Crest-Gate Dam Fortifies Tempe Town Lake
The dam, made up of eight 260,000-lb gates, replaces a rubber bladder on the lake’s west end to hold back the Salt River
One of the largest hinged crest-gate dams of its kind in the country is under construction in the Salt River in Tempe, Ariz.
Also one of the largest hydraulically operated steel-gate dams in the country, each gate on the $45-million project is 106 ft wide, 17 ft tall and weighs close to 260,000 lb, says Stewart Vaghti, senior project manager at Gannett Fleming Inc. Gannett Fleming is the engineering consultant and construction manager on the project, with PCL Construction as the prime contractor for owner, city of Tempe.
The new dam permanently replaces a rubber bladder which had captured some of the flows of the Salt River and formed Tempe Town Lake since 1999. Since opening, Tempe Town Lake has become a major attraction in the city, with an estimated economic impact of roughly $1.5 billion. The lake flanks high-rise office buildings for major corporate employers, including Amazon, Silicon Valley Bank, KPMG, Microsoft and State Farm.
The new dam consists of eight hydraulically operated steel gates supported by seven piers, two abutments and a 21-ft thick concrete foundation.
“The gates can lay flat in a major storm to drain [excess water] into the river,” says Chris Kabala, Tempe senior civil engineer.
The hydraulic cylinders support the gates at a 70° angle to maintain the lake. Massachusetts-based Steel-Fab Inc. created the crest gates for the dam. The company also created gates for several similar dams in Oklahoma City.
In order to avoid the cost and logistical problems associated with shipping eight massive gates across the country, Steel-Fab, of Fitchburg, Mass., worked with Phoenix-based Schuff Steel to fabricate the gates locally. Steel-Fab sent quality control and key fabrication personnel to Phoenix to oversee the fabrication process.
Steel-Fab produced smaller key components, such as hinge brackets and seal plates, that had to be fabricated under tight conditions in Massachusetts and shipped to Arizona.
“It was nice to have [the gates fabricated locally], because they would bring them out at two or three in the morning, and we’d stage them on site,” says Adam Gordon, senior project manager at PCL Construction.
Crews used a specialty Goldhofer trailer from Southwest Industrial Rigging when it was time to bring the gates down to the construction site in the riverbed for installation.
While the dam receives much of the attention, the project consists of several other features as well, including a nearly 1,000-ft scour wall that will prevent water from reaching the dam’s foundation during a release.
Because of the groundwater conditions, the project team went with a diaphragm wall construction that allowed crews to construct the wall in wet conditions without a mass excavation. It was both a time- and a money-saving maneuver, says Gordon.
Sacramento-based contractor Magnus Pacific Corp. installed the wall in 40-ft sections. First, crews dug 40-ft-long panels and filled them with bentonite to keep the excavation stable before installing two end-stop beams and a reinforcing cage. Finally, crews used two pipes to pump a highly-plasticized concrete mixture into the panel from the bottom up.
The process displaced the bentonite into a holding tank, and it was eventually reused to install the remaining sections of the wall. The bentonite became thick and difficult to pump during the excavation process, so crews used diaphragm pumps to force it through a belt-filter press to remove sand. The process resulted in the bentonite having a sand content of about 1%.
Magnus Pacific also installed a cement-bentonite cutoff wall that went under the foundation with a depth that varied from 30 to 80 ft. The low-permeability curtain wall ties into bedrock and was installed to prevent water from creating uplift pressure on the dam.
The construction team built the wall using a specialty Komatsu PC1250 mass excavator with a custom extension. While excavating, the team constantly mixed a specialty mixture of bentonite and concrete on site and then filled the wall with the bentonite cement and left it in place.
It was a constant operation to get that wall up as crews worked on that wall 24 hours a day for 36 days straight.
Phoenix-based Cemex provided concrete for the project and its Yuma division batched and installed approximately 18,000 cu yd of the roller-compacted concrete foundation.
Constructing the dam and ancillary structures was no easy feat as PCL Construction and subcontractors had to work through excessively wet conditions in the drained lake and riverbed.
