Cochran Basin Stormwater Conveyance

By Chris Gandy | May 3, 2024

Excavation Shoring

 

Utility work is far from glamorous. Utility systems are regularly overlooked, possibly due to the contents of the pipes or the fact that it literally is underground and unseen. Though these systems rarely get the recognition they deserve, they are an integral part of the health and safety of our communities. Throughout my professional career, much of my work has had some link to these systems. While an intern during college, I worked to plan, bid out, and oversee large utility works for a municipal engineering division. Now at D.H. Charles, I am able to see utility work from the installation side, including all the challenges that come with it.

 

Last year, I had the opportunity to work with our team on the Cochran Basin Stormwater Conveyance project in Spokane, WA where the contractor needed to set two underground utility structures to bolster the stormwater infrastructure of the city. These structures, with a total volume equal to 12 Olympic sized swimming pools, