Adding a specialized synthetic turf component to an exposed geomembrane cap can significantly improve membrane protection, accessibility, wind resistance uplift and aesthetics. Additionally, the turf provides for a combination of savings on upfront construction cost and post-closure maintenance cost when compared to traditional cover systems.
By Mike Ayers
Sustainability for landfill closures has a dual meaning. One meaning relates to the physical stability and long-term performance and maintenance, which is a problem that has long-plagued the landfill industry. The other meaning is associated with reducing environmental impacts such as carbon footprint and water quality.
Many traditional soil cover systems are destined to fail on steep landfill slopes as a result of excessive erosion, gas pressure buildup, earthquake loads, poor maintenance and/or inadequate post-closure oversight. Closures have shown to lose the integrity of its intended function after site closure and in absence of onsite personnel. The initial construction and reconstruction an activity of the cover destroys land to obtain borrow, creates sedimentation issues, consumes significant fuel, and produces significant emissions from trucking and operation of heavy equipment.
An Improved Closure System
The goal of an improved closure system should be to first increase the performance and reliability of its intended function of protecting the environment from groundwater releases, surface waters impacts and landfill gas emissions. Sacrificing functional integrity for the sake of environmental sustainability is pointless or even harmful from the macro goal of protecting the environment.
A step in the right direction has been the reliance on exposed geosynthetics. Exposed geomembrane cover systems (EGCS) have been successfully used for closures at landfills in the U.S. for several years. However, even though the use of EGCS minimizes veneer stability issues and eliminates the impacts associated with a soil cover, they can come with their own set of challenges. Covers using just an exposed geomembrane can have negative aesthetics, require numerous anchor trenches to resist wind uplift, are prone to physical damage and can cause serious storm water management issues. Access can also be very difficult as well as dangerous during post-closure care operations.
A new cover system deals with similar concepts as the EGCS but goes a step further by combining a patented system of impermeable liner with an engineered synthetic turf. The approach is to use the benefits of an EGCS and eliminate the negative aspects of frequent anchoring, poor accessibility and aesthetic issues. This is achieved by providing additional layers of protection, higher friction angles and homogeneous ballast (as opposed to point loads at anchor trenches) with high drainage capacities. The turf simply serves as aesthetically pleasing, uniform ballast that also provides accessibility, membrane protection and surface water control.
The system also provides for a new approach in landfill gas collection (LFG) by using a geocomposite material at the foundation layer to direct and relieve gas pressure to collection points. This surficial gas collection approach can dramatically improve the landfill gas collection efficiency early in the life of the facility due to the ease of installation and ability to cover as soon as first waste grades are achieved.
Improving Performance
It is common for engineers to design traditional synthetic and soil covers with a very low factor of safety against veneer failure (factors of safety of 1.2 are many times accepted under dynamic conditions). The post-closure maintenance period also accepts the fact that the system is heavily reliant on replacing soil loss, re-vegetation, fertilization and storm water repairs. Simply improving the performance from this standard traditional design is in and of itself adding to the environmental sustainability of the cover. The design approach presented here uses synthetic turf and structured geomembranes for final landfill covers. The system requires minimal anchoring and provides a drainage system that can handle intense wind and rain. The components also work together to allow for traffic access during post-closure care and provide for an aesthetically pleasing surface.
The proprietary cover system of this approach is designed from the bottom up with a lower impermeable layer placed over the soil intermediate cover comprising of: (1) a structured geomembrane with integrated studs on top for drainage and aggressive spikes on bottom for stability; (2) an engineered synthetic turf that covers and protects the underlying geomembrane and; (3) a sand or cementious infill layer that used to ballast the material and protect the system against wind uplift. The internal friction angle of the system components provides for a highly stable system without expensive anchor trenching.
The new system does not present stability issues from sudden failures such as sliding and washout, or from long-term failures caused by soil and wind erosion since there is no vegetated cover. Facility owners and operators of these closures can potentially realize significant cost savings by constructing a cover system with the synthetic grass that does not require the vegetative maintenance, soil grading and replacement that are common with traditional closures.
Through the combination of these unique geosynthetics components, the landfill operator can achieve a more reliable and stable cover along with a more environmentally friendly cover system during construction and post-closure maintenance.
Optimizing Sustainability Through Closure Phasing
In addition to saving soil, the engineered synthetic turf cover can be constructed at least 50 percent faster than traditional covers. The lower construction costs and simplicity of the installation allow the operators to close in smaller increments (3 to 5 acres at a time) once the final grades have been achieved. This results in earlier collection of the landfill gases as opposed to waiting a longer period of time to close large areas (20 to 50 acres typically for traditional covers) in order to obtain economies-of-scale. Typically, it is impractical for operators to mobilize heavy equipment and procure soil borrowed to construct less than 20 acres of closure. The proposed new closure can be installed in an economical way in an area as small as 3 acres because of the low mobilization costs and the minimal earthwork required.
New geosynthetic applications can improve the reliability and performance of landfill closures. In particular, adding a specialized synthetic turf component to a geomembrane can significantly improve membrane protection, accessibility, wind resistance uplift and aesthetics. Additionally, the turf provides for a combination of savings on upfront construction cost and post-closure maintenance cost when compared to traditional cover systems. | WA
Mike Ayers is the the President of Watershed Geo, a geosynthetics company specializing in engineered products that solve earth’s greatest challenges with solutions that reduce risks, require virtually no maintenance and stand the test of time. Watershed’s patented ClosureTurf® product is used across a variety of landfill applications, including MSW, construction and demolition, coal ash and industrial. Mike can be reached at (770) 777-0386, e-mail [email protected] or visit www.watershedgeo.com.