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ALCOSAN Drilling In Preparation For Possible Underground Sewage Overflow Tunnel

ALCOSAN will spent nearly $300,000 to drill down 300 feet to test soil in anticipation of building an underground sewage overflow tunnel.

For the next two months, the Allegheny County Sanitation Authority will send drilling rigs to 13 locations around the county to take samples as deep as 300 feet. 

Drilling started Monday at ALCOSAN’s Ohio River treatment facility. The other 12 drilling sites are also in non-residential areas. The authority is spending approximately $280,000 on the project, said ALCOSAN Director of Engineering and Construction David Borneman.

The work is being done to prepare for construction that could be mandated by the federal Wet Weather Consent Decree, which aims to cut down on sewage overflow into bodies of rivers, like Pittsburgh’s many rivers and streams.

The wet weather plan originally called for building deep tunnels to hold overflow sewage on rainy days. ALCOSAN is also exploring the efficacy of other plans that would involve less “grey infrastructure,” such as holding tanks and pumping stations, and more “green infrastructure,” like green spaces to catch parking lot drain off.

But, while ALCOSAN awaits federal approval for such infrastructure, it’s moving forward on soil testing to get a better sense of what’s underground, Borneman said.

“We know there’s a coal seam down there,” he said. “We want to make sure we understand where that’s at.”

He said he also wants to know if there are pockets of methane gases or hydrogen sulfides along with layers of ground water, which would affect costs later down the line.

Most of ALCOSAN’s existing infrastructure is buried 100 feet, or less, underground. Borneman said ALCOSAN is testing well below that depth to prevent any extra work down the line.

“If you go into a certain depth and later you find out you have to go deeper, then you have to go back and redo the whole thing,” he said. “So initially, it’s a smart thing to go even deeper than you may need to be.”

This is just the first step in what will be more detailed testing, if ALCOSAN decides to move forward with creating deep tunnels.

“Any time you are doing any type of a tunnel-type construction, you usually try to create a geotechnical profile,” Borneman said. “That would entail much more investigation, much more engineering.”

At least five of the drilling sites sit over the Freeport coal seam.

Drilling work will be restricted to between 7 a.m. and 5 p.m. and is expected to wrap up sometime in June.

(Photo via Robert Strovers/Flickr)