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has worked on a handful of projects with the Portland Water Bureau in downtown, so he knows what crews can run into.

鈥淵ou find something unique every time you dig downtown,鈥� Moore Excavation project manager Matt Posey said.

For an upcoming pipe replacement project around Southwest Naito Parkway in downtown, Moore Excavation will confront one big obstacle: a road tunnel hidden since the 1960s. After weeks of research and preparation, crews will start work this week.

In the second phase of the Westside Header project, a 5,000-foot-long stretch of 36-inch water transmission main will be replaced near the west bank of the Willamette River in Portland. The header connects pipes that cross the Willamette River, so that service isn’t disrupted if something happens to one of the connections, said Scott Bryan, a civil engineer with the water bureau.

鈥淚f we do our job right, no one knows we’ve done our job,鈥� he said.

Phase two will connect work done in phase one, near the Ross Island Bridge, with downtown mains constructed five years ago.

Now, old pipes are being replaced and the header is being relocated. In many cases, that means routing the pipe so that it doesn’t run under buildings. This serves two purposes: increasing accessibility to the pipes and reducing seismic risk to those buildings, said Nanci Snyder, an engineer with the water bureau.

鈥淚f a 36-inch transmission main breaks, it would take a building down鈥� with the force and amount of water, she said.

After some research and calculations, water bureau engineers found that the best straight-line stretch with the fewest underground conflicts is along Naito Parkway.

鈥淚t’s the worst from a traffic standpoint, but the best straight shot (for the pipe),鈥� Bryan said.

A straight shot is important, particularly for such a big pipe. With a small pipe, navigating around an obstacle or turning a corner is less complicated: 鈥淵ou just go to the hardware store and buy a bend,鈥� Bryan said.

However, the 36-inch pipe used for the West Side Header project is manufactured specifically for the water bureau by a company in California. If an unexpected obstacle is encountered, new pipe must be ordered, manufactured and shipped from California.

鈥淲ith this pipe, you don’t just move around stuff,鈥� Bryan said.

Though Naito Parkway is a straight shot, its ground holds the 鈥淕rant Street Tunnel鈥� 鈥� a former street connection that was partially backfilled, hidden and forgotten when Interstate 405 was built in the 1960s, Bryan said. The water bureau discovered the tunnel in its research, and then located it underground. The ends of the tunnel were backfilled with dirt, concrete and gravel, but the middle was still open.

Crews will run the pipe through the tunnel, so holes will be bored on both ends and a support structure will be constructed in the middle. Crews are working to reinforce the holes they make in the walls, so they don’t compromise the tunnel’s structural integrity. They’re also preparing to keep the pipe intact if the old tunnel collapses, Snyder said.

Digging through the walls of the tunnel is not an easy task. It runs at an angle to Naito Parkway and the structure itself is complicated.

鈥淚 think the ceiling is flat, but nothing else is,鈥� Snyder said.

Posey’s crews will have trouble knowing where to bore holes and at which angle without a real frame of reference, he said.

The Naito Parkway portion of the project will require reducing traffic to two lanes for part of the road during two weeks, Snyder said. The $3.8 million project is scheduled to wrap up in March.

Crews are preparing the ground and testing along the length of the project to probe for other potential obstacles.

鈥淲e prepare as well as we can (and) then cross our fingers and start digging,鈥� Snyder said.