Completion Date: Aug. 15, 2016
Fabric Name: Geogrid
Fabric Producer: Tensar International Corp.
Fabric Name: HDPE Geomembrane
Fabric Producer: Groupe Solmax
A large multinational company closed a manufacturing facility in North Carolina. Sludge was one of the byproducts of the manufacturing process. As part of the closure protocol, the company needed to cap its 12-acre (5-ha) sludge-storage basin. The semiliquid sludge presented characteristics that prevented the use of traditional methodology for dewatering and capping.
Adding to the complexity of how to cap the semiliquid sludge, the project site had an average rainfall of more than 57 inches (145 cm) per year, so the cap design needed to manage a large volume of rainwater. In addition, the coastal location of the site was subject to high winds and potential hurricanes; therefore, the cap needed to be designed for windy conditions.
The purpose of capping was to eliminate rainfall from entering the sludge basin and to prevent leachate from developing. The company and its engineers needed an effective and economical solution to capping the sludge with a design that also incorporated the highest safety standards for the installation crew.
The sludge depth ranged from 7 to 9 feet (2 to 3 m) across the basin. Normally, sludge can be dewatered, which makes it possible to haul the solids off-site and then install a low-permeability cap over the basin. However, existing polymers in the supersaturated sludge resisted dewatering. As a result, the sludge did not have any structural integrity or strength to support a work crew, much less support a heavy clay cap layer. If the sludge was subjected to any significant load, it would be quickly displaced, causing the denser and heavier material on top to migrate downward. The engineering firm determined the best approach was to cap the solids in place.
The engineering firm solicited more than 20 geomembrane companies for the tender. Part of the prequalification process was to evaluate each company’s experience and approach to capping the in situ solids. Following the vetting process, the engineering firm chose an innovative three-layer capping approach.
The company responsible for the three-layer design sent its engineering team to the project site to demonstrate its solution to both the client and the client’s engineers. The demonstration was set up in a section of the basin to try out this new design. The demonstration proved successful, meeting the objectives.
The first layer of the design was a floating geogrid system installed directly over the sludge. The geogrid was installed to distribute surface loads across a wide area to reduce sludge displacement.
A secondary layer was installed directly over the geogrid. A patented floating modular cover system provided the needed structure to further distribute weight across a larger area of the sludge. The modular cover was constructed of high-density polyethylene (HDPE) geomembrane that encapsulated a proprietary foam in an airtight module. The buoyancy of the modular cover also provided the safe working platform needed for the installation crew and its equipment.
In consideration of the intense ultraviolet rays and high summer temperatures of North Carolina, crews installed a reflective 80-mil (2-mm) HDPE geomembrane across the basin. The material was double-fusion-welded to create a monolithic flexible cover that protected the solids from rainfall. The system was designed to stay afloat, enabling rainwater to be collected and diverted off the cover. A series of strategically placed ballast weights were installed across the cover to facilitate the draining of rainwater to collection points. The geomembrane was buried in a specially designed anchor trench. The trench prevented rainwater from entering the basin and prevented wind from getting under the cover. Both were important design factors for the project due to the frequency of hurricane winds and heavy rainfall.
The solids in the basin were largely inert, producing minimal biogas. However, as a design precaution for the potential need to collect biogas in the future, the system incorporated gas laterals. The field crew welded the gas laterals to the underside of the cover to enable gas to migrate to specified locations where the gas can be expelled through a series of valves. The valves can be opened by operators if the need later develops.
The project was deemed successful and cost-effective by the client’s engineer. Costs normally associated with dewatering and hauling solids were eliminated by using the innovative approach. The capping project stayed within budget and was completed several weeks ahead of schedule without any safety violations. The cover system has since been awarded a United States patent.