Laguna Treatment Plant

By Nick Britten | May 23, 2024

Excavation Shoring


In Santa Rosa, CA, as is true in many cities, it’s important to maintain and upgrade infrastructure to meet the demands of a growing city. One of these needed upgrades was the replacement of a nearly 25-year-old UV disinfection system at the Laguna Treatment Plant to provide a more sustainable and efficient system. The project would not only replace the existing UV disinfection system, but also add an effluent diversion pump station and pipeline to provide greater flexibility. With the rise in large weather events, this project was particularly needed to account for the peak wet weather sewer flows.


D.H. Charles Engineering was tasked with designing multiple components to help with the construction process. This ranged from various temporary supports for existing utilities to a large temporary excavation shoring system around the new UV Influent Channel and the new Diversion Facility. Adding to the complexity of the designs, the plant had to be fully functional until the new structures and facilities came online.


For the large temporary excavation shoring system, our scope consisted of designing cantilever and braced sheet pile shoring walls ranging in depth from 10’ to 26.5’. The diversion facility sheet pile shoring pit had a footprint of ~125’-long x 82’ wide and was braced with W24x162 walers and 24” diameter pipe struts. With one side of the pit being relatively shallow, there were discussions to slope out one of the long sides of the pit and make the shoring (3)-sided.  (4)-sided or (2)-sided shoring systems typically work by pushing against one another to provide counterbalance. With the potential switch to a (3)-sided system, it meant one side would not have the typical resistance of an opposing wall, meaning the side walls would need to be analyzed to act as shear walls. After an initial analysis was conducted, it was determined that the overall system wouldn’t be stable enough to support this condition. This was particularly important because the long side opposite the proposed open side was supporting (2) 48” diameter existing pipes that were operational and vital to the treatment plant. We then decided to go with the (4)-sided system seen in these photos.

As a company that works in all 50 states and much of Canada, it isn’t too often that you get to work in the city in which you live, so I really cherish working on local projects that will improve the community.  It also helped to work with a contractor that was a good partner that takes pride in doing things right.

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