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Gatehouse Anchorage System for Prettyboy Dam Project

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Gatehouse Anchorage System for Prettyboy Dam Project 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 1 | Page 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. 2 | Page 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. 3 | Page 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,
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