That’s a page from a slide deck presented earlier this month to the Levy to Move Seattle Oversight Committee in a briefing on levy-funded earthquake-resistance work for bridges around the city. We’ve already reported on two of the projects on the list, Admiral (listed twice because technically it’s two bridges) and Delridge. But the “silt removal” project for the low bridge was something we hadn’t previously heard about. We asked SDOT for more information – at the committee briefing, it was just a quick mention – and got this explanation:

This project will remove silt that has built up around the western bridge foundations and replace rusted steel collars that are secured around the piles that support the bridge pier.

The western pier is supported by several piles surrounded by a steel casing. There is a circular gap between the support piles and the steel casing which allows the bridge to freely move during an earthquake, reducing the force and stress on the pier foundations. There is a steel collar that fits around the top of each of the steel casings to prevent debris from entering the circular space. Over time, the collars have rusted and the gap has filled with silt. A contractor specializing in underwater construction will remove the silt and replace the collars so that the bridge can move as it was designed to do during an earthquake.

We expect to complete this work later this year, and we do not expect that this will have any effect on travelers or vessels. The estimated project cost is about $650,000. This is one of 16 bridge seismic reinforcement projects being funded by the Levy to Move Seattle.

The silt problem apparently goes back quite some time – we found a mention in this 2019 SDOT report.