Fast, Simple, and Satisfying
This was supposed to be a simple project. A cooling water intake is just an underground pipe that extends through a riverbank and out into a river, where a perforated segment connects to make a T-shape. Pumps suck water through the pipe and into the power plant. Driven piles normally support the submerged pipes, and piles installed upstream act as debris barriers.
Because of other project constraints, the in-water work period was about to close before geotechnical boreholes could be drilled and a cooling water intake designed and installed. To accelerate the schedule, UAM hired us to design the in-water support piles using the Observational Method, basically collecting data from pile driving that could be used to analyze and design the piles that were being driven. It’s fast-paced work that we find particularly satisfying.
And Then the Rains Came
Just as the divers started connecting cross-braces and pipe saddles, heavy rains in the central Midwest caused unseasonal flooding on the Red River, raising water levels and forcing the crew to stop construction. Woody debris, mostly logs and branches but sometimes stumps and root wads, rode the floodwaters downstream and impacted the partially-completed foundations. The unbraced piles were vulnerable to debris loads until the braces could be installed, and it was not safe to perform underwater construction during the flood. It was possible that the flood would damage the supports so badly that the intake could not be installed during the in-water work window, forcing a several-month delay onto the project.
An Uncommon Solution for an Unexpected Problem
We performed emergency analyses of the pile stability under the unexpected river loads. We eliminated all conservatism and just focused on soil failure and pile bending. We identified that debris loads high up on the piles were the most likely problem, and worked with the contractor to develop safe methods of towing reachable debris off of the barrier piles and around the structure.
After the floodwaters receded we found that a number of the barrier piles had been bent just above the mudline. There were no soil failures and no detectable damage to the pipe supports. The intake pipes were installed on the undamaged support piles without delay, and a plan was made to repair the barrier piles during the subsequent in-water work period.