Living on the Edge

Precast concrete saves the day on a project where significant site constraints, bridge weight limits and a lack of surrounding soil make an aboveground evapotranspiration system absolutely necessary.

By Bridget McCrea

Situated about 65 miles northeast of Phoenix on the Salt River, Horse Mesa Dam is a 305-foot-high concrete thin-arch structure constructed in the 1920s. The dam forms a 245,138 acre-foot (just under 80 billion gallon) reservoir and includes an auxiliary tunnel, regulating gate, gatehouse and operating mechanism for controlling the tunnel.

Horse Mesa Dam also includes a camp that workers live at and operate from while working onsite. Constructed nearly a century ago, the camp’s septic system was well past the end of its useful life. The camp also needed greater wastewater treatment capacity in an area where space was limited, difficult to access with a vehicle and situated on rocky terrain.

A Team Effort

As the engineer of record, George Cairo Engineering reached out to Jensen Precast’s Phoenix plant for help designing and building precast evapotranspiration ponds – which use the combined process of water surface evaporation, soil moisture evaporation and plant transpiration – to replace the camp’s traditional septic system.

Because the site was mostly rock with very little soil, the team decided that an aboveground treatment system would best handle the wastewater flow. Rob Larson and Brian Hartsfield, both Jensen project managers, worked together with the engineering firm to nail down the details of the project.

“They really didn’t have the luxury of doing a lot of excavation, digging or earth work in the area where the evapotranspiration ponds had to be installed,” Hartsfield said. “So they had to build the ponds on top of existing ground.”

Together, the two firms developed a design for three separate evapotranspiration ponds – one upper, one middle and one lower – that were manufactured at Jensen’s plant and then installed on a hillside near the camp. Hartsfield said the design team decided on various configurations for the ponds because of the site’s layout. A liner was put down first, and the ponds were filled with soil and other medium to replicate a leach field.

For the installation work, George Cairo Engineering recommended Scholz Contracting of Mesa, which ended up winning the bid.

“We hadn’t worked in the area before, and the site was unique,” said Jared Scholz, Scholz Contracting vice president. “I don’t think anyone really had much experience working out in that extremely remote area.”

Working Off the Side of a Cliff

Jensen constructed the three new evapotranspiration ponds in three unique sizes and shapes with varying angles. The component pieces that were used to create the ponds had to be small enough for a contractor to get them down a hill and set in place with very little equipment and a small vehicle. Unable to access the site with a standard tractor trailer, Jensen delivered the products to a staging area for the contractor, which then carried the pieces to the jobsite one or two at a time using a small truck.

“This isn’t a typical jobsite,” Hartsfield said. “To get to it, you have to take a long, winding dirt road down the side of a mountain.”

Scholz concurred, adding: “We were basically working off the side of a cliff.”

Complicating things further, the contractor also had to transport the ponds’ pieces and their equipment over a bridge that has a 40-ton limit.

“We basically had to offload at the top of the mountain – which is about five miles from the site,” Scholz said. “And then walk the equipment down to the site.”

The precast pieces built at Jensen and customized around these constraints were the clear choice for the project. Being precast, the pieces could be constructed in various sizes to ensure an accurate fit within the footprint.

“There were a lot of different sections and pieces, and each unique piece had a different SKU (stock-keeping unit),” Scholz said. “Getting all of that organized, scheduled, produced and out to the customer in the appropriate order was a major undertaking.”

Knowing that product weight and size had to be factored into the pond design, Jensen designed a solution that would work using available forms. The precaster decided to make L-shaped pieces using its existing box culvert forms, building bulkheads onto those forms to create the ponds’ L-shaped panels.

“This provided the versatility that we needed along with the structural engineering and design to support the approach that we wanted to use,” Hartsfield said. “This strategy allowed us to produce the ponds in small enough sections for the contractor to haul them down to the site.”

Precast: The Only Choice

Scholz said there were other reasons precast was the only choice for this project.

“There’s no way we could have taken a concrete truck down that hill,” he said. “These factors made precast the logical choice.”

Scholz coordinated closely with Jensen to ensure that the contractor could transport even the project’s largest precast pieces on trailers and handle them using smaller pieces of equipment.

Scholz said the contractor “really had to think through” the installation process for the ponds, each of which is situated at a different level on the land. The lower field is about 15 feet lower than the middle field, and the middle field is about 50 feet lower than the upper field.

“As we set them, we basically had to work our way out; there was no way to go back in after the fact,” Scholz said.

This required coordination of what beams the contractor needed first, the placement of those beams and having a good plan on which beams needed to be placed where.

“All of these considerations had to be factored into the overall project strategy and timeline,” Scholz said.

Durable, Strong, Versatile

With the final stages of construction still underway on the project, both Hartsfield and Scholz say the early planning and coordination has paid off. Once the new evapotranspiration ponds are fully installed, they will operate as leach fields – with liquid flowing out of the septic tank and then into each pond for further treatment. With no soil in the area to use for a traditional leach field, the evapotranspiration pond is designed to capture all of the liquids from the wastewater. The liquid then is consumed by evaporation plus the vegetation and grasses that will be planted on top of the pond.

“Everything worked out well on the precast manufacturing side particularly in terms of using our existing box culvert framework to streamline the project,” Hartsfield said.
Precast provided the needed durability, strength and versatility to contain the camp’s new wastewater treatment system.

Bridget McCrea is a freelance writer who covers manufacturing, industry and technology. She is a winner of the Florida Magazine Association’s Gold Award for best trade-technical feature statewide.