ARE is currently helping a Shortline Railroad client with an issue on a major bridge crossing. Our engineers received a call in January 2019, about an old swing span bridge that had moved overnight. ARE performed an emergency inspection and survey of the pier the next day. The center pivot pier had moved and began to lean downstream. As a result, the swing span had become unlocked. Our inspectors determined that a dam upstream had released a large amount of water due to recent heavy rains. This resulted in the pier movement from the force of the high water. In short, there was a 200-year storm event with maximum flow being around 62,000 cfs (cubic feet per second). Normal flow for this river is around 12,000 cfs.
The first action was to get commercial divers in the water to look at the scour. The velocity of the river was too high, and our team had to coordinate with the water authority that controlled the dam upstream. They slowed the water flow down to less than 20,000 cfs, and this process took two weeks. Once the divers were able to inspect the pier they found quite a bit of scour. With this removal of sand and gravel from bridge abutments caused by the moving water, about 10-15’ of the footing had been exposed.
ARE engineers investigated several viable methods to repair this pier and determined two options to stabilize the foundation and mitigate the scour. These two options were: micropiles (a small-diameter, drilled and grouted non-displacement pile that is typically reinforced) and jet grouting (a cross between soil mixing and grouting). Because of an access issue getting to the pier at this remote location, micro-piling from a barge was chosen as the preferred alternative.
The micropiles will be battered to provide lateral restraint and resist the force of the water. They will then be installed in the front and the back, acting as tension and compression piles. A concrete pile cap will be constructed and serve to attach the piles to the existing pier.
The process of obtaining borings to finalize design and investigation is currently ongoing, as well as the design on how to mitigate the scour.