Working deep inside a remote canyon near Prescott, Ariz., contractors are battling accessibility and logistical challenges as they plug two leaky valves at the bottom of a 100-year-old cast-in-place arch dam.
Just 10 minutes from Prescott, Granite Creek Dam’s limited access comes from its location in the Granite Dells, a geological formation of exposed bedrock and granite boulders. Getting to the dam’s downstream side requires a nearly mile-long hike over bulbous rock.
On this $2.7-million improvement project, contractors will cap one of the two original valves while updating the other with new equipment. Crews will also install a ladder and platform to improve access to the dam’s downstream side. Contractors are working both underwater and at the bottom of the downstream side.
As heavy equipment access is nearly impossible on both sides of the dam, crews used a barge as the staging point for work and as a way station for concrete and other materials. Nearly every piece of equipment and construction material was brought to the dam via barge and lowered down by crane or divers.
Wood is the design-build project team leader and designer, with Fann Contracting performing site and civil work, while Arizona Commercial Diving Services is conducting specialized diving operations, says Eric Bay, utilities manager for the city of Prescott.
Brian Hamrick, project manager at Wood, says using design-build methodology trimmed nearly two years from the project. Design was complete in October 2017, and the project is expected to be finished in December.
The dam was built in 1917 and previously owned by the Chino Valley Irrigation District until it was sold to the city of Prescott in 1997, says Bay.
A Leaky Matter
Bay says the leaks were a well-known issue. He estimates the non-functioning 48-in. and 36-in. valves—which had been mechanically frozen for unknown decades—had been allowing about 1 to 2 cu ft per second of water to enter Willow Creek unabated. However, the water amounts met release requirements owed to downstream water-rights holders.
The city of Prescott and the Arizona Dept. of Water Resources (DWR) had endeavored to fix the leaks for decades, and the city finally allocated funds for the project in late 2017. After a feasibility report and a bathymetric study, the engineering team designed the fix to include total abandonment of the 48-in. valve. A new 36-in. valve will be installed and augmented with a 10-in. branch valve for finer control on water releases.
Bay says the rationale behind abandoning the 48-in. valve arose from the depths of the sediment and lake level management. The 48-in. valve is approximately 55 ft from the high water level, which puts it below the recreation requirements at Watson Lake. The 36-in. valve is about 38 ft from the high water level.
The first phase of the two-phase project began in late April and lasted about six weeks. Phase two will begin in early fall after the new bonneted knife gate valve is manufactured by DeZurik.
Barge In Charge
Kurt Hankes, project manager at Arizona Commercial Diving Services, says Flexifloat barges were brought out in staggered groupings and supported ready-mix trucks carrying 10 cu yd of concrete and a truck-mounted 20-ton crane. The barge also held the hyperbaric chamber for the underwater construction workers and other support equipment.
According to Hankes, since underwater work was only performed on the 36-in. conduit at 38 ft, much of the decompression was done in-water, but the decompression chamber was used when needed. He adds that since the dam’s elevation is nearly a mile high, oxygen levels are regularly an issue even out of the water.
Most of the underwater work in phase one surrounded the installation of a 52-in. stainless steel gate in front of the 36-in. conduit. Hankes says crews first removed accumulated silt and the original bar gate, then built a grout bed to create a flat surface in order to attach the gate’s thimble in front of the conduit. The assembled thimble and gate was then lowered down as a single unit where divers secured it to the grout bed. Hankes says crews worked in zero visibility, or about 3 in.
“It’s much like blind people reading Braille,” Hankes says.
The gate is operated by a hand crank or by a drill hooked up to the gearing system that is accessible on the top of the dam.
During phase one, on the dam’s downstream side, crews performed a significant amount of concrete work. Contractors placed an 8-ft-long, 13-ft-wide platform below the 36-in. valve to create a safe access point for installing the new valve unit. At the 48-in. valve, a thrust block was cast in concrete in front of the valve, the first step in permanently sealing both the valve and conduit to DWR requirements, Bay says.
The approximately 16 cu ft of concrete that was used to create the platform and thrust block was lowered to the bottom of the downstream side of the dam via the crane on the barge. Bay says to aid the crane operator, two white stripes were marked on the dam edge to mark the needed location of the crane’s boom in order to lower the hopper of concrete to the appropriate position.
When phase two begins this fall, crews will close the new slide gate for the 36-in. valve so that no water can enter the conduit going through the dam, Bay says. Crews will then remove the original valve, core into the dam to install a new portion of conduit and then use the conduit as the stabilized connection point for the new 36-in. and 10-in. outlet valves.
Crews also will install enclosed access ladders and fall-prevention features on top of the dam for workers maintaining the valves. To complete the abandonment of the 48-in. valve, crews will install a coffer dam on the dam’s wet side in order to access and pressure grout the conduit shut, Bay says, adding that DWR requires that when a dam outlet is abandoned, a pressured conduit cannot remain.
Hankes adds that he expects significant clearing of silt and debris. Nearly 200 yd of material was moved during phase one, he says.
“We are now ready for phase two, and luckily we didn’t encounter major delays in phase one,” Bay says.