Water under the bridge

Tunnel beneath the Sandy River will increase safety and capacity of water supply

By Shannon Wells

The Sandy Post, Jun 23, 2009, Updated Oct 30, 2009

Sealed and delivered

Below the surface of the water bureau outpost, the tunnel is dug approximately 220 feet of its ultimate 435-foot length. Crews of seven per 10-hour shift stay busy in the cool, damp shaft.

As the machine digs away “like a crab,” as Carlson says, to clear more earth, workers install arched steel girders every 4 feet to support the ceiling. A high-velocity hose sprays a rough cement substance they call “shotcrete” to seal the subterranean environment from water.

Amid the hiss and roar of equipment, Carlson says the soft rock geology is ideal for the project.

“The mudstone self-supports long enough for us to install our supports,” he says.

While drilling core sample holes 10 feet into the ceiling, it’s not uncommon to hit water-filled fissures, most of which drain their charge pretty quickly, he notes.

When the tunnel reaches a second vertical shaft on the opposite side of the Sandy River, crews will turn their attention to installing the two 6-foot-tall conduit pipes, section by section. Elbow joints will connect the pipes, as they come out of the shafts, with their horizontal counterparts where they emerge from the riverbanks.

As the sections are laid in and bolted together, the tunnel will be gradually filled in. The pipes themselves can be monitored and entered for future inspections and maintenance.

“We can acoustically determine where there’s a leak or problem,” Collins says.

As all this action goes down, area water customers shouldn’t notice a single change in their service.

Clearly proud of their product, Collins and his fellow bureau employees point out that the quality microbrews and coffee the area is known for have one common ingredient.

“This is the best water in the world,” he says.