Last month we reported that the “Diller Island” project will initially cost taxpayers $35 million to build two bridges out to the island, and in the long run will make the Hudson River Park pay for the maintenance of the piling structure below the floor surface.
This month, WestView News wanted to explore the size and costs of the proposed project by talking to pile driving firms and researching past projects and studies.
The pilings supporting the island will, according to Diller, vary from 15 to 64 feet above water. In Heatherwick’s design, these pilings, often referred to as “mushrooms” because of the shape of the concrete cap, are essentially very large, very expensive pieces of sculpture. Taking into consideration that the tides change the water levels by more than four feet daily, and these piers are far from shore because they were originally built to dock large cargo ships, the depth of water may vary from 15 to 35 feet in that area.
In addition, the pilings must also be driven far enough into the bedrock to support the structure, and this depth also varies. New York 2000, published by Monacelli Press, states that the Golf Club at Pier 59 has “support piles which were driven 250 feet below the riverbed.” That’s just a little north of Pier 55 site, and can help estimate the depth necessary.
The typical marine piling consists of H-shaped steel members driven into the riverbed with large pneumatic machines. Greater depths can be achieved by adding H-piles on top of one another. The drivers are mounted on barges floated, or “mobilized” to the site. The US DOT website gives a typical cost of about $300/foot length to drive marine pilings, plus additional costs for mobilization and de-mobilization.
Above the riverbed, the pilings need to be protected from water and weather. The pilings for Pier 55 would likely have a concrete form poured around the steel, as well as the concrete “mushrooms” at the top to support the deck. In our example, we will use $500/foot as an estimated cost for the concrete.
Given all these figures, it is possible to estimate a piling at 300 feet (80 feet above the riverbed, 220 feet below into bedrock). A piling that size would need about 80 feet of concrete to encase it and create the mushroom. This is just a “schematic guesstimate” but doing all the math (see sidebar) adds up to about $137,000 for just one of these pilings. To build all 300 pilings, that’s about $41,000,000!
Contrast this to a recent engineering study of Pier 40 pilings—done by McLaren Engineering Group and commissioned by the Durst Organization—which determined that only some of the 3,463 steel H-piles there needed to be repaired, lengthening their useful life by at least 30 years with little maintenance.
The recommended repairs involve filling a fiberglass wrapping with grout, from just below the riverbed and up about 17 feet to the cap. Below the riverbed, the pilings don’t require repair. The typical repair would cost somewhere around $8,500 to as much as $12,750 for a ‘severe’ rehabilitation.
Several architecture firms have conducted studies of how the 15 acre Pier 40 could be reconfigured for new uses, raising the floors above flood levels, rehabbing parking spaces, and adding recreation fields as well as commercial offices. It would not be hard to include entertainment performance facilities as well.
WestView concludes it makes more sense to repair Pier 40, which will bring more money and utility to the community, than it does to waste more than $41 million to build piles for this proposed “mushroom island.”
Mushroom Math at a Glance
| $300/ft. piling x 300 feet of structure to drive the piles | $ 90,000 |
| $500/ft. x 80 feet of concrete to pour the concrete | $ 40,000 |
| Mobilization/demobilization of pilings | $ 2,000 |
| Mobilization/demobilization of concrete | $ 5,000 |
| Schematic guesstimate per mushroom | $137,000 |
| 300 mushrooms | $41,000,000 |
