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 , 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 , 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 custom- modified 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 core- drilling, 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 cost- effective manner.

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