Schwing Boom and Trailer Pumps Complete World’s Largest Circular Prestressed Water Tank 2005
From Concretepumping
DYK Inc., El Cajon, CA, a leader in the design and construction of prestressed concrete water storage tanks, is reaching the final phases of construction on the 35,000,000-gallon capacity Earl Thomas Reservoir at the Alvarado Water Treatment Plant in San Diego, CA. Working under general contractor C.E. Wylie Construction Co., San Diego, CA, DYK called on operators and concrete pumps from American Standard Concrete Pumping, Fontana, CA, to install 20,000 cubic yards of structural concrete into the tank. To complete vertical prestressing and the exterior finish of the 35,000,000-gallon structure, DYK utilized one of their five WP 1000 XP trailer-mounted concrete pumps in an automated wet-mix shotcrete process.
The upgrade and expansion project is part of the City Water Department’s Capital Improvements Program to ensure San Diego area water will meet all current and future drinking water standards and provide maximum protection of public health. Measuring 1289 feet in circumference with a 406-foot interior diameter, the $27 million reservoir will be the world’s largest circular prestressed concrete tank of its kind. The addition will increase the storage capacity of the plant to a total 77-million gallons of drinking water, servicing 500,000 citizens. DYK’s construction, prestressing and shotcrete divisions and pumping contractor American Standard began work on the reservoir on August 21, 2003.
Over their 40-year history, DYK is responsible for engineering, constructing or building more than 2.5 billion gallons of low maintenance prestressed concrete tanks. The company is known for embracing technological advancements to improve the construction of these tanks for water treatment, distribution, storage, and the retention of storm water runoff from industrial sites. Each phase of the Earl Thomas Reservoir tank construction reflects DYK’s dedication to building strong, attractive tanks with greater seismic reliability, less leakage, low maintenance and longer life spans for municipal, industrial and commercial users.
In the early months of construction, DYK maximized production by simultaneously pouring the foundation, floor and walls on one section of the Earl Thomas Reservoir while grading was being completed on an adjacent section. Engineering and design plans by Malcom Pirnie/Richard Brady & Assoc., San Diego, CA called for a highly reinforced membrane concrete floor with a slight upward slope to the center of the tank. This conical hopper bottom design facilitates collection and removal of sediment at the perimeter. American Standard’s KVM 52 and KVM 45 concrete boom pumps completed 12 separate floor pours, pumping between 400 and 500 yards at a time. Southern California Sales Manager Louis Arcia said that with over 170 feet of vertical reach, the contractor’s 52-meter pump was able to complete 10 of the 12 total pours alone.
To complete vertical prestressing and the exterior finish of the Earl Thomas Reservoir, DYK utilized one of their five WP 1000 XP trailer-mounted concrete pumps in an automated wet-mix shotcrete process.
Curved steel wall forms were then erected along the outer circumference of the tank. The core wall was cast continuously to the full height and in equal sections. Vertical prestressing tendons cast inside the walls provide compression and minimize cracking and deterioration. American Standard completed the tank’s 27 wall sections using their 23-meter boom pump manufactured by Schwing America. Equipped with an 87-foot three-section boom, the concrete pump easily completed the 39’-3” tall core wall sections. To reduce bending caused by hydrostatic, thermal, backfill and seismic forces on the tank wall, the top and bottom of the core wall is separated from the roof and floor by neoprene bearing pads to provide an unrestrained connection. A continuous PVC bulb waterstop at each wall-roof and wall-floor connection prevents leakage.
Columns on the Earl Thomas Reservoir were completed concurrent to wall construction. To support the tank’s 18-inch thick, two-way flat slab concrete roof crews completed 227 round columns measuring 30 inches in diameter.
The next phase of the tank’s construction involved 12 separate roof pours, each pour requiring an estimated 700 to 800 yards of concrete. American Standard utilized their KVM 52 and S 58 SX boom pumps to place concrete into the tanks “cap.” Forms created the desired slight upward slope to the roof’s center.
Once the roof was completed, DYK utilized their state-of-the-art circumferential prestressing equipment in a three-phase process to produce a durable, crack-free, leak-free storage structure.
To create an ideal bonding surface on the corewall for the initial layer of shotcrete, DYK utilized their newly developed automated hydroblasting process to clean and etch the tank wall instead of conventional abrasives like sand. DYK’s hydroblaster is carried, powered and controlled by a mechanized tower that travels around the circular wall at a controlled speed. The machine recycles the water used by the operation.
Next, wet-mix shotcrete was applied on top of the blasted core wall from a nozzle mounted on the same mechanized tower. At the Earl Thomas Reservoir, DYK’s WP 1000 XP remained stationary while 500 feet of line measuring 2 1⁄2 inches in diameter feeds the end nozzle mounted on a mechanized tower. The tower travels around the circular wall at a controlled speed of an estimated 250 feet per minute. The arm of the tower moves up the side of the 39’ 3”-high tank in a uniform spiral path, applying an estimated 10 cubic yards per hour. DYK applies an aggressive shotcrete mix consisting of ten-sack cement, concrete sand, steel fibers, air and retarders, formulated to be high in PH and rich in alkaline to prevent strand corrosion. The mix had a four-inch slump.
DYK Prestressing Project Manager Randy Smith commented on the advantages of the automated system. “In comparison to hand-applied shotcrete, the automated system minimizes several variables concerning distance from the wall, applied thickness and the shooting angle. If the consistency of any of these is compromised, it can diminish final quality. Automation provides a uniform cure and prevents shrinkage and cracking.”
The third step of DYK’s state-of-the-art prestressing technique, external circumferential strandwrapping, supplies the corewall compression. The galvanized 3/8”-diameter seven-wire strand was applied to the wall with the same mechanized tower, this time equipped to provide stringent stress-tolerances and electronic winch controls to assure accurate strand spacing. DYK utilized 58 reels of strand, measuring 242 miles long. If placed end to end, the strand measures approximately the distance from San Diego to Santa Barbara, California. The strands withstand 14,950 pounds of force with a -/+1.5% differential.
Utilizing the same system as the initial grout application, crews then apply several protective layers of the same shotcrete mix, with the addition of polypropalene fibers for uniform cure and cracking prevention, over the prestressing strand. The full cover of the exterior is built up in numerous thin layers. Shortly after the final shotcrete layer is applied and soaked with water, plastic sheeting is lapped and sealed as necessary to retain moisture and properly cure the concrete. Upon completion, the exterior wall of the Earl Thomas Reservoir tapers from 38-inches thick at the base to 12-inches at the top. A total 750 cubic yards of shotcrete mix was applied during the prestressing and exterior construction phases.
Smith commented on the World Pump’s twin circuit hydraulic system: “On most pumps, there are short periods of time when the supply hesitates while the valve switches cylinders,” said Smith. “That sacrifices quality and affects productivity. We’ve tried equipment from just about every manufacturer, and the Schwing 1000 trailer pumps have the performance we’re looking for. Our pumps are equipped with Rock Valves, and I’m convinced they perform the quickest, most aggressive cylinder switch in the industry.”
The Earl Thomas Reservoir is scheduled for completion in Summer 2005. Beginning in October 2004, general contractor C.E. Wylie commenced on the installation of piping and pumping components.
