The Downtown Commons sits next to U.S. Bank Stadium, the new home of the NFL's Minnesota Vikings. Game day and community events are held on its "Great Lawn," a large oval grass field that serves as an open public space.

The larger eastern block also has a stage area for performances. The smaller western block is more intimate and tranquil, and features a rolling landscape and more trees, benches and tables.

Peterson Companies, Chisago City, Minn., was the landscape contractor for the Commons, and Blackstone Contractors, LLC, Corcoran, Minn., was hired as a subcontractor to install Porous Pave's XL and XLS products.

The XL (50 percent aggregate and 50 percent fine-cut, 1/8-1/4 inch recycled rubber chips in the tan color) was installed at a depth of two inches atop a 2" aggregate base for the paved permeable areas. The workers applied two inches of XLS directly over the soil of the tree wells.

The installation of the permeable pavement and tree surrounds was one of the last steps in the landscaping process.

Before constructing each of the seven sections of permeable pavement, the crew measured the base of each tree, drove in metal stakes to define the circumference of the tree wells, and affixed heavy-duty cardboard around the stakes to establish and protect the areas for the tree surrounds.

The workers put down a two-inch base of compacted aggregate in the sections to be paved and covered the edges of bordering sidewalks with plastic sheeting.

The recycled rubber chips and the dry, clean, chipped granite aggregate for the XL come in pre-measured bags. The binder is in five-gallon pails. In a portable mortar mixer, the installers mixed the rubber and aggregate chips with the binder in the proportion specified by the manufacturer.

Workers wore gloves and were careful to protect adjacent surfaces from the binder, which is very sticky and begins to set up quickly.

They dumped mixed batches of the XL into wheelbarrows, spread the material with concrete come-along rakes, screed it with oil-coated boards in a back and forth motion to achieve a depth of two inches, guided by two-inch measuring screed boards. They finished the corners and rolled the edges with trowels, and then smoothed the surface with well-oiled bull floats.

The liquid binder is moisture-cured. A light spray of water on the completed permeable pavement helped to accelerate the curing process.

For the tree surrounds, the Porous Pave XLS all-rubber formulation was mixed with its special binder, shoveled into the wells atop the soil, and the workers finished it with trowels. Blackstone tapered and thinned the material at the tree trunks to allow for the expanding diameter of the trunks as the trees grow. Since the XLS was also used for tree surrounds in nearby streetscapes next the Commons, the Blackstone crew took small batches of the material in wheelbarrows for those trees.

All tools and equipment were rigorously cleaned with bio-diesel and stiff brushes.

Environmentally Friendly Approach to the Project
Porous Pave is an eco-friendly, highly porous and durable poured-in-place paving material. According to Ryan Potvin, operations manager, Peterson Companies (Chisago City, Minn.), the landscape contractor for the Commons, Porous Pave was selected for its permeability, durability and ease of installation and maintenance. The shredding and processing of discarded tires produces the recycled rubber chips incorporated into the surfacing product. The Commons installation includes rubber recovered from approximately 5,000 scrap tires.

In contrast to permeable pavers that infiltrate stormwater only in the spaces between them, the entire surface of Porous Pave is permeable. It infiltrates stormwater at a tested rate of up to 6,300 gallons per hour per square foot.

The Commons brings much needed green space to the downtown and improves stormwater management, especially important with the Mississippi River just four blocks away. Reducing harmful stormwater runoff and pollution was an important goal of the project.

"As is typical for highly urbanized areas, much of downtown Minneapolis is impervious. The Great Lawn is a prominent element of the Commons, designed as an open space and for deep infiltration of stormwater," said Mary Lydecker, RLA, LEED AP, senior associate, Hargreaves Associates (San Francisco, Calif.), the landscape architecture firm that led the project. "Extensive native and adapted tree and groundcover plantings, as well as various permeable surfacing materials, make the Commons very effective at decreasing stormwater runoff."

"The Downtown Commons is one of our largest projects to date in the Upper Midwest," said Dave Ouwinga, president, Porous Pave, Inc. "It helped us reach an important environmental milestone. In 2016, we surpassed a total 7.5 million pounds of recycled rubber used in manufacturing our permeable paving material."