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Design, Inspection and Rehabilitation of the New Croton Aqueduct, New York City


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Author A Noble, D Roberts and A Fareth


Constructed between 1885 and 1891, the New Croton
Aqueduct (NCA) in New York State is a 50 km long (31 mi) brick-lined tunnel
conveying water from the Croton watershed north of New York City to distribution
systems in the Bronx and Manhattan. Except at two siphons, the aqueduct operates
as an open channel conduit from the New Croton Reservoir downstream 39 km (25
mi) to the Jerome Park Reservoir in the Bronx. The remaining 11 km (6 mi) is
operated as a pressurised conduit and includes a major siphon, of circular
cross-section, under the Harlem River. The aqueduct has a flow capacity of
approximately 1100 million litres of water per day (13 m3/s or 290 million
gallons per day) and supplies on average ten per cent of New York City’s
drinking water. The City’s water supply system is operated and maintained by the
New York City Department of Environmental Protection (NYCDEP).
This paper covers
the work done during the three main stages of the program to rehabilitate the
entire NCA: inspections, design and rehabilitation. It also includes some of the
more interesting structural findings of the original construction as well as
documents and drawings from the NYCDEP archives. The rehabilitation program has
been underway in phases since 1993, with inspections scheduled to minimise
outages, allowing this strategic aqueduct to remain in service during critical
seasonal periods.

The open channel flow section of the aqueduct in the north
is horseshoe shaped, with inside dimensions of 4.1 m (13.6 ft) in width and 4 m
(13.5 ft) in height. The pressurised section is 3.7 m (12.3 ft) in diameter, and
includes a 90 m (330 ft) deep siphon under the Harlem River. A total of 163
million bricks were used in the construction, enough to build a 48 km (30 mi)
wall around Manhattan Island, 3 m (10 ft) thick and 15 m (50 ft)
Between 1993 and 1997 a series of in-tunnel investigations
were performed in the open channel portion of the NCA, consisting of field
inspection, non-destructive geophysical testing, and coring of the brick

Between November 2004 and September 2005, a major inspection program was
conducted to assess the condition of the 11 km (6 mi) long pressurised section
and all shafts, headhouses and blow-off structures along the entire 50 km (31
mi) alignment. Inspection methods included using an underwater remote operated
vehicle (ROV) equipped with sonar and cameras to inspect the deep siphon, fibre
optics examinations of probe holes drilled through and beyond the brick lining,
and core holes and geophysical inspections to assess properties of the liner and
behind-the-liner materials. A water pressure and test grouting program was
conducted to assess methods and expected grout takes for the rehabilitation work
in the pressurised sections. The rehabilitation program is anticipated to be
completed in 2010.