Erosion Control March/April 2017 : Page 46

and the grass has camouflaged the GrassProtecta so that it looks like a soccer field.” Journey to the Center of the City Connecting the communities of the New York metro area will take a bit longer than one good growing season, and it will also take more than bridges. In addition to its already-famous bridges, New York is also digging tunnels to move its millions of visitors and residents around with comfort and efficiency. The project is part of one of the largest transportation infrastructure initiatives currently underway in the US. It includes more than 11 miles of tunneling that encompasses work in multiple locations in Manhattan, Queens, and the Bronx. The vast majority of the project is in an urban set-ting, making material delivery difficult due to severe space constraints and the scope of the job. A section of the project called the East Side Access, connecting the Long Island Rail to New York City’s MTA, will create a series of tunnels underneath the city streets of Queens. One of the early steps in digging and embedding the transit tunnel network, starting at 150 feet below street level, was to dig a hole 67 feet deep to provide a subterra-nean access platform for staging equipment and personnel during the tunnel construction. In layman’s terms, it’s a huge hole in the earth used to access or to connect two tunnel lines underground; in the subsurface transport industry, it’s known as a bellmouth. With a bustling cityscape above it to support, the con-struction of this bellmouth would require a 67-foot-high, mechanically stabilized earth (MSE) temporary retaining wall with a 10-year design life to span the construction phases of the overall project. “It’s one of the largest infrastructure projects in the coun-try,” says Craig Bell, regional manager for Strata Systems, who provided onsite assistance. “They cleared the entire city block. They keep the traffic live on the streets above, and created an access point that gets deeper and deeper as it approaches the East River,” resulting in a platform to sup-port “a working area perpendicular to the tubes.” The tubes for the transit vehicles, he notes, will eventually be situated several hundred feet deeper. Explaining the importance of a reliable geogrid structure to support this work, Bell says, “A lot of the area is filled material. We’re talking about an area that was built up —elevated, essentially. There is bedrock as well, and there was some blasting, but it is primarily fill.” A Diff erent Kind of Wall The Strata Wall was built to shore up the boundaries of the nearly 70-foot hole incorporated hot-dip galvanized 4×4 wire form facing elements, with geotextile and geogrid facing wrap. While the original project specifications and plans called for a steel-reinforced wire wall, that option would have been both cost prohibitive and difficult to construct given the tight work-ing area. Materials had to be delivered with a crane into the hole where the MSE wall was constructed. Because StrataGrid is rolled and easy to handle, material transport and delivery are much more efficient, which also helped with the strict time 46 EROSION CONTROL and budget constraints on the project. The fill specified for backing the geogrid consisted of 1-inch-minus select, well-draining material, which Bell describes as a mixture of small rock and fines. “It’s a good material to work with. It compacts well. It’s easy enough to spread around.” He cautions, “With geogrid reinforce-ment you want a material that’s well graded, but not too big. Anything bigger than 2 inches is not a preferred material for geogrid. It can be done, but it’s not ideal.” There were real challenges from a constructability stand-point with regard to placement of the materials, says Bell. However, the builder, Tutor Perini Corporation, employed a novel solution. “They had a loader on street level feeding the fill material through a conveyor that led to a chute that dropped it a hundred feet down, and they would guide the chute in order to fill the area. Then they would spread and compact the material over the geosynthetic. They’d do that, lift, and then come back and do the next one, and the next one, and the next one. That’s how they got the area all the way up to 67 feet tall.” Bell describes building the MSE wall as a sequential process repeated many times until it reaches the desired height. “They’ll place the wire form, they’ll place the primary reinforcement, which is our StrataGrid 700, and then they’ll place the high-survivability geotextile face wrap at the wire form. After the structure is filled, all you really see is the wire form and the geotextile.” Although in appearance it is very different from a geogrid application used to reinforce a slope, “the technology is virtually the same,” says Bell. For this application, he explains, “We just steepened up that structure to vertical, or near vertical—in this case it’s about a 3-degree setback.” The wall’s designer, Chad Clark, principal with Clark Geotechnical LLC, says, “The quality of Strata’s products and services allowed us to design the temporary retaining wall system with confidence. This was important to us, given the significance of the overall project and critical nature of the temporary wall. And the structure is likely one of the tallest single-height MSE temporary walls constructed in the US using welded-wire form facing and geogrid reinforcement.” As with many geosynthetic applications, the East Side Access’ bellmouth temporary MSE wall will soon be hidden from public view. With the continued progress of the overall transportation infrastructure initiative, once the tubes are constructed, the hole will be filled, and new development—a constant in the City of New York—will likely be constructed on the site. Thinking nothing of the massive wall and all the effort that has gone into it, the transit riders, pedestrians, and motorists passing through Queens will continue to enjoy life in the city that they love. Whether supporting society by reinforcing efficient landfill operations, supporting neighborhoods by preserving parklands and recreation, or undergirding massive urban infrastructure improvements—and whether they are seen or hidden from view—geosynthetic materials and products are quietly inspiring innovation all across the country. EC David C. Richardson is an award-winning writer on science and public policy. WWW.EROSIONCONTROL.COM

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