Despite the property damage and personal tragedy inflicted on South Central Florida, the hurricanes of 2004 provided valuable information for those involved in landfill research.

By Mark Roberts, P.E., Valerie Bonilla, Rebecca Kelner, and Allan Choate, P.E.

Figure 1. Polk County NCLF Aeria.l

Polk County lies in the heart of South Central Florida, approximately 60 miles east of the Gulf of Mexico and 85 miles west of the Atlantic Ocean. Its North Central Landfill (NCLF) (Figure 1) is a 45-acre, 1,800-ton-per-day Class I facility extending over 150 feet above grade. In 2000, an exposed geomembrane cap (EGC) final cover was constructed over 16.5 acres of the facility. Polk County, the Florida Department of Environmental Protection (FDEP), and Jones Edmunds & Associates Inc. (Jones Edmunds) worked together to design this alternative approach to traditional landfill closure systems. Ana Wood, the Polk County Solid Waste director and a major proponent of the EGC for the NCLF, saw the EGC as an opportunity to provide the county value on multiple fronts. “The EGC not only provided Polk County with savings at the time of closure construction, it has since proven to be low maintenance and relatively easy to inspect, and has provided economic and environmental benefits to the county by avoiding periodic mowing, fertilizing, soil replacement, and channel clearing after storms.”

Figure 2. Hurricane Charley.

Figure 3. Hurricane Frances.

Figure 4. Hurricane Jeanne.

2004 Hurricane Season
When Hurricane Charley (Figure 2) roared inland from the Gulf of Mexico onto the Florida Peninsula on the evening of August 13, 2004, high winds inflicted damage to trees and man-made structures within a 50-mile radius of the hurricane’s eye. At landfall, Hurricane Charley was a Category 4 hurricane with wind speeds of 140 mph near the eye. Charley’s center passed just 25 miles south of the NCLF and produced peak wind gusts at the landfill of 115 mph and sustained winds between 75 and 90 mph for about 45 minutes.

Three weeks later, over the Labor Day weekend, Hurricane Frances (Figure 3) crossed through the state from the Atlantic Ocean to the Gulf of Mexico. The NCLF again took the brunt of this storm with the eye passing within 13 miles. Peak wind speeds at the landfill were again over 100 mph and rainfall was measured to be over 7 inches on the day of the storm.

A third hurricane, Jeanne, entered Polk County on September 26 (Figure 4), packing wind gusts greater than 90 mph and bringing further national attention to the county for its meteorological bad luck. The three major storms provided a rare opportunity to collect detailed information and observations on the ability of this new approach to landfill cover systems, the EGC, to perform under even the harshest of circumstances.

Throughout all three storms (See Figure 5), Polk County’s EGC withstood approximately 20 inches of heavy rainfall and hurricane-force winds originating from all points of the compass. The personnel involved with designing and maintaining the EGC at the Polk County Solid Waste Department (Polk County) and the engineers at Jones Edmunds saw this as the first major opportunity in the history of EGCs to demonstrate that, if properly designed, EGCs could withstand multiple violent storm events without incurring significant damage.

Figure 5. State Map and Paths of Hurricanes.

EGCs and Traditional Subtitle D Caps

Figure 6. EGC and Traditional Subtitle D Cover System. Click Here for larger view.

At first glance, an EGC appears to have little defense from a major storm when compared to a traditional landfill cap (Figure 6). Since an EGC does not have a protective soil layer covering the geomembrane, some may see the exposed slopes of the EGC as providing a less-rigorous method of protecting the landfill from damage. However, the EGC takes advantage of this misconstrued weakness by turning it into the foundation of its design strength. There are a number of design advantages inherent to exposing the geomembrane, as well as benefits to not having a layer of soil on top of the geomembrane that can potentially slip, slide, and pull away from the liner when the soil becomes saturated.

