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CU-Structural Soil for Pavement Loads Kensico Dam Plaza
Extrapolated from edited case study by Len Phillips and Richard Gibney


The Kensico Dam Plaza work involved installation of granite posts and pipe railings, four brick plazas totaling about 15,000 sq. ft., seating areas with benches, and the planting of 42 new trees: red and white oaks, American and 'Hop' hornbeams (pictured) and Hackberries. At heart, the project was about installing protective layers of CU-Structural Soil beneath the hardscape to ensure the successful growth of the new trees.

The Kensico Dam Plaza is a 20-acre part of the 98-acre Kensico Reservoir. The southern extension of the reservoir is located in Valhalla (Mount Pleasant), in Westchester County, New York in the Catskill Mountains, 32 miles north of New York City. Sleepy Hallow is just a five-mile drive to the west, for you fans of The Headless Horseman.

The reservoir was built as an earth and gravel dam in 1885, then constructed as a masonry dam from 1915-17. It is 307 feet high and 1,843 feet long. Kensico Dam Plaza is at the southern end of the reservoir. The property was acquired as parkland in 1963 from the New York City Watershed Commission and remains the property of the New York City Department of Environmental Protection. It is listed on the National Register of Historic Places as part of the Bronx River Parkway Reservation.

In 2005 the New York City Department of Environmental Protection (DEP) commenced a $31.4 million project to rehabilitate the dam infrastructure, reconstructing portions of the dam, spillway, stone masonry surfaces and reconstructing the dam's plaza, which included regrading, beautification and landscaping.


For the new trees to survive the pavement loads, the landscape architect specified excavating 2-3' of soil and replacing it with CU-Structural Soil(R) (aka CU-Soil). This structural soil was developed at Cornell University to bear pavement loads after compaction and allow root penetration and vigorous tree growth. The loam contained at least 20% clay to maximize water and nutrient holding capacity, and was mixed with 1" dia. crushed stone to create void spaces. A small amount of a hydrogel was added to prevent the soil and stone from separating during the mixing and installation. The proportion of soil to stone was approximately 80% stone to 20% soil by dry weight. This proportion insured that each stone touched another stone, creating a rigid skeleton, with the soil filling the pore spaces between the stones.


The structured soil was installed in layers and compacted after each layer was added. This continued to the final grade just before a layer of stone dust was applied. After the CU-Soil was installed, the new trees were planted with regular soil immediately around the root balls. The clay brick pavers were set atop the structural soil with no mortar joints to allow water to infiltrate into the structural soil, thus requiring no automatic irrigation system. Photo: Ken Uhle, RLA

Landscape Work on the Plaza
Installation of granite posts, pipe railings and 15,000 sq. ft. of brick plazas was likely to have a major detrimental effect on existing and the 42 new tree plantings. The landscape architect thus specified excavating 2-3' of soil and replacing it with CU-Structural Soil to allow root penetration and vigorous tree growth. The special soil was developed at Cornell University to safely bear pavement loads after compaction The installation was overseen by the university and its licensed representatives to be sure the procedure was correctly done and in the best interest of the newly planted trees. All of the newly planted trees are likely to survive the plaza environment and live for a normal lifespan.

The plaza/park is the start of the bike and jogging trail that winds along the Bronx River Parkway. The plaza also features "The Rising," Westchester's monument to the county residents who died at the World Trade Center on 9/11. The plaza offers a wide variety of public activities: concerts, fitness classes and picnicking. There's also a renovated playground with safety surfacing, in-line skating, widened walking and nature study trails and opportunities to study nature and the trees.

As seen in LASN magazine, June 2018.

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October 15, 2019, 4:52 am PDT

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