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Opportunity
In 1917 and 1925, two water mains (known as siphons) were built to carry drinking water from Brooklyn to Staten Island. Located at depths of 56 and 60 feet (17 and 18 meters), they were laid into trenches at the bottom of the harbor in 12-foot (3.7-meter) lengths connected by flexible joints.
The Port of New York and New Jersey is the third largest port in the country, handling almost 40% of the East Coast shipping trade. The port provides more than 279,000 jobs to the local economy and $12 billion in annual wages.
The port’s Anchorage Channel was 45 feet (14 meters) deep — not deep enough to accommodate the next generation of cargo mega-ships. Dredging operations would deepen the channel to 50 feet below the mean low-water level over a 19,000-foot (5.8-kilometer) stretch from the Verrazano Bridge to the Port Jersey Channel.
Because the deeper channel might compromise the two aging water siphons, the New York City Economic Development Corporation launched a project to replace them with a larger and deeper siphon. The project required boring a tunnel under full hydrostatic pressure, through highly variable clays and sands.
Solution
In 2005, Mott MacDonald was retained as a Joint Venture partner to evaluate alternatives for replacing the existing siphons with one new 72-inch-diameter steel pipeline at a sufficient depth. Alternatives included wet trench cut-and-cover, horizontal directional drilling, microtunneling, and tunneling either through relatively shallow soft ground or through very deep rock.
We considered environmental impacts, impacts on shipping traffic, permitting challenges, scheduling risks, cost risks, and operations and maintenance challenges.
We recommended the use of a 72-inch-diameter (1.8 meters) steel siphon inside a tunnel with an overall diameter of 12 feet (3.7 meters). The outer tunnel would be bored through soft ground at a depth of 100 feet (30 meters) and constructed with precast segmental lining.
Our recommendation was accepted, and Mott MacDonald was retained to provide overall project management for the Joint Venture. We were responsible for the design of the tunnel and siphon, the structural design of all valve chambers, and the abandonment of the existing siphons and their associated chambers. In addition to the water crossing, the project included several thousand feet of water transmission mains on land, to connect the new siphon to existing pipelines.
Water mains on the land side included 20-inch (51-cm) ductile iron mains, and 60-inch (1.5-meter) and 72-inch (1.8-meter) steel mains. Several chambers contained butterfly valves, gate valves, pressure-regulating valves, blow-offs, and flow meters.
The project also required two trenchless crossings of the Staten Island Railroad, with 60-inch steel pipe installed in 84-inch (2.1-meter) steel casings. Passive cathodic protection was provided for all land-side steel pipelines.
The project also included a new chlorination station on Staten Island, able to treat 150 million gallons (568 million liters) of water per day. We performed electrical, instrumentation, structural, and architectural design of the chlorination station, and quality control reviews of all elements of the project designed by our Joint Venture partner.
The tunnel was the first major underwater soft-ground water tunnel in New York City, and the first tunnel of any kind built under the harbor in many decades. A mixed-face earth pressure bound tunnel boring machine was used to bore the tunnel, with train tracks transporting workers, equipment, and excavated soil to and from the tunnel. We coordinated the participation of agencies including the New York City Economic Development Corporation, New York City Department of Environmental Protection, Port Authority of New York and New Jersey, and the US Army Corps of Engineers.
Outcome
Ocean shipping is one of the most economical and environmentally sustainable means of transporting freight. According to the New York City government, new and larger “Post-Panamax” vessels offer additional environmental benefits by carrying more cargo in fewer ships and making use of cleaner fuel technology.
The new 100-foot-deep (30 meters) water siphon enables these bigger vessels to navigate New York Harbor. The new siphon provides five million gallons (19 million liters) of water per day under normal conditions and up to 150 million gallons (568 million liters) per day in an emergency.
Speaking in April 2012, Mayor Mike Bloomberg said, “New York Harbor has been a critical part of our economy since the founding of our great city some 400 years ago. And if we want New York City’s economy to stay competitive, we must accommodate new mega-ships and their cargo. This investment in our infrastructure will spur economic activity all along our working waterfront.”
On October 29, 2016, Mayor Bill de Blasio and other New York City officials announced the completion of the project. According to the mayor, "Our city is better prepared to tackle 21st century threats like Sandy today than ever before. This tunnel is one measure that will help Staten Island spring back to action in the event of a disaster that would disrupt the water supply. Measures like these are being implemented across the city, from Red Hook to the Rockaways. Our city is becoming safer and more resilient every day."