Imagine a retaining wall made of columns built into the bedrock of the Ohio River. The wall is only 50 feet long, but it pierces 25 to 40 feet into the ground -- completely underwater.
It may seem small, but the U.S. Army Corps of Engineers Pittsburgh District recently finished the secant pile wall for massive construction to move forward at Montgomery Locks and Dam in Beaver County, Pennsylvania. Now that it is complete, the corps can enter the next phase of the Upper Ohio River Mega Project.
“This is the low stage of the crescendo,” said Tom O’Buckley, resident engineer for the U.S. Army Corps of Engineers Pittsburgh District’s Upper Ohio megaproject resident office. “Getting these little things done and in place is setting the stage for this huge billion-dollar project.”
A secant pile wall is not constructed like a brick wall from the ground up. Instead, contractors used giant drills to burrow holes and fill the columns, known as piles. Each pile is 4 feet wide. The columns overlap, resulting in a top-down view of the wall resembling a series of interlocking rings, each circle overlapping the next just enough to create a seal. To build the wall, contractors first placed gaps between the primary piles. The primary piles contain a steel H-beam at their core and reach bedrock 25 feet down. Once the piles cured in place, construction crews added taller secondary piles to fill the gaps. Filling the gaps required drilling away the edges of the primary piles. This subtraction from the primary piles is where the structure derives its “secant” name.
The completed wall consists of 17 piles. It will remain in place, though its purpose will end once the mega project is complete.
O’Buckley oversaw the construction of the wall. The Montgomery facility is one of three in the mega project, where the corps is reconstructing larger locks at Montgomery, Dashields and Emsworth dams.
Commercial tows use the lock chambers on the Ohio River year-round to transport commodities across the country. The corps built Montgomery L/D more than 80 years ago when boats and quantities moving on the river were smaller. That means Montgomery’s smaller chambers cause a significant bottleneck for industries transporting goods through the Pittsburgh region.
The longer and wider lock chambers will allow more barges to lock through at once.
The facilities required increasing maintenance for several decades to ensure the chambers did not break down or stop river traffic.
“This is the end of the band-aid approach to trying to get the work done,” O’Buckley stated. “To get our money’s worth out of it, we are building it bigger and better.”
O’Buckley compared the district’s river improvements to the generational improvements in roadways.
“Some roads were built for the width of a cart. Well, the width of the carts nowadays is pretty big,” he said. “As you saw this morning, barges have to break down to get through this little lock. As big as the lock is, it’s relatively small.”
Once the new lock is complete, it will become the primary chamber.
This upcoming construction is only possible with the completed secant pile wall first. “We built a retaining wall before we need to retain something,” O’Buckley said. “There’s a five-foot-thick concrete apron that runs the entire width of the river. Below that, there’s soil and that goes down to rock.”
When the corps finishes the new lock chamber, contractors must cut into the concrete apron and down into the ground to build the new river wall. The secant pile wall will hold back the river and support the apron.
The project required many partnerships between multiple Pittsburgh District divisions, Buffalo District’s geotechnical drilling, contractors, the maintenance fleet, and surveyors.
With so many organizations present on site, O’Buckley joked that some days there wasn’t even a place to park.
“This has been a very successful project,” said Alex Benedict, who worked as the contracting officer’s representative, often managing the day-to-day affairs to facilitate the Pittsburgh District partnership with the contractor.
It took eight months to build the wall, a timeline that experienced delays due to weather.
“The river has a vote,” O’Buckley said.
While they couldn’t build partnerships with the weather, O’Buckley said the district’s success relied on partnerships with contractors. USACE regularly uses contractors for specialized work.
“The contractor here specializes in this type of construction, not only in a marine environment but also on land. This is their bread and butter,” O’Buckley said.
Whenever the project encountered problems, the Pittsburgh District and the contractor partnered to work through challenges together.
“Every project experiences problems. How you resolve those problems is a result of quality partnerships and how you fix those problems by working together,” O’Buckley said.
Next, the corps will expand the land facilities at the lock to support ongoing construction. This includes erecting buildings for supervision and administration of the work for the government and contractor sides. All of that construction will occur while the facility remains functional.
They will then build a concrete batch plant, like the one at Monongahela River Locks and Dam 4, at Charleroi, Pennsylvania.
Regarding the upper Ohio locks, O’Buckley said the dams were there for nearly 100 years, and the upcoming mega project will help ensure they will be there 100 more.