was overlapped just enough to allow for a common row of staples. Intermediate staples then were inserted in the blanket's center at the rate of approximately one staple per square yard, forming a number "5" pattern.
To simplify installation, the ECB was furnished in two custom lengths: 112 feet and 225 feet. These special lengths conformed to specific cap locations and eliminated the need for cutting rolls and transporting them back to the top of the slope.
McNamara Brothers (MB), North Harwich, Mass., was selected by the general contractor, Zecco Inc., to install the landfill blanket.
"The longer rolls simplified the installation once they were positioned for roll-out," said Chris McNamara, MB's project superintendent. "However, because of the heavier weight, removing them from the semi-trailer presented a challenge." As a result, the manufacturer now delivers the blankets on flatbed trailers.
Generally, the seed is installed before the ECB. In some cases, however, it is desirable to seed after the blanket has been installed, particularly if the onset of winter weather or mid-summer drought conditions are close at hand.
The final touch an ECB can add is color. For example, Costello used a sod green blanket, which, as it faded in color, was replaced by green blades of grass. According to Costello, ECBs are cost effective at approximately $1 per square yard.