In conjunction with the construction of a new filtration plant, the New Croton Aqueduct is undergoing restorations that will allow another 100 years of supplying drinking water to New York City.
By Jim Rush
The history of New York City is – like many great cities – inextricably linked to water. The natural, protected harbor surrounding the City served as an attractive stopping point for trans-Atlantic travelers and traders, while the Hudson River provided convenient passage inland.
Although ideal for transportation, the brackish estuarine waters around the City were not suitable for drinking water, leading New Yorkers to seek an alternative to wells, springs and cisterns for drinking water supply as the population continued to grow and spread across the region.
In 1842, the City completed the construction of the original Croton Aqueduct, a near-surface aqueduct that brought in fresh water from the Croton watershed about 40 miles north of Manhattan. By the late 1800s, the City sought to increase capacity and began the construction of the New Croton Aqueduct, a 31-mile tunnel system that was completed in 1893 and still serves as an important component of New York’s drinking water system today.
The City continued to build out its drinking water infrastructure with the construction of the Catskill and Delaware aqueducts, completed in 1916 and 1945, respectively, that, combined with the Croton Aqueduct, supply more than 1 billion gallons of water a day to some 9 million residents. This vast network, managed by the City’s Department of Environmental Protection (DEP), comprises more than 6,000 miles of pipes, tunnels and aqueducts stretching from rural upstate New York into the City.
Renewing the Croton Aqueduct
The Croton system was design to deliver unfiltered water from Croton Lake to the City. It did this for more than a century before being taken offline in December 2008 in conjunction with a project to build the new, 290-mgd Croton Water Treatment Plant. Plant construction was needed as a result of a federal order requiring that water from the Croton system be filtered.
As part of the plant construction, which is being built in Van Cortlandt Park in the Bronx, the New Croton Aqueduct was taken offline for renewal. (In a separate project, twin tunnels were constructed between the New Croton Aqueduct and the filtration plant – one to bring raw water to the plant and the other to return filtered water to the aqueduct. That project was completed by Schiavone-Picone in 2009.)
“With the construction of the plant, we needed to ensure that we had a reliable source of raw water, and a reliable source for discharging the treated water,” said Gerard Cox of DEP’s Bureau of Engineering, Design and Construction. “The aqueduct is over 100 years old, and we would like to get another 100 years out of it, so we’re putting in the effort now to keep it in good condition.”
The New Croton Aqueduct comprises three sections: 24 miles of 13-ft high, horseshoe shaped brick tunnel that carries water by gravity from Croton Dam to Jerome Park Reservoir; 7 miles of 12-ft, 3-in. brick-lined pressure tunnel that connects Jerome Park to the distribution system at 135th Street in Manhattan; and a 1,300-lf siphon under the Harlem River.
The renewal of the entire 31-mile New Croton Aqueduct was completed in phases, beginning with the inspection and evaluation of the tunnel and shafts – an effort led by Parsons Brinckerhoff and MWH – that began in 1993 and ended in 2003 (CRO-196). Frontier-Kemper supported the initial aqueduct inspection from 1997 to 1998. The next phase (CRO-333), which consisted primarily of renewing the gravity section of the aqueduct, was completed by AFC Enterprises from 2004 to 2006. The current phase (CRO-334), which began in 2009, is being completed by a joint venture of Frontier-Kemper, Schiavone and Picone (FKSP). It achieved substantial completion in January 2013.
During the initial phases of the project, inspections took place to identify areas in need of repair as well as the type of repairs to be performed. Visual inspections, sounding and probe drilling were performed based on the previously performed non-destructive testing that included soundings, scanning with ground-penetrating radar, and ultrasonic testing. Common problems identified included missing bricks, leaks and voids. Other tasks included ladder removal from shafts, site restoration and restoration of historic structures located on the surface (stone headhouses and stairs).
Getting to Work
After being awarded the contract in January 2009, FKSP began work on shaft sites in Westchester, followed by Bronx tunnels in May 2010 and Manhattan tunnels in September 2010. The fact that the work area was spread out across different parts of the alignment, and within various jurisdictions, added a wrinkle to the project. Agencies that had a stake in parts of the project included city and state historic agencies, the New York State Department of Environmental Conservation, New York State Department of Transportation, the U.S. Army Corps of Engineers and New York City Parks.
Challenges involved with the project included moving men and material into and out of the tunnel from limited access sites, as well as providing ventilation, lighting and power. FKSP selected or developed narrow equipment and used specially designed and built bridges to allow equipment to pass each other.
Grouting on the project consisted of contact grouting and consolidation grouting. Contact grouting consisted of injecting grout at low pressure to fill voids and cavities behind the brick liner and to reduce seepage out of the aqueduct using 2- to 4-ft deep drilled holes. Consolidation grouting used 6- to 12-ft deep holes into which grout was injected under low pressure to fill joints and small cavities in the rock foundation and natural and blast-induced fractures in the surrounding rock. This was done to alleviate concerns about water seeping out, as well as ground contaminants seeping in. The grout pressure was a concern since DEP did not want to over-pressure the brick liner.
Equipment used for the grouting process included pneumatic jackleg, electric rotary hammer drills, modified Brokk for drilling, grout pumps and packers. More than 64,000 bags of cement grout were used, keeping crews busy along the length of the alignment.
“We had more than 100 people working on the project at its height – including crews working in five or six different locations spread across 33 miles from Westchester to Manhattan,” said Lonnie Jacobs of FKSP.
Nearing the End
One additional component of the contract was the construction of a concrete plug to separate the raw water section north of the filtration plant and the treated water section south of the plant. Construction of the plug was added as a change order to the contract and was completed from October 2011 to July 2012. The plug – 65 ft long – was cast in three sections. Officials were concerned about controlling the heat of hydration to reduce the potential for shrinkage cracking. It was paramount the concrete plug was non-permeable and did not convey water from one side to the other to ensure non-treated water did not mix with and contaminate the treated water on the downstream side. Extensive testing was performed after the concrete plug placement to validate the concrete plug was non-permeable and satisfy the New York State Department of Health requirements.
According to DEP’s Cox, preparation on the front end is key to managing a project of this magnitude. “The more planning and investigation beforehand makes the job so smoother,” he said. “We knew what we were getting involved in so there were no surprises. If, all of a sudden, you come along with an unexpected issue, it can stop a job.”
Additionally, the knowledge and cooperation of the people involved can make or break the project. “When you have knowledgeable people who can make decisions, that makes the job go much easier – and that is true for the contractor, designer, CM, trades people and management,” Cox added.
This article was written by Jim Rush, editor of TBM. Sources include “Rehabilitation of a Brick-Lined Aqueduct,” by Jacobs, Pannuti, Heffernan and Roberts, published in the NAT 2012 Proceedings; and “The New Croton Aqueduct Gets an Overhaul after 115 Years,” by Lonnie Jacobs, published in Frontier-Kemper’s Crosscut.
Owner: New York City Department of Environmental Protection
Designer: Aecom/Hazen and Sawyer (filtration plant); Parsons Brinckerhoff/MWH (aqueduct rehabilitation)
Construction Management: URS/Haks JV
Contractor: Frontier-Kemper/Schiavone/Picone JV (CRO-334)