Upper Stillwater Dam
- Location: Duchesne, UT
- Owner: Bureau of Reclamation, Provo District
- General Contractor: ASI-RCC
- Technique(s) Utilized: Asphalt Grouting, Chemical Grouting, Close Tolerance Drilling
- Subsurface Conditions: Roller compacted concrete dam
The Upper Stillwater Dam, located in Duchesne County, Utah, is part of a larger water control and irrigation system designed to make use of Utah's share of the Colorado River. The dam has a maximum height of 292 feet (crest EL. 8,182 feet), crest length of 2,650 feet, crest width of 29 feet, a maximum base width of 183 feet and a reservoir capacity of 32,496 acre-feet. The Upper Stillwater Dam was constructed between 1983 and 1987 using roller compacted concrete (RCC) and is classified as a gravity dam.
PROJECT BACKGROUND
Shortly after construction on the Upper Stillwater Dam was completed, a series of full transverse cracks developed through the dam structure. The transverse cracks were continuous and irregular from the upstream face to downstream face. Crack widths on the face of the dam ranged from hairline to approximately 0.5 inches. Water flowing through the cracks ranged from a few gallons per minute up to nearly 2,800 gallons per minute.
Five of the cracks that were of primary concern had previously been grouted above the gallery floor elevation with hydrophilic urethane grout and below the gallery floor with cement grout. Both grouting applications were unsuccessful in sealing the cracks.
In 2003, the Bureau of Reclamation issued a solicitation for repairing the cracks that had been plaguing the dam since its construction.
The principal components of the Bureau's remedial program included injecting chemical grout into seven of the transverse cracks in the dam (cracks less than 20 gallons per minute;) designing, manufacturing and installing membrane systems for three transverse cracks in the dam (large flows;) and drilling holes from within the foundation gallery to drain 14 transverse cracks in the dam. Based on the severity of the crack, a treatment program was to be implemented involving one or more of the following scenarios: 1) drill a series of holes to drain the crack, 2) grout and drain the crack, and 3) grout and drain the crack, and install a membrane liner extending across the crack plane and from the crest of the dam to the underlying bedrock.
The membrane treatment option was considered extremely important because it would provide a long-lasting water barrier system.
The Bureau's solicitation required a detailed technical plan for the design and construction of the membrane slot, the extracting of concrete and the installation and sealing of the membrane. Design considerations for the interior membrane system had to include life expectancy, movement capability, ability to withstand expected hydrostatic pressure, crack movement and cold temperature properties.
THE WORK
The proposal submitted by ASI Constructors, Inc. (ASI) and Nicholson was selected.
Work on the Upper Still Water Dam began with the chemical grouting treatments using polyurethane resin. The grouting established effective water barriers and reduced seepage at each of the crack locations. Small diameter injection holes were drilled to intersect the crack plane at the midpoint of the specified treatment zone. The chemical grouting was followed by the drain hole drilling, which included the drilling a series of three-inch drain holes in a fan-shaped pattern at each crack location. Each drain hole was designed to intersect the crack plane at specified depths and inclinations as measured from a designated drill station.
The drilling for the membrane slot holes, which would be the most unique and challenging portion of the project, began after the drilling of the drain holes was complete.
The membrane slots, each measuring 14-feet long with a minimum opening width of 4.5 inches were installed to depths of 158 feet, 182 feet and 240 feet, respectively. Construction of a typical slot consisted of drilling a series of overlapping drill holes using a percussion drilling system modified with a unique guide system. An initial pilot hole was core drilled to the depth of the slot followed with a percussion drilling system fitted with a special guide sleeve designed to follow the profile of each sequential slot hole to ensure a high degree of verticality and bearing alignment with adjacent slots.
Constructed of stainless steel, the membrane was designed with accordion folds (zig-zag) designed for lateral expansion as the dam structure expanded and contracted. Pre-fabricated components of the membrane systems were delivered to the site and field-fabricated into final assembly for the individual slots. After an assembled membrane was lowered into its final position, the bottom seal was placed.
Finally, the upstream and downstream annular space of the slot membranes were back filled with a special hot asphalt/Portland cement mixture specifically designed for adhesion to stainless steel and concrete.
THE RESULT
Nicholson's work proved to be a unique and durable solution to a long-standing challenge at Upper Stillwater Dam. The initial scope of work for the project was to chemically grout seven of the dam's existing cracks. Due to the success of the grouting operation for the first seven, the scope of work doubled, for a total of 14 chemically grouted cracks.
Upper Stillwater Dam remains one of the world's largest existing RCC dams.
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