When work is complete on the Tempe Town Downstream Dam in early 2016, it will be a modern marvel and the world’s largest hydraulically controlled crest-gate dam. The new dam will replace the existing rubber dam, which has failed in recent years owing to age and UV damage. This project has seen its share of challenges, but PCL’s commitment to client satisfaction has kept the dam rehabilitation on target to meet goals.
The 950-foot steel structure across the dry Salt
River bed will be complete in 2016.
Construction is well underway on the Tempe Town
Lake Downstream Dam. Once it’s complete, it will
become the world’s largest hydraulically controlled
GLASS HALF FULL
The project has proved to be a showpiece for a number of PCL solutions, including elements of dewatering, demolition, earthwork, roller-compacted concrete, electrical work, cast-in-place concrete, coatings, cement-bentonite walls, scour walls, gate fabrication and erection, hydraulic systems, and geosynthetic clay linings.
The project team knew going into the construction project that the dewatering work was going to be challenging; until crews began the excavation process, however, no one knew just how difficult an undertaking this would be. “We had a lot of water coming from the North slope, which was expected but not at the rate it was flowing,” said project manager, Adam Gordon. “This project is also 100 feet downstream of an existing dam that holds approximately one billion gallons of water. Water is coming from every direction.”
To help alleviate headaches, the project team worked with the City of Tempe, providing multiple cost models and value engineering solutions to ensure client satisfaction. Value engineering solutions help maintain budgets, making both the client and project crew happy.
One value engineering option that saved the City a substantial amount of money was the construction of the downstream scour wall. This 1000-foot-long, 25-foot-tall wall was originally proposed as a reinforced cast-in-place wall that required an extensive amount of excavation, forming, backfilling, and dewatering efforts. Working with subcontractors, the PCL project team developed a plan to install the wall using bentonite, an absorbent aluminum phyllosilicate clay, as a means to keep the excavation stable so the downstream scour wall could be placed in an excavation instead of installing it by traditional methods. Forty-foot-long panels were dug and filled with a bentonite slurry to keep the excavation stable during construction. Upon completion of the panel excavation, custom end-stop beams and the reinforcing cage were installed. Finally, a high-slump concrete mixture, used to maximize flow without sacrificing strength (as would happen if water were added at the jobsite) was pumped up from the bottom of the panel while the bentonite was displaced into a holding tank so it could be recycled and used on a future panel. This process was repeated for a total of 22 panels.
On-site use of construction technology is crucial to project success on this project because of the gate anchor bolts that need to be set perfectly in place. Crew members use Building Information Modeling (BIM) to model embeds for crest gates and hydraulic cylinders. The BIM tool allows them to model individual gate components and concrete design elements of the dam and anchor bolts to make certain they’re in the exactly right location so no modifications need to be made after placement.