What do you do when you discover severe corrosion on the foundation of 69 guyed mast towers on a 230kV line that's also carrying high-priority fiber optic communications? Did we mention that these towers run through a protected wetland, and wetland mitigation bank credits are not available for this habitat?
So what do you do?
You make a list of your options. Don't worry, it's a short list.
the line to a new right-of-way
Cost: $$$$ + the cost of removing the old line which includes excessive wetland mitigation and permitting
the line in the existing right-of-way
Cost: $$$ + the cost of removing the old line which includes excessive wetland mitigation and permitting, AND you must de-energize the line for an extended period of time.
the towers by encapsulating the foundations with concrete
Cost: $$$ ...and there are significant challenges associated with transporting, mixing, and pouring concrete in a protected wetland
4. Repair the towers by reinforcing the foundations with steel
Wait, Start from the Beginning
Routine aerial inspection revealed corrosion on the foundations of a majority of the guyed mast towers on a 230kV line running through a protected wetland. Detailed inspection of the structures by boat revealed the severity of the corrosion and made the project a top priority. The initial assessment made it clear that the right remediation program would have to not only provide a long-term, reliable solution, it would also have to account for the difficult-to-access location of the towers and the restrictions associated with working in a protected wetland area.
Reinforcing the H-pile foundations with steel fit the bill. It was the most economical and environmentally feasible way to quickly and safely restore the towers to code-mandated strength.
What's the Deal with Wetlands?
This particular line runs through a waterfowl sanctuary within flood protection levees. Wetland environments present some interesting challenges to say the least. Low water levels and heavy vegetation make the use of large airboats impossible. Everything - equipment and personnel - must be brought in on marsh buggies and small airboats to ensure minimal damage to the surrounding flora and fauna. Beyond concern for the environment, the wetlands also make accessing the foundations difficult. Coffer dams are required in order to create a suitable working area around the foundation of the tower.
Tell Me More About These Repairs
Remember those detailed inspections mentioned earlier? They included excavation of the towers which helped determine a number of things. (1) The H-piles showed the greatest deterioration in the area from the splash zone (the area of the foundation where the water level fluctuates) to the mudline. At around 18" to 24" below the mudline, the foundations were generally intact. Why? Because the conditions necessary for steel to corrode don't typically exist that far below ground. (2) Both the web and flange of the H-pile were corroded, and in some cases, so was the bearing plate.
Step 1 - Bring in equipment and supplies via airboat/marsh buggy
Step 2 - Secure structure for safe work environment
Step 3 - Install coffer dam and pump water out of work area
Step 4 - Clean and prep the existing steel for coating
Step 5 - Coat existing steel for corrosion protection
Step 6 - Clamp, drill, and bolt repair plates into place
Step 7 - Apply coating to finished repair for corrosion protection
Step 8 - Install cathodic protection for added protection
Phase 1 of the repairs included priority structures. The level of corrosion on these foundations required the use of a crane to prevent possible catastrophic structure failure. As such, the line had to be de-energized and a new load path had to be created. Fortunately, Phase 2 towers were able to be restored without the use of a crane and therefore without an outage.
The Moral of the Story
Cyclical inspections addressing the below-grade condition of steel assets can not only help mitigate corrosion and extend the useful life of the asset, they can identify corrosive conditions before they become severe, helping to prevent dangerous situations that require emergency action.
As a turnkey solution provider, Osmose can assist utilities
with every aspect of implementing a steel infrastructure maintenance program.
We have the industry expertise to develop effective corrosion and concrete
programs, the engineering resources to design and customize complex repairs,
and the field personnel to thoroughly assess structures and install repairs,
cathodic protection, and protective coatings.
For more information on structural foundation repairs, please contact your local Osmose professional. Not sure who your local Osmose professional is and how to contact him or her? Call 716-319-3423.