The NCA 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. 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 liner.

Between November 2004 and September 2005, a major inspection program was conducted to assess the condition of the 11 km long pressurised section and all shafts, headhouses and blow-off structures along the entire 50 km 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. Part 1: planning and design

The overall concept for rehabilitation of the NCA was developed in the early 1990s, at which time a far-reaching program for achieving a logical and practical approach to the work was established. Since outages could only take place seasonally, contracts for any program had to be appropriately sized to coincide with the seasonal outages, while taking into account the operational needs of other water supply aqueducts owned and operated by NYCDEP.

The design for the NCA rehabilitation was initially based on the results of previous investigations performed within the aqueduct’s gravity flow section in 1993 and 1996. The field investigation program in 1993 included:

• Visual inspection of 9.6 km of tunnel
• Geophysical surveys of 9.6 km of tunnel
• Remote operated vehicle (ROV) inspections
• Gould’s Swamp siphon
• Harlem River siphon
• Probe and core drilling
• Fibre optic scope inspection.

The field investigation program in 1996 included:

• Visual inspection of the remaining 29 km of gravity flow tunnel
• Geophysical surveys of 29 km of tunnel
• Probe and core drilling
• Fibre optic scope inspection
• Test grouting program
• Epoxy mortar lining – test section
• High quality video recording
• Walk-through inspections in 5.3 km length of pressurised section of NCA.

In general the design phase focused on restoration of the brick lining rather than its replacement with concrete or shotcrete. Where significant defects had been observed in previous inspections and were determined to require larger-scale repairs, in situ concrete was used. (Figure 2). The design phase also considered temporary structures, such as the design of standby bulkheads to fit inside the shafts in the pressurised section of the NCA. These temporary bulkheads would serve as a means of restoring the service of the NCA should it be required to provide water to the city in the event of an emergency situation elsewhere in the water supply system. In addition, the design phase included mechanical engineering for the inspection and replacement of ageing infrastructure such as pipes, gates and valves.

Part 2: inspection

Objective The objective of the shaft and tunnel inspections was to locate and document defects. Information gathered during the visual inspection was used to establish focal areas for both future repair contracts and further field investigation, which included coring, probe drilling, fibre optic scope inspection and test grouting.

Inspection of the tunnel and shafts

The scope of the inspection work under the construction contract included assessing the condition of the 9.6 km long pressurised section of NCA and inspection of shafts, headhouses and blow-off structures in both the pressurised and gravity sections.

Tunnel inspection was performed between stations. Brick-lined portions of the tunnel and shafts were sounded, and the entire tunnel alignment was visually inspected. Potential voided areas were recorded in the field log and marked on the wall for possible future probe hole drilling. The lowest part of the invert was typically covered with water 100 to 200 mm deep, preventing inspection; but defects were noted when observed.

The siphon under the Harlem River extends to a depth of 122 m. The purpose of performing an ROV inspection instead of dewatering the siphon and performing a walk-through inspection, was to minimise the risk of the lining buckling due to external water pressure acting on an empty lining. The siphon has never needed dewatering since it was brought into operation in 1891. The ROV was equipped with high resolution video and dual imaging sonar instruments. The objective was to verify shaft and siphon lining materials and construction features, to assess sediment levels and debris accumulations, to locate defects and to obtain video and sonic records of the structures.

Testing the ground

Non-destructive geophysical testing was performed within Gould’s Swamp and a fibre optic testing program was conducted at locations that appeared from the visual and geophysical investigations to merit further investigation.

Continuous diamond core drilling was performed on selected areas of the aqueduct to obtain samples of the tunnel lining, mortared and grouted rubble, and foundation bedrock for material identification and laboratory testing to determine engineering properties. Test grouting was performed in both the Bronx and Manhattan pressurised tunnels and at two shaft locations.

The comprehensive field investigation program was used to assess the current condition of the NCA and to design a rehabilitation program. Overall, the tunnel and shafts are in very good condition, with reparable leaks or defects at some locations. The visual inspection identified tunnel defects and the geophysical inspection determined the condition of the brick liner and the grouted and mortared rubble backing. Visual and geophysical inspection findings frequently complement the laboratory test results. The test grouting program successfully stopped or decreased inflows into the test areas. The locations of abandoned shafts were verified using geophysical methods. Investigations of the ‘soft rock zone’ determined that the area is geologically stable.

There are a total of 49 shafts along the aqueduct alignment, ranging from large chambers in cut-and-cover sections, to small diameter shafts up to 113 m deep. Seventeen of these shafts were filled at the end of construction and are difficult to detect from the ground surface or from within the tunnel. Eight shafts were lined with a combination of brick and Cast Iron. Most were rimmed with granite collars, typically 350 mm thick, at the surface. Collars at the tunnel intersection are up to 600 mm thick. Most shafts had steel ladders from the ground surface to the tunnel crown which often exhibited oxidation, tuberculation and/or section loss. The shafts were in very good to excellent condition, overall.

The Harlem River siphon and associated structures were successfully inspected by a ROV and found to be aligned according to historic drawings and without evidence of major displacement or defects. Part 3: rehabilitation

The goal of the project was to restore the gravity flow section of the aqueduct to optimal operating condition, thus extending the lifespan of this significant water supply for the New York City metropolitan area. A large-scale contact grouting program was performed in the horseshoe-shaped gravity flow sections of the aqueduct, with the objective of filling identified voids behind the liner, filling fractures in broken mortared and grouted rubble and foundation bedrock materials, and reducing water infiltration into the tunnel.

Cover depths of the aqueduct vary considerably, ranging from a few to hundreds of metres, typically consisting of hard rock. Primarily mined using conventional methods of the late 1800s, construction of the NCA also employed cut-and-cover methods in several low-lying sections of the alignment, totalling approximately 1.6 km. The NCA passes through numerous lithologic changes, fault/shear zones and under several significant water bodies, including the Pocantico and Saw Mill Rivers and the Tarrytown Reservoir. It also passes under the Harlem River as a siphon at a depth of approximately 122 metres below grade.

Soon after construction, reports on the NCA from The Aqueduct Commission documented prominent defects in the tunnel, such as large voids behind the lining. More recently, large-scale inspections of the gravity flow section, performed during the 1990s, and a 2004 inspection of portions of the pressurised sections, have revealed additional defects, such as open and/or deteriorated masonry joints/cracks, leaks ranging from trickles to several gallons per minute into and out of the aqueduct, missing bricks, formed openings, and in one place, a rupture through the liner with discernible offset. Despite these defects, the generally good condition of the NCA is remarkable. It is not uncommon to traverse several miles through the aqueduct without noting any significant defects.

Major liner repair

Major liner repair work consisted of cast-in-place reinforced concrete for filling of existing formed openings located at or near the crown of the tunnel and areas of brick heave located within the tunnel invert. Areas requiring brick replacement were minimal, totalling only 14.8 square metres.

The successful rehabilitation of the 38 km long gravity flow section of the New Croton Aqueduct resulted from good management and positive working relationships among the owner, the engineering consultants and the contractor. The rehabilitation was completed on schedule within approximately 17 months, with crews working a typical eight-hour work shift five days a week.