Dear Neighbor: 

Three separate contractors are working to prepare the College Hill site for tunneling and vertical construction. Scroll for details and join us on July 24 at 10 a.m. for coffee and conversation with Project Manager, Laura Farthing, P.E.  

Tank Prep

Like the demolition phase, contractors are working from the south section of the site to the north. They are spreading a black geotextile fabric topped with a uniform layer of gravel for the south tank. They are also pouring concrete for a box housing the inlet and outlet pipes at the base of the south tank. This work will be repeated for the north tank next.

Subfloor for south tank.

Box for inlet/outlet and drain pipes at the base of the south tank. 

Tunnel Prep

On the north side of the site along Lincoln Street, a separate contractor is preparing for tunneling. They have excavated the shaft for the tunnel boring machine and are now lining the bottom and side of the shaft with concrete. This will give the boring machine a firm surface from which to launch. 

Launch shaft for tunnel boring machine.

Why is tunneling necessary?

There is a hill on Lincoln Street that crests just north of the project site on the way to 23rd. If the team tried to install the pipeline with a traditional open cut method, it would create an approximately 35-foot-deep trench in that location. Having that deep of a trench is not practical or safe in a residential construction area.

Tunneling will enable EWEB to bore under the hill, increasing safety and decreasing disruption. By the time it reaches the intersection of 23rd and Lincoln, the tunnel will be approximately 12 feet deep, which will allow for the installation of the remainder of pipeline via open trench. 

General process for tunneling 

The tunnel boring machine will use a cutter head to grind through soil and rock underground. As it grinds forward, it will simultaneously install a tunnel casing to reinforce the excavated area.  

Material that is excavated from underground is called “spoil.” The contractor is building a separation system to process the spoil and separate rocks from mud and debris. The rocky material may be reused as fill on the site, depending on the quality of the material after processing, and the mud and debris will be transported for disposal.

Separation system.

Steel tunnel casing.

The tunnel will be filled with grout to stabilize the pipelines 

Once the tunnel casing is in place, pipelines will be installed through the tunnel to the intersection of 23rd and Lincoln. The tunnel will then be filled with grout to protect and reinforce the pipes.  

Pipes in the tunnel will feed into a vault just outside of the tanks. From there, water will be routed to the individual inlet/outlet pipe boxes for each tank. The pipes connecting to each tank are smaller diameter (24” for inlet/outlet and 18” for drain) as they fill/drain water for a single tank.

Street Closures 

The intersection at 23rd and Lincoln will be closed during tunneling to allow contractors to excavate a shaft to retrieve the tunnel boring machine. Lincoln will be closed to through traffic, likely at 26th Avenue. Residents who live along Lincoln will be able to use the road to access their properties throughout the tunneling phase. 

Closures will be coordinated with tunneling, which is tentatively scheduled to start on July 28 and last until the end of November. Dates may shift as the start of tunneling is contingent on the completion of all necessary prep work.

Signage will be placed ahead of time and neighbors should plan alternative routes. Updates will be posted to the tunnel/pipeline webpage.

Project Team

• Laura Farthing, Principal Engineer & Project Manager
• Jen Connors, Public Affairs & Communications Manager
• Claire Wray, Communications Specialist
• Hanna Ashcraft, Communications Specialist
• Chris Irvin, Water Engineering Supervisor 
• Mike Masters, Water Operations Manager