EGC Installation
During the EGC design process in 2000, Polk County and Jones Edmunds selected a 60-mil textured HDPE geomembrane for use on the 16.5-acre closure. Vertical anchor trenches were also used so that the geomembrane was anchored regularly along the slope of the landfill, as opposed to horizontal anchor trenches used in traditional Subtitle D closures along only the top and bottom of the slope.

Anchor trenches in traditional Subtitle D closures are commonly used to terminate edges and layers of the closure system by sealing off the layers and preventing stormwater intrusion. These same anchor trenches are used to accommodate the thermal expansion and contraction of the geosynthetic materials. Simply put, anchor trenches used in traditional Subtitle D closure applications typically are not an effective element of the structural design of the cap system.

Figure 7. EGC Panel Installation.

In an EGC system such as the one designed for the NCLF, vertical anchor trenches are a major component of the structural design, and are used to protect the geomembrane from wind uplift (Figure 7). The distance between the vertical anchor trenches is determined by the designer’s assessment of the relative risk of major storm events and the probable maximum sustained winds associated with them. For the NCLF, Polk County and Jones Edmunds staff decided on a vertical anchor trench spacing 100 feet apart. This anchor trench design provided resistance against uplift of the geomembrane in winds of at least 100 mph. Jones Edmunds also included a 1.5 Factor of Safety in the anchor trench design.

Figure 8. EGC "T-vent".

Figure 9. Hurricane-related erosion at a Florida landfill.

Figure 10. Wash-out on a landfill cap after Hurricane Jeanne.

Wind Uplift
Wind uplift is a major design consideration unique to the EGC system. The geomembrane component of the EGC must have sufficient tensile strength and must be sufficiently anchored to the landfill slope to resist the tensile stresses caused by wind uplift. From experience on a number of EGC design projects, Jones Edmunds has designed special vents that were used for the EGC at Polk County to further resist wind uplift. These vents extend under the EGC system and penetrate the geomembrane. Each vent rises above the surface of the EGC by approximately 1 foot and terminates in an open-ended T (Figure 8). Steve Laux, vice president of Jones Edmunds, says, “The Ts can be capped on both sides and used for gas pressure relief if necessary, but their unique value comes into play during high winds.” When wind speed increases, the caps are removed and the wind blows through the T section, causing a venturi effect with a relative vacuum generated in the vertical section of the pipe. The vacuum induces a negative pressure that locally compresses the geomembrane against the landfill surface and thereby helps counteract the uplifting effect of the high winds. Based on field observation, the harder the wind blows, the greater the vacuum compression is on the liner around the vent.

Erosion Control
A problem encountered by operators of landfills using traditional Subtitle D closure systems is that major storm events often wash away significant amounts of soil cover material from the side slopes, creating ruts and channels and eroding the initial thickness of the overall cover material (Figures 9 and 10). The eroded material must be replaced and re-grassed before the next storm event or failure may occur. Erosion control on sideslopes is a common problem for operators at open landfills as well as those conducting post-closure care. In direct contrast, an EGC system does not rely on the exposed grass and soils to protect the underlying geomembrane liner, and therefore there is no potential for wash-outs or erosion from extreme rains.

In many traditional Subtitle D landfill caps, heavy rainfall events increase the head on the underlying geomembrane cap, thereby increasing the probability of loss in stability of the overall cap system. Many reports have documented the soil veneer failure that occurs when the exposed soils become saturated on a landfill slope. EGCs are not subject to this type of failure because 100% of the rainfall that contacts the landfill cap becomes stormwater runoff. Stormwater on the EGC immediately sheet flows upon contact directly down into lined perimeter channels leading to a drainage basin, leaving no head on the liner. Also, since there are essentially no storm-related erosion issues, the stormwater runoff requires no substantial residence time for sedimentation before discharge from the drainage basin.

Figure 11. NCLF EGC two days after Hurricane Jeanne (View 1).

Figure 12. NCLF EGC two days after Hurricane Jeanne (View 2) .

