While all eyes were on the National Museum of African American History and Culture opening in Washington, D.C., in late September, another project was also nearing completion: The $40 million, seven-year reconstruction of the iconic central grassy spine of the Mall between Third and 14th streets.
The area consists of eight grass panels totaling 18 acres. The first three, on the Capitol side, were completed in 2012. The fences have come off the other five, but barriers will continue to keep people away from the grass to allow its root system to grow over the next few months.
What caused the bare spots and the need for the repairs? Roughly 3,000 permitted events and up to 33 million visitors per year, says Teresa Durkin, senior project director of the Trust for the National Mall.
“It was in pretty bad shape,” Peter Landschoot, a professor of turfgrass science at Penn State and a consultant on the project, told The Washington Post.
The grass panels, lawns and American elm trees together form the grand avenue connecting the monumental core of the nation’s capital. The Mall also functions as America’s common space, where people gather for events or just to hang out.
To improve and renovate this high-traffic and highly visible lawn, coarse sand was used in the mix over regular loam soil to promote vigorous root growth. Sandier soil resists physical compaction and contains more oxygen that roots need to grow, says Norman Hummel, a consulting soil engineer who worked on the project.
But because sandy soil dries out fairly quickly and doesn’t hold nutrients like clay soil the strategy needed a complex subterranean infrastructure to ensure success. This is made up of a 4-inch sublayer of pea gravel with an 80 percent sand, 10 percent topsoil and 10 percent peat mix on top. Each panel received 3,000 tons of the new soil mix. Sod was then laid to help the lawn take root at a time of year when seeding may not become as lush as quickly and so the lawn is ready for January’s inauguration. The sod is made up of 90 percent turf-type tall fescue and 10 percent Kentucky bluegrass. BrightView did most of the installation work on the project.
Drainage is another key in making the project work.
Brian Vinchesi with Irrigation Consulting Inc. and his team were irrigation consultants on the project, which includes a very sophisticated rainwater/stormwater harvesting system, pump station, control system and irrigation system. The new system allows the grass to receive as much as 4 inches of rain without waterlogging; when the water in the soil reaches a certain level, the suspended water drops into the gravel layer below.
Each panel is edged with French drains and a grid of drainage lines. Rain and irrigation water is collected and stored in four new 250,000-gallon underground cisterns where it is then filtered and disinfected with UV treatment before being reused in the irrigation system. The sprinkler system is made up of sprinkler heads that can throw water as far as 90 feet and is linked to an automated weather station that monitors temperature, humidity, wind and soil moisture.
An aggressive maintenance program is also in place to ensure the new lawn’s success and to counter continued effects of traffic and use.