Water line being threaded through solid rock
The pipe marks a stark contrast to its predecessor, laid nearly 100 years ago to begin carrying water — untreated those days — down into McMinnville from Haskins Creek.
In 1917, laborers hand-dug a tunnel through a rocky ridge. In it, they laid lengths of 12-inch pipe fashioned of tar-coated wood. Digging from both ends to speed things along, they met in the middle. At the lower end of the tunnel, they connected it to a pipe covering the final 10 miles into town.
That tunnel is still in use today. However, back in the 1970s, McMinnville Water & Light city replaced the original wooden pipe with a 24-inch pipe made of ductile iron.
Now, Water & Light is undertaking a $5.6 million dollar project designed to boost the delivery line's capacity as well as its earthquake resistance. It requires cutting a new tunnel of larger diameter through the same ridge, but there's not a shovel in sight.
A massive boring machine is being used to cut through the rock in successively larger sizes, from 8 inches in diameter up to 48 inches. The tunnel must be cut successively wider, Dietz said, because it would require too much energy to move such a massive volume of rock all at once.
A slurry of bentonite clay and water is pumped into the hole to cool and lubricate the drill. It runs back down the tunnel, where it is pumped into a separator that removes the ground rock fragments. Then it is recirculated back in.
The massive boring machine is flanked by enormous backhoes, dump trucks, excavators and trailers. A smaller boring machine is creating a three- to four-inch tunnel to carry fiber-optic lines down from Water & Light's treatment plant to a vault on Panther Creek.
Meanwhile, a pair of welders specializing in pipe joints are finishing up the joining of 2,200 feet of 36-inch steel pipe, laid along a road carved out of the woods to accommodate its length.
A bright blue coating creates a smooth surface. That will help crews slide it through the 48-inch tunnel, in addition to protecting the steel from gouges and rust.
After the pipe has been placed into its new resting place, debris and soil will be pushed back over the roadway. The gash will be replanted with trees in the fall.
John Dietz, Water & Light's engineering and operations manager, said the entire stretch of pipe will be pulled through the tunnel in a single day. He estimates it will take about 20 hours.
“Once they start, they won't stop,” he said, as workers can't risk getting the pipe stuck somewhere mid-tunnel.
The pipe will be filled with water to give it some buoyancy. The same clay slurry used in the drilling operation will be pumped in to provide lubrication.
Despite the pipe's massive size, the operation is expected to smoothly.
The original tunnel will continue to operate, Dietz said, giving the utility a measure of redundancy. In the event something goes wrong with one line, it can continue to deliver water through the other.
Meanwhile, having both in operation increases the utility's delivery capacity.
Currently, Water & Light is capable of treating 30 million gallons per day, but it is only capable of delivering 14.7 million gallons per day. Daily use has only averaged 5.1 million gallons per day over the last 10 years, but can hit 13 million gallons on a hot summer day.
Dietz said the existing 24-inch ductile iron pipe carries water from the treatment plant through the ridge. On the other side, it connects to a pair of delivery pipes, one 24 inches in diameter and the other 16, that bring the water down to storage reservoirs on Fox Ridge Road.
Because the new 36-inch steel line will connect to the same set of pipes, it won't increase capacity all that much on its own. However, plans call for eventually replacing the 16-inch line with one twice that diameter, in stages over several years.
The Water & Light Commission chose steel for the section running through the tunnel because it is the strongest and most durable material available. But it's also the most expensive, so the commission opted to go with ductile iron for the remaining sections of its delivery system, which are located underground, but don't have to pass through rock.
Cast iron is very brittle, and that makes it susceptible to breakage. Ductile iron is a form developed in 1943 to combat that problem.
Ductile iron isn't as flexible as steel, but is more flexible that traditional cast iron, and extremely strong. It is often used in water and sewer lines.
Dietz said the project is slightly ahead of schedule right now, and is expected to be completed in October.