If you’ve driven on Interstate 70 through Jefferson County, Colorado, recently, chances are you’ve noticed the big blue pipes lined up in a field near Golden.

These large sections of pipeline — 50 feet long, over 5 feet in diameter and weighing 25,000 pounds — are a critical part of a $90 million pipe replacement project for Denver Water’s northern delivery system.

“The pipeline will play a big role in how we get water from the mountains to our customers in the city,” said Jim Light, design project engineer at Denver Water.

The new water line will replace an existing pipeline that was built in the 1930s. The pipeline carries water from Ralston Reservoir north of Golden to the Moffat Treatment Plant in Lakewood.

An assessment of the pipeline in 2012 found signs of wear, cracking and concerns with the joints.

“The old pipe served us well, but it's showing its age,” Light said. “We want to replace it now before there are any major problems.”

The pipeline replacement is part of Denver Water’s nearly $600 million North System Renewal — a renovation project to update the infrastructure that brings 20 percent of Denver Water’s water to customers in the metro area.

The large row of pipes sitting in the field near I-70 and state Highway 58 will be installed as part of the first phase of the construction project.

“We wanted to get the pipes underground before this area is developed,” Light said.

The first phase also includes digging tunnels under state highways 93 and 58, as well as I-70 and two railroads.

“Installing pipelines was easier back in the 1930s when this was all rural farmland,” Light said. “Now we have to work around major highways, homes and businesses.”

The tunneling work is being done by Michels Corporation — a construction company that builds tunnels for water infrastructure projects around the world.

“Digging tunnels is always a challenge,” said Johnie Halkyard, tunnel project superintendent. “We go through a lot of rock and have to keep our machine going straight so it doesn’t stray off course.”

The tunnel boring machine is a circular piece of equipment with a diameter of nearly 9 feet. The tunneling crew uses different types of boring machines depending on the type of soil and type of tunnel.

“The machine going under I-70 uses an excavator arm in the front to hack away at the rock and soil,” Halkyard said. “The debris goes on a conveyor belt, into a cart, down a track and out the end of the tunnel where we lift it out with a crane.”

Halkyard said the machine can dig about 40 to 60 feet per day when conditions are good.

“We’ve got a team of seven people down in the hole, including an operator running the machine,” he said. “It can get a little scary when you’re down there for a long time, but we take plenty of precautions to make sure everyone is safe.”

The tunnel under I-70 will be nearly 1,600 feet long and also goes under a shopping area before reaching 32nd Avenue in Wheat Ridge. Crews hope to complete tunneling by the end of 2018.

Another challenging part of the project includes placing pipes under Clear Creek near Highway 58.

“We had to divert the creek so we could remove the old sections of pipe and install the new ones,” said Bill Ianiro, project inspector for Denver Water. “These are some of the biggest pipes in our water delivery system, and it’s not easy getting them in the ground.”

Engineers hope to complete the five tunnels, a creek crossing and installation of the first section of pipes by the end of 2018.

Once the tunnel phase is completed, trenches will be dug along the 8.5-mile corridor through Jefferson County, Arvada, Lakewood and Wheat Ridge to install the rest of the pipeline.

Another reason the pipe is being replaced is to make it compatible with Denver Water’s new water treatment plant being built next to Ralston Reservoir north of Golden.

“The old pipeline carried untreated water,” Light said. “The new pipe will deliver water that’s safe to drink from the new plant. We can’t run treated water down the old pipe because it would impact the water quality.”

The entire pipeline is expected to be completed by 2020, but it won’t go in service until the new Northwater Treatment Plant is operational in 2024.

“We’re always looking to the future,” Light said. “This project will help make sure we can deliver water safely for the next 100 years and beyond.”