Figure 13. NCLF EGC two days after Hurricane Jeanne (View 3) .

Post Hurricane Assessment
The Polk County NCLF withstood the tests of Hurricanes Charley, Frances, and Jeanne exceptionally well. Despite extending over 150 feet above level ground into the fury of the storms, the cover system experienced not a single storm-related problem from any of the hurricanes (Figures 11, 12, and 13). Additionally, no post-storm cleanup of the EGC was necessary, particularly with regard to erosion or channel clearing. Ana Wood explains, “It was helpful that the EGC area of the landfill did not require any post-storm attention and that the solid waste management staff could concentrate on immediate storm-related cleanup needs.

These storm events illustrate how the EGC has enabled our staff to concentrate on production work for our residents as opposed to reactive work of cleaning up around the landfill.”

Nearby buildings, trees, crops, and telephone poles did not fare as well as the EGC. It is estimated that high winds contributed to over $900 million in losses to structures in the county, including everything from pole barns to apartment complexes. Factory-built homes fared the worst in the final estimate, with over 500 of them destroyed and approximately 6,000 receiving major or minor damage. Almost 120 single-family, site-built homes were destroyed, while an additional 11,825 suffered major or minor damage. Seventy-one commercial buildings were destroyed according to a post-storm survey. Polk County may have experienced stronger hurricanes than Charley, Frances, and Jeanne, but none that have caused so much damage.

The inherent advantages of the EGC system, coupled with the design decisions made by Polk County and the Jones Edmunds staff in 2000, paid off four years later when three strong hurricanes passed through the county in quick succession. By enhancing the strength of the overall system through implementing vertical anchor trenches into the design and augmenting the hold-down properties of the EGC with T vents, Jones Edmunds designed the cover of the closed landfill with a cap liner system that, when the winds died down and the sun came out, looked, in one observer’s words, like a “well-made bed in an Army boot camp.”

The EGC concept is an exciting new development in landfill cover design. The EGC not only provides savings to landfill owners due to relatively low construction cost, but also permits regulators to easily inspect the exposed geomembrane for damage and/or degradation. In comparison, it is difficult to determine how a traditional Subtitle D cap is performing unless a failure condition occurs.

Jones Edmunds engineers are designing EGCs to outperform conventional Subtitle D designs for a variety of sites. For example, EGCs have been implemented in intermediate or temporary cover situations. Jones Edmunds has found that EGCs work well and are cost effective as temporary cover for slopes that have only intermediate cover and cannot sustain an adequate stand of grass. EGCs also have been found to work well for gas containment and odor control. In bioreactor landfills, EGCs work well to contain moisture within the landfill (no seeps). On a land-use note, EGCs allow the landfill to be designed steeper than traditional landfills, adding valuable airspace to the landfill and optimizing the value of the footprint.

Hurricanes Charley, Frances, and Jeanne provided a valuable test of the performance of an EGC under high sustained winds and heavy rainfall scenarios. The Polk County NCLF EGC performed as designed and withstood without any damage three of the most damaging and expensive hurricanes to impact Florida in recent memory. By using the vertical anchor trench design and vacuum-inducing vents, Jones Edmunds made maximum use of the inherent tensile strength and erosion resistance of the geomembrane material. Ana Wood states that she is “very pleased with the EGC’s performance not only through these recent violent storms, but also in its overall performance during the last four years. The EGC can provide landfill owners and operators value in many different areas. Polk County has seen savings related to the EGC with lower construction and maintenance costs, and fewer compliance-related issues, and has contributed to the overall efficiency of our operations. It is important as a public entity to take a leadership role in showcasing successful innovations such as the EGC closure system, and contributing to the evolution of engineering technology in the solid waste industry.”

Mark Roberts, P.E., is project manager, and Valerie Bonilla and Rebecca Kelner are project engineers for Jones Edmunds & Associates. Allan Choate, P.E., is an environmental engineer for Polk County Solid Waste Division.

MSW - March/April 2005