The soil conditions were challenging because the upper 15 ft of soil on the site was a layer of sand, gravel and cobble, says Gordon. There was groundwater 4 to 5 ft below the existing grade, and crews had to mitigate the flow of water through the area. Some of that water came from Papago Park and lakes north of the Loop 202, both of which are also directly north of Tempe Town Lake.
Dealing with the water during construction was difficult, especially on the north side of the lake, where the presence of sewer lines and electrical utilities made it difficult to install a dewatering system. The team says part of the solution was to create wells in on the north side of the lake, but these did not work well because water perched on top of a cemented rock layer.
Below 15 ft, where the dam foundation was going, crews dealt with basin fill that consisted of low-permeability material and some cemented rock.
“When excavating, we had to dewater 2 ft below the structure,” says Gordon. “Down at the dam foundation, the biggest challenge was that you have portions of foundation that are cemented, hard material, and it had [pressurized] water springing through cracks in the rock layer.”
PCL used small pumps to control the water until it created a concrete leveling mud pad. Then, the team brought in filter sand for the foundation to manage the water.
In other areas of low-permeability material where it was easier to dig, PCL used wells that would dewater the area, but that tactic often failed to dewater the entire footprint of the foundation area. Crews subsequently moved well locations and used French drains to dewater.
Because the construction site was situated 100 ft downstream from the location of the existing rubber bladder dam, construction crews also had to constantly contend with the threat of flooding due to inclement weather.
“[We’re] working in an active river,” says Gordon.
The crews contended with multiple weather events that flooded the site, including three tropical storms from the gulf of Mexico in 2014. As as result, the project experienced about six weeks of delays.
During that time, construction crews were putting in shotcrete walls on the north and south sides of the lake, which required exposing 4-ft lifts that were vulnerable to potential floods.
“Well, it’s kind of a balancing act there,” says Gordon. “You’re trying to manage potential rain flows that come up [quickly]; then you have this exposed 4-ft lift all across the north and south side that could potentially wash or undermine and cause damage and cause a failure to your wall system.”
With the Tempe Center for the Arts located just 50 ft from the far corner of the excavation site, the stability of this area was a constant concern. If a rain event was likely, construction crews pushed dirt berms up against the area or put in extra time to finish the shotcrete walls to offer protection to prevent erosion.
The sheer size of the dam is not its only unique feature. The city also tailored the permitting process to peculiarities of the job.
“The permitting process was unique, one of a kind,” says Kabala. “The Arizona Dept. of Water Resources resolved the permits in three phases when [these projects] are typically permitted all at once.”
Of the over 20 technologies considered by the city, it chose the steel crest-gate dam because it is a proven technology. The project team expects the dam to last “50 years and maybe longer,” says Tempe City Engineer Andy Goh.
In order to construct the dam, the city drained a significant portion of the lake in February and March. This required a vast amount of coordination with other local and state agencies in order to preserve the wildlife in the lake and make sure the discarded water did not go to waste.
SRP, an Arizona public utility, helped drain the lake into one of its canals and used the water to irrigate parks and lawns. The construction team also interfaced with SRP and the flood control district to anticipate oncoming inclement weather and potential flood conditions.
The Arizona Game and Fish Dept. removed fish from the lake prior to construction. The city took several precautions, including removing fishing limits in the lake, moving some fish to a protected portion of the lake where water would be retained during construction and not stocking fish as usual beginning last fall.
While the dam project began receiving widespread attention earlier this year as the city drained the lake, construction crews and engineers actually began working on the project all the way back in 2014. Gannett Fleming finalized the design process in March of that year, and crews began early work in mid-2014.
Since the beginning of the project in May 2014, PCL Construction ran 10-hour shifts six days a week. Throughout most of the job, PCL maintained a crew of roughly 60 workers on site, including subcontractors. Toward the end of the job, that number decreased to 30 workers.
The city of Tempe also contracted PCL to remove the existing rubber bladders that sat at both ends of the lake. The eastern side of the lake has a grade-control structure to maintain water levels.
With both design and construction considered, the dam will cost $45 million upon completion. PCL began testing both computer and manual controls on the dam in April and began refilling the lake later that month.
The city expects the dam to begin holding back the lake’s 977 million gallons of water in time for Tempe Town Lake’s grand reopening, scheduled for May 14.