From Landfill To Athletic Fields
"From day one the words 'excavate,' 'dig' and 'cut' were outlawed. We had to do everything above existing ground level and not cut into the clay cap," explains Todd Smith, division manager for American Civil Constructors (ACC) Inc. on converting Rooney Landfill in Jefferson County to waterless athletic fields. The $2.9-million construction now under way experienced four months of documented weather delays with winter snows and spring rains, but today is only 2-1/2 months behind schedule with an expected completion date of September 15, according to Smith.
Sealing the landfill and extracting methane
The Rooney Landfill, located in Jefferson County near I-70 and Green Mountain on the western edge of the Denver metro area, has been closed for approximately 25 years. In 1999 Jefferson County asked KRW Consulting Inc. of Lakewood to do a site characterization study. The study answered questions such as "How deep is the refuse?" and "What are the moisture conditions?" and "What is the methane gas production?"
"When we conducted the study, even though funding was not yet available, Golden was considering developing the land for waterless soccer fields," says Dave Douglass of KRW. "Knowing the future use, KRW solicited advice on possible field configurations from Golden, Jefferson County and Table Mountain Soccer Association and included the information in its planning."
From the data collected, KRW recommended installing a gas control system. In 2004, while enhancing the clay cap, Winslow Construction Co. of Englewood installed 25 methane extraction wells with casing down to bedrock, sometimes as deep as 80 feet, and piping that carries the collected methane to a flare that burns it off.
"At this point, operating a generator with the methane or marketing it is not economically feasible," Douglass explains. "In our dry climate a landfill will release lower concentrations of methane for decades, while in a wetter climate the methane release is faster and more concentrated."
The cap enhancement Winslow installed is site fill of recycled tire shreds in 2-inch to 4-inch chunks, compacted low permeability clay, then a geosynthetic clay liner (GCL), a manufactured fabric containing bentonite. Then Winslow brought in and graded the soil so rainfall would run off.
The current phase one athletic field construction begun last fall will develop a third of the master plan for the 80-acre site with five full-sized soccer fields, access roads, parking, restrooms, and two smaller athletic fields, according to Smith. KRW's planning and placement of the methane extraction wells was so concise the piping for only one wellhead had to be moved for the fields.
ACC knocked down vegetation and the native grass, and brought in 50,000 cubic yards of fill to further level out the field areas. ACC spread the fill anywhere from 0 to 3 feet deep to make the fields flat with a slight 1-percent slope for drainage. The field base above the fill consists of aggregate. Academy Sports Turf of Denver rolled out the artificial turf much like carpet and anchored it to the concrete perimeter curbing for each field. Then laborers in-filled the turf with recycled rubber tire chips or crumbs for softness, much different than earlier abrasive synthetic turfs.
Since all landfills settle as refuge decomposes, Golden and Jeffco expect to repair the fields when dips appear. According to Smith, it's a simple matter of cutting back the turf, filling the depression and gluing the turf back in place. Compared to the maintenance costs of watering, fertilizing and mowing natural fields, these periodic repairs should be a small expense item.
Because the Rooney Road Recycling Center is still operating, ACC built a new access road to isolate a central area for the park itself and redirect the recycling traffic around the athletic field areas. The parking lots for the athletic complex will be surfaced with recycled asphalt millings known as RAP. Normal asphalt paving would buckle with the decomposition of the landfill beneath. The 4-inch to 6-inch deep milling layer will lock together and form a solid dust-free surface for parking. When the landfill moves, the surface can be graded back into place and rolled to restore the surface.
Two of the slabs for restroom areas will be constructed of a rubberized concrete developed by the Colorado School of Mines. This concrete has some flexibility to cope with earth movement. Since the site is waterless, portable toilets will sit on top of the rubberized slabs. The third restroom is a permanent vault toilet installed in 3 feet of fill that was brought in.
ACC was involved with the project starting in 1999 when Golden, Jefferson County and the Table Mountain Soccer Association planned the fields. Decisions were challenging in these early years, working out the intergovernmental agreements and who was responsible for what, according to Smith.
"You have many chiefs who have different regulations and different goals, trying hard to work together," he says. "However, it's worked out well. We have all these points of view in our weekly meetings, which is good, so we get everything first hand."
The methane being emitted by the landfill necessitated numerous restrictions on both the construction and the finished site. "No Smoking" signs will be prominent. KRW monitored methane emissions during construction to safeguard the workers. Even when ACC was only digging as deep as the fill it brought in, the county required methane monitoring in the event methane was leaking from the cap and infiltrating the new piles of fill.
Just one spark can set off a fire if methane is present. The safety plan required the construction crews to cut all rebar for the curbing and restroom slabs and metal such as drainage piping off-site. When one of the drainage pipes needed to be trimmed back, the crew had to dig up the 20-foot-long section, take it off-site and cut off 2 feet, then bring it back and reinstall it.
As Smith says, "We brought the material on-site, measured it, took it off-site to cut it, and then brought it back."
Because moisture can increase methane emissions, the field drainage system is designed to carry water away from the landfill cap. According to Smith, the first quarter-inch of rainfall will fill gaps in the turf's rubber infill. The next quarter-inch of rainfall will penetrate through the backing of the turf. There it will fill voids in a 6-inch cross section of aggregate. The next half-inch will flow through the rock to the field edge where it enters a perforated pipe 12 inches to 18 inches below grade and flows north to a detention pond.
ACC has installed numerous synthetic turf fields, although not all on landfills with these challenging construction restrictions. In particular, mountain communities are discovering the turf extends the season for its playing fields into late fall, well beyond the growing season. Because the turf is typically hotter than the ambient temperature and rarely freezes, the University of Colorado at Boulder was able to plow snow off one of its fields for a winter soccer practice when it was 19 degrees and snowing.