gatehouse anchorage system for prettyboy dam

ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
PROJECT DESCRIPTION AND HISTORY
Dam Description. Prettyboy Dam was built during the early 1930s in Baltimore, Maryland and is owned by the
City of Baltimore. It is a concrete gravity dam which is about 150 feet
high and 700 feet long, and is classified as a large high-hazard dam. The
dam is founded on rock, which is primarily a foliated micaceous schist,
and supports a bridge which carries Prettyboy Dam Road.
The dam creates the Prettyboy Reservoir, which has a design storage
volume of about 58,000 acre-feet. Together with the downstream Loch
Raven Reservoir, the reservoir provides about 60% of the water supply
for the 2.7 million residents of the Baltimore metropolitan area.
Control of water flow through the dam is via a concrete gatehouse which is monolithic with the dam.
Dam Deficiencies. By 1978, extensive cracking was observed in the gatehouse and the adjacent main body of the
dam, along with substantial water leakage into the gatehouse stairwell, to
the extent of requiring staff to wear rain
jackets at times. This cracking was
observed
abovewater,
with
the
expectation that extensive cracking
existed underwater as well, in conditions
that would be difficult to both inspect
and rehabilitate. To respond to this
concern, continuing until 1994, six
investigations of the cracking were
performed by five different consultants, but with inconclusive and/or
inconsistent findings, thus leaving the situation a mystery.
Inspection, Testing, and Monitoring. At that point, Alvi Associates participated in
a comprehensive multi-phase dam
investigation involving many tasks:
exhaustive review and summary of all
available
records,
abovewater
inspection, underwater inspection using
divers and a remote-operated vehicle
(ROV), precise mapping of defects throughout the exterior of the dam as
well as inside the gatehouse, crack monitoring during gate testing
operations, concrete coring and testing, analyses and evaluations, and
preparation of a 300-page study report with recommendations.
Forensic Investigation. Alvi Associates played an integral role in all aspects of the investigation, including leading
the forensic structural and geotechnical investigation of the gatehouse cracking, which was the most critical
deficiency of the dam. After an intense process of detective work involving many long collaborative sessions, the
mystery was solved via the insight of discerning that the cracks clustered into eight distinct groups, and likewise
discerning three distinct general causes of the cracking, with each cause contributing in varying degrees to each
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ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
crack group. In other words, a “cause-effect matrix” was developed, thus transcending the usual assumption of a
simple one-to-one influence of cause to effect. The three identified causes of the cracking were as follows:
•
Vertical Flexure of the Dam – In this mechanism, the dam acts as a beam-on-grade in a vertical plane
parallel to the dam axis. The dead load of the dam is the primary load involved, which results in
settlement by compressing the foundation rock. The primary factors affecting the settlement are the
varying height of the dam, stiffness of the
foundation rock, and creep behavior of the
concrete and foundation rock. The flexure
produces compressive stresses toward the
top of the dam, related lateral tensile strains
due to the Poisson effect, bending and shear
in the gatehouse walls (due to a “pinching”
effect), and concentrated compressive and
tensile stresses at the gatehouse stairwell
due to the discontinuity the stairwell void
creates in the surrounding compressive
stress field.
•
Differential Settlement – In this mechanism,
the differential settlement which would
normally occur between the gatehouse and
the main body of the dam is restrained by
the monolithic connection of the two. The
key loads involved are dead load of the main
body of the dam, dead load of the gatehouse, lateral water pressure against the upstream face of the
dam, uplift pressure under the dam, and uplift pressure under the gatehouse. The differential settlement
produced by these loads is magnified by creep effects. This differential settlement produces flexural,
shear, and deep beam stresses in the gatehouse and in the vicinity of the gatehouse/dam interface.
•
Bridge Deformation – In this mechanism, the reactions from the bridge carried by the dam include
negative moments at the ends of the spans adjacent to the gatehouse. These moments produce local
flexural and shear stresses in the walls of the gatehouse near the stairwell.
Our hypothesized causal matrix was quantitatively validated by analyses of stresses and deformations of the dam,
gatehouse, and bedrock, and the resulting predictions were found to fit the observed cracking remarkably well.
To our knowledge, this type of complex multicausal model represents a relatively novel approach to forensic
engineering investigation.
Stability Analyses. The next step was to assess the implications of the cracking. We performed stability analyses
considering various mechanistic assumptions and a wide range of potential failure surfaces. Several of these
scenarios were found to have a factor of safety close to 1.0, thus indicating a significant risk that the cracks could
interconnect to the extent of precipitating a catastrophic gatehouse stability failure, with associated inability to
regulate water flow through the dam. Our solution to address this concern is described below.
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ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
Rehabilitation Design. To stabilize the gatehouse, Alvi Associates designed a $6 million anchorage system
consisting of 38 post-tensioned steel threadbar anchors
oriented approximately horizontally. The anchors were
core-drilled with lengths of up to 70 feet into the dam,
all while working underwater in water depths up to
100+ feet.
The precise geometry of each anchor was selected to
carefully dodge many fairly tight constraints within the
gatehouse, including intake openings, gate chambers,
wet wells, a float gage well, a stairwell, a valve
chamber, various other chambers and galleries, and of
course the downstream face of the dam.
To provide adequate stabilization force while working
around these many constraints, two different
anchorage systems were used in combination:
•
One system consists of 26 anchors, with the
anchors directly attached to the face of the
gatehouse.
To prevent damage to the
gatehouse concrete during post-tensioning, these
anchors were tensioned to a modest load and
grouted in two stages, with the second grouting
stage being bonded to increase the ultimate capacity
of the anchors. A special rubber “wiper” detail was
developed to enable this two-stage grouting in the
relatively deep underwater conditions such that the
bond zone would be fully grouted, while avoiding
leakage of the grout into the free-stressing zone.
•
The other system consists of 12 anchors arranged in
pairs, with each pair of anchors transferring load
through a large 32-ton high-strength prestressed
concrete beam which bears against the mid-width
portion of the gatehouse through a pair of
elastomeric pads. These anchors were each grouted
in a single stage, and relatively precise simultaneous
jacking was required for each pair of anchors.
The anchorage system was expertly constructed by Brayman
Construction, with Alvi Associates leading the review of
contractor technical submittals and closely partnering with
the contractor to resolve technical issues during
construction.
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ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
Construction Aspects. The following are examples of some of the team’s noteworthy challenges and
accomplishments during construction:
•
Nearly all of the work needed to be performed underwater, in water depths reaching more than 100 feet.
This required rotating teams of divers using two
decompression chambers, underwater cameras on
the diver’s helmets, and continuous audio
communications between the divers and supervisors
at the surface.
•
For the anchor drilling, a core-drill was custommodified so that it could “breathe” underwater and
was equipped with sensors to monitor hydrostatic
water pressure. The drill was positioned on a
specially designed drill cart equipped with four
cameras, and was operated remotely from the
surface. The drill and drill cart were precisely
positioned to meet the tight construction tolerances
by attaching them to an extensive drilling template consisting of vertical and horizontal steel H-beams.
•
Two anchor holes in deeper water severely failed to pass watertightness tests and had high grout takes
during pregrouting, thus indicating substantial leakage through the dam. To address this, the team
worked together closely and quickly to investigate a
wide variety of potential solutions. As our “Plan A,”
we agreed to add polypropylene fibers to the grout
mix, with corresponding adjustments to the grouting
equipment and procedures, and we found that this
solution was successful.
•
To ensure adequate bond strength after coredrilling, the anchor holes were carefully roughened
using roller bits, with a special underwater camera
used to inspect the holes and evaluate the level of
roughness.
•
As noted above, specially designed rubber “wipers” were used to allow underwater two-stage grouting,
and simultaneous jacking was required for the pairs of anchors at the cross-beams.
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ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
Verification of Design Reliability. To verify design reliability, along with proactively preventing construction
problems and damage to the dam, the contract documents included many innovative measures:
•
Only prequalified contractors were allowed to bid on the project. Because of the uniquely challenging
nature of this project, we contacted more than 100 contractors around the US to alert them to the project
and to encourage them to submit a prequalification package. The result was that only four contractors
became prequalified.
•
It was evident early in the design process that the complexity of this work would require a great deal of
pre-mobilization research and planning, with meticulous attention paid to potential pitfalls and
development of specific measures to prevent and effectively respond to any such issues which might
arise. To accomplish this, rather than allowing piecemeal shop drawing submissions, we required
submission of a single coordinated shop drawing package covering all aspects of the work, with the entire
package needing to be resubmitted if any element needed to be revised. Further, the contractor was not
allowed to mobilize until the shop drawing package was accepted. Both the owner and contractor initially
had concerns that these requirements were potentially too stringent but, at the end of the project, there
was a consensus that this rigorous shop drawing process had proven to be highly beneficial, if not vital.
•
While the shop drawing review process was a necessary step to ensure that the construction process had
been carefully thought through, empirical evidence that the process would be successful was also
necessary, given the complexities involved. This need was met by developing a program of four
preproduction test anchors to be installed in a location of the dam away from the gatehouse, so that any
problems encountered during preproduction testing would provide a useful learning experience without
risking permanent damage to the gatehouse or other sensitive portions of the dam. A few issues were
encountered during preproduction testing – which allowed fine-tuning of the shop drawings – but
fortunately all of these issues were quite manageable and none were close to representing fatal flaws.
•
The cracking pattern in the gatehouse was such that, with increasing water depth, the cracks were less
dense and less wide. In view of this pattern, we decided to “zip up” the gatehouse by specifying a
particular anchor stressing sequence which generally involved working from the bottom up.
•
Given the challenging conditions of working underwater, underwater inspections were performed at key
milestones during the construction process. These inspections enabled close monitoring to ensure that
the construction process stayed on track, thus allowing any issues to be promptly identified and
effectively corrected while still at a readily manageable stage.
Because of the cooperative professional attitude and solid capabilities of all parties, the construction process
went exceptionally well – the project was recently completed ahead of schedule and under budget, and Alvi
Associates and the contractor both described it as their best experience ever on a construction project!
Based on our consultation with international experts in anchoring and dam rehabilitation during both the design
and construction stages, we understand this project to be the first of its type in the world.
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ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
PROJECT BENEFITS
Population Benefitting from the Rehabilitation. The Prettyboy Dam and its associated Prettyboy Reservoir are
located along the Gunpowder Falls, upstream of the Loch Raven dam and its associated Loch Raven Reservoir.
Together, these two reservoirs provide about 60% of the water supply for the Baltimore metropolitan area, which
has a population of 2.7 million residents.
Also, most of the Gunpowder Falls, including the reach
downstream of Prettyboy Dam, is part of Gunpowder
Falls State Park, one of the most scenic parks in
Maryland. This park is a resource for a variety of
recreational activities such as hiking, fishing, canoeing,
kayaking, and birdwatching for a large number of
people.
Economic Savings.
The anchorage system for
Prettyboy Dam was designed to prevent a catastrophic
failure in which a substantial portion of the gatehouse
could literally “break off” and fall into the reservoir due
to instability resulting from a shear failure. Such a
failure would prevent control of water flow through the gatehouse, with a spectrum of potential consequences.
At one end of the spectrum, the flow through the gatehouse could become inadequate, particularly if the
gatehouse valves could not be opened, and this scenario could jeopardize the water supply to the Baltimore
metropolitan area as well as the downstream Gunpowder Falls State Park, with an associated cascade of
economic impacts. At the other end of the spectrum, the downstream flow could become excessive, thus causing
potential flooding impacts, as well as loss of water storage if the
downstream Loch Raven Reservoir was already filled to
capacity.
In addition to these impacts to the downstream park and
Baltimore’s water supply, the cost of repairing the gatehouse
could easily reach several tens of millions of dollars. And last
but not least, the loss in public confidence and political
consequences associated with a gatehouse failure would be
very substantial.
While many of these impacts cannot readily be quantified in
terms of dollars, $100 million is perhaps a reasonable ballpark
estimate of the consequences of failure (assuming no loss of life). Considering that it reliably prevents these
consequences, we believe that the $6 million spent on the dam rehabilitation represents a very cost-effective
investment.
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ASDSO 2010 National Rehabilitation Project of the Year
GATEHOUSE ANCHORAGE SYSTEM FOR PRETTYBOY DAM
Awarded to
Alvi Associates, Inc.
September 2010
Environmental, Aesthetic, and other Considerations. As noted above, the dam is located along the Gunpowder
Falls State Park, and therefore mitigation of environmental impacts was a high priority. The following are some
key measures which were implemented to meet this need:
•
In coordination with the Inland Fisheries Division of the
Maryland Department of Natural Resources and the City of
Baltimore Reservoir Natural Resources Section, a stream
monitoring program was developed to protect aquatic
habitats, which are home to a carefully controlled population
of blue-ribbon trout, among other species. The stream
monitoring program included stations both upstream and
downstream of the dam and included monitoring of pH,
temperature, and turbidity.
•
A variety of measures were implemented to prevent water
contamination, including use of biodegradable and
environmentally-safe hydraulic fluids in all underwater equipment, as well as innovative methods to
contain drill shavings and grout overflow, pipe these waste materials to the surface, and then isolate
them for safe disposal.
In addition to these environmental considerations, the dam is also eligible for the National Register of Historic
Places. To maintain its historic integrity, Alvi Associates therefore designed the anchorage system to minimize its
aesthetic impact. This included use of concrete cross-beams with a color similar to the dam concrete, painting
steel hardware with a similar color, and embedding anchor heads in grouted pockets where possible.
Contribution to Advancement of the Community. This project contributes to advancement of the community by
preventing a failure which would greatly impact the community across many dimensions: loss of water supply,
economic consequences, environmental damage, loss of public confidence, and damage to a historic resource,
among others. We are delighted that our team’s innovative approaches to forensic investigation, design,
construction, and construction management – combined with the outstanding working relationships we
developed – resulted in a unique project which reliably prevents all of these adverse consequences in a very costeffective manner.
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