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Installing Two 20-inch Sub-Aqueous Water Mains to City Island

Installing Two 20-inch Sub-Aqueous Water Mains to City Island

Dual Compound Curve Brings First-Ever NYC Technology to Long Island Sound 

In 2002, the City Island Bridge was determined to be in a state of serious deterioration by the New York City Department of Transportation (DOT). The city proposed to replace the bridge with a state-of-the-art cable-stayed bridge on the same alignment of the old bridge, which was built in 1901.

“The bridge was extremely old, and New York City had to replace it,” says Patrick Ronan, PE, a senior project manager in Dewberry’s New York City office. Next to the bridge underneath the waterway was an old, failing, and non-redundant water main. To provide a temporary bridge crossing during construction of the new bridge, the old water main would have to be moved. “The multifaceted nature of the project is why it was managed through New York City’s Department of Design and Construction, the city’s capital construction manager in the five boroughs,” adds Ronan. 

City Island is a small island off the coast of the Bronx in New York City. It’s a thriving recreational and commercial shorefront area with more than 30 restaurants providing social and economic benefits for the Bronx and the city. The island, a well-known water-front community and popular destination during the summer months, used to be a ship-building area. Today, it’s a fishing community with only one land transportation access on and off the island. 

New York City’s 6,800-mile water distribution system delivers more than 1.2 billion gallons of water to residents, businesses, and visitors daily. The much-anticipated replacement of the 114-year-old City Island Bridge — which connects the Bronx with City Island and serves as the sole access point for vehicles, cyclists, and pedestrians — necessitated the replacement of the existing sole source of water for the island. The project provided an excellent opportunity to design a new trunk water main connection to improve potable water capacity and redundancy. The oft-engaged community was sensitive to how the new bridge would impact the area’s seashore aesthetic. This made installing a water main on the bridge a less than desirable option. The landings of the planned subaqueous waterway crossing had some difficult physical constraints that required the creative use of trenchless technologies in the design. 

Ronan adds, “the bridge replacement is what initially drove the need for the new water main. This was fortunate because the old main was antiquated and, if it ruptured, it would have left the entire island without water.” 

A temporary bridge was erected in 2015, and the design for the water main was completed before December 2016. During the project’s construction, an interim water main was installed over the temporary bridge. Once approved in fall 2018, construction started on the water main project.

A Collaborative Effort

The project design included the replacement of the existing water supply to City Island by installing two new, 20-inch diameter, restrained joint ductile iron sub-aqueous water mains inside two separate, 32-inch diameter steel sleeves. 

“The Department of Environmental Protection (DEP) requested that there be two water mains instead of one for redundancy,” says Emilio Barcelona, an associate staff engineer with Dewberry. “That’s where we came in and studied what type of construction needed to be done.” 

The project team developed three alternate methods for installing the new water mains, including jacking, micro-tunneling, and horizontal directional drilling (HDD). 

The challenge was to plan and design an approach to water main installation by boring through hard rock underneath the Eastchester Bay riverbed. Due to tight space constraints on either side of the channel, coupled with the need to reduce impacts to the environment, the design required an innovative approach to water main installation. 

Ultimately, the New York City Department of Design and Construction (DDC) and New York City DEP, in conjunction with engineering consulting firm Dewberry, planned an alignment using horizontal directional drilling to drill the water main while curving in two dimensions to hit the targeted landing zone. HDD was ultimately selected as the preferred option and final plans and specifications were developed for bidding.

According to Barcelona, HDD is not a common method for installing water mains. “HDD is typically used for gas mains,” he says. “Other utility companies will be using this method of installation, because they have a lot of their cables going under the seabed.”

Earlier water main and sewer projects in New York City used traditional methods. Recently, new innovative trenchless technologies are gaining the attention of municipalities. Trenchless technologies, the construction method of HDD, played a critical role in executing this project by providing economical, practical, and environmentally responsible ways to install buried pipeline systems. The HDD process requires drilling a small pilot hole (6 to 9 inches) and subsequently enlarging the bore to the desired larger diameter, ultimately pulling the final size sleeve into place. A water main is installed by pulling the pipe through this sleeve. A critical part of the design was specifying the proper water main joints that could be used in this process and be curved.  

As with any major design project, developing the best team with the proper expertise is key to project success. The Dewberry design team was well supported by key expertise from their partner subconsultants Dawn Underground Engineering based in Flemington, New Jersey, and McLaren Engineering Group who has a presence in New York City. 

“With the major trenchless technology projects in particular, the need for a team atmosphere and the proper expertise is extremely important for successfully seeing the design through construction. Another factor for the success of this project was the flexibility of the entire team working together to solve problems. The contractor, Northeast Remsco, and the resident engineering team of McMillen Jacobs deserve much credit for working with us as a team to focus on the success of the project,” says Ronan. 

Challenges and Community Engagement 

The major challenges associated with this project included interconnections and staging with existing water mains, minimizing impacts to old growth trees on Parks Department land, preventing interference with utilities, deep water borings and bathymetric surveys, selection of staging areas to avoid wetland impacts, and installation of the sub-aqueous watermains by trenchless technology. 

“What’s different about this job is the dual compound curve. It’s curving in both directions – the horizontal plane and the vertical plane – because we have such a narrow target to hit on the island side,” says Ronan. “That’s what is new and has never been done in New York City on a water main.” Ronan says the reason this dual compound curve was chosen was to avoid trees, wetlands, city streets, and seawalls. 

The team worked consistently throughout the project to keep the island’s tight-knit and well-informed community members onboard with project goals and the construction schedule. 

Improved Water Capacity and Infrastructure Resilience

The project improved water capacity and infrastructure resilience for City Island by increasing the size of the main, resulting in more potential waterflow. The first water main was tested and put into service last fall. 

Ronan says one of the project’s biggest takeaways is a blueprint that can be applied in the future for tunneling under waterways throughout the city in areas with extremely tight launch and landing point conditions. These previous experiences can assist the designers, contractors, and municipalities with future decisions and help to carry out studies and construction to the next level. 

The project received the American Council of Engineering Companies (ACEC) New York Engineering Excellence Diamond Award – a top honor presented by the organization.