Expanded Shale Utilized at World Trade Center 9/11 Memorial

On October 26, 2010, New York City Mayor, Michael Bloomberg, and 9/11 Memorial President, Joe Daniels, planted the 50th tree on the plaza of the 9/11 Memorial, which opens in less than a year. Tree plantings began on Aug. 28 when the first 16 trees were planted. The final design for the plaza calls for more than 400 trees. By the time of the 10th anniversary, about 250 trees will be in place. The swamp white oak trees have been growing in a New Jersey nursery since 2007. The average height of the trees is currently 30 feet and they are expected to reach heights of 80 feet.

Above: Left to right, 9/11 Memorial President Joe Daniels,
Mayor Michael Bloomberg, Starr Foundation Chairman
Maurice Greenberg and 9/11 Memorial architect Michael Arad
plant the 50th oak tree on the plaza of the 9/11 Memorial.
(Photo credit: Amy Dreher)

The trees have been selected within a 500-mile radius of the World Trade Center site, with additional trees coming from locations that were affected in the attacks on September 11, 2001. These trees will come from Pennsylvania and the Washington, D.C. area.

“I think this is just another step. It’s a small one, it’s a symbolic one, but it shows that the World Trade Center site is progressing, and particularly the memorial which will be done on time and on budget,” Bloomberg said after the planting.

Memorial Plaza

The Memorial will include a treed plaza area that sits atop a 70-foot below-grade building that houses a museum, a train station and other facilities. The trees are being planted in a structural soil mainly consisting of a 3/8” x No. 8 expanded shale lightweight aggregate manufactured by the Norlite Corporation located in Cohoes, NY. The expanded shale was blended with a natural sand and an organic component offsite and then delivered to the World Trade Center Site for use as the soil medium in the planting beds.

In order to grow healthy trees in this context, the trees on the Memorial quadrant will be installed in a suspended paving system that has been devised by Peter Walker and Partners. The paving of the plaza will rest on a series of pre-cast concrete tables that “suspend” the Plaza over troughs of planting soil that run the full width of the Plaza.

The suspended paving system will allow the structural soil to remain uncompacted and aerated, since the walking pathways are separated from the planting soil below. Many urban trees live in stressful conditions because they are planted where the roots are under pedestrian pathways and vehicle pavements, thereby compacting the soil so severely that the roots cannot grow or gather water and nutrients. The Memorial paving system as well as the expanded shale soil design will enable the trees to flourish because of a healthy root zone.

Memorial Pools

In addition to the plaza area, the Memorial will have two massive pools set within the footprints of the Twin Towers with the largest manmade waterfalls in the country cascading down their sides.

The names of the nearly 3,000 individuals who were killed in the September 11 attacks in New York City, Pennsylvania, the Pentagon and the February 1993 World Trade Center bombing will be inscribed around the edges of the Memorial pools.

Sustainable Design

The Memorial Plaza will be one of the most sustainable, green plazas ever built. The Memorial project is pursuing the Gold certification under the LEED for New Construction (LEED-NC) program of the U.S Green Building Council and is designed to satisfy the requirements of New York State Executive Order 111 and the WTC Sustainable Design Guidelines.

Unlike other green roofs, the Memorial has been designed to support almost 400 large indigenous trees. The irrigation, storm water harvesting and integrated pest management systems will ensure sustainable treatment of the site and conserve energy, water and material resources. This large urban forest will link to adjacent green spaces at Battery Park City, City Hall Park, the churchyards at Liberty Church and St. Paul’s Chapel, Liberty Plaza, and the new Liberty Park just to the south of the Memorial, providing habitat and green space within Lower Manhattan.

The Plaza has been designed to collect the stormwater in tanks below the Plaza surface. The storage potential will exceed the irrigation needs of the Plaza. Daily and monthly irrigation requirements will be met by the harvested stormwater.

Construction will continue throughout the winter, spring and summer, in preparation for the opening of the Memorial in 2011 on the 10th anniversary of the attacks.