Water under the bridge
Tunnel beneath the Sandy River will increase safety and capacity of water supply
A new tunnel being constructed underneath the Sandy River will house two large pipes that will replace the older, smaller above-ground pipes that currently transport water from the Bull Run watershed to Portland.
Shanda Tice / Sandy Post
Driving across an old green iron bridge at Dodge Park, a crane and a large cylinder that juts from the ground contrast sharply with the idyllic natural surroundings along the Sandy River.
There’s a project going on, all right. But the real action is under the radar of passers-by on Southeast Lusted Road.
Eighty-five feet below ground – and river, in fact – a machine with whirring blades rapidly pummels the porous Sandy River mudstone in its path. Each time a front-loader carts out the resulting gray muck, a 30-foot diameter tunnel extends ever closer to its goal at the west bank of the Sandy. All told, it’s a 435-foot journey.
As the river flows seemingly obliviously above, through layer after layer of rock substrata, hard-hatted crews toil away down under. Their labor with heavy equipment gains around 16 feet of earth “on a good day,” says Jarrett Carlson, project supervisor with Kiewit Pacific Co.
The company is charged with transferring two Portland Water Bureau conduit pipes spanning an 1894 iron bridge to a manmade 30-foot-high cavern deep under the Sandy.
While the vintage bridge is structurally sound, federally mandated studies pinpoint the crossing as a key point of vulnerability – due to seismic, weather or human-caused damage and catastrophe – in the Bull Run watershed system, says Tim Collins, Portland Water Bureau project manager.
“The goal is to improve our capacity and reduce the vulnerability of the conduit,” he says.
An upgrade to larger, 72-inch diameter pipes will also allow greater capacity, up to 165 million gallons per day, for future water demands.
“That’s not what we can deliver right now,” Collins stresses. “But we want to make sure this conduit could accomplish anything in the future.”
Crews broke ground on the $21 million project in late 2008. By April 2010, the bureau expects the underground conduit to be, well, down and running.
“We plan to do the (conduit) bridge removal next summer,” says Carlson of the ancient span that parallels a vehicular bridge for Lusted Road. It will remain intact and open to traffic.
A visionary pipedream
Thanks to the ingenuity of engineers and planners more than 100 years ago, mountain-pure water flows 26 miles – without assistance from pumps – from the Bull Run watershed near Sandy to reservoirs at Powell Butte, Mount Tabor and the hills of Southwest Portland.
The system provides unfiltered, minimally treated drinking water to more than 800,000 residents in the greater Portland area, according to water bureau literature. The bureau monitors flow for more than 200 contaminants, including those regulated by the Environmental Protection Agency.
In its 114 years of operation, the water-delivery system has proved uncannily reliable, Collins notes. Burying the conduit will, however, guard against incidents such as a 1995 landslide along Bull Run that temporarily disabled two of the three conduits delivering water to Gresham and Portland.
A third conduit crosses the Sandy on a bridge just downstream of the 1894 bridge. Unlike some long-distance municipal water supplies that rely on a gigantic single tunnel, the Bull Run system was designed with redundancies to protect against failure at any one spot.
“That redundancy is thanks to the vision of our forefathers,” Collins says of the system’s designers.
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.