North American Special - Trenchless International

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April 2009 - Trenchless International
• 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.
Diagram showing the horseshoe-shaped
brick liner of the New Croton Aqueduct in
the gravity flow portions of the alignment.
The profile on the left shows a typical cutand-cover
section: the profile on the right is
typical of the mined sections. Source: The
City of New York Aqueduct Commission,
1895.
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
Test grouting operations in the Bronx pressurised
aqueduct.
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.
Heavy groundwater inflow encountered
after drilling into a water-filled void. Inflow
rates subsided over the following 48 hours
and were significantly reduced after grouting.
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.
This is an edited version of a paper by A. Noble,
D. Roberts and A. Fareth. Please refer to the paper
for more detailed information, acknowledgements
and references.
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April 2009 - Trenchless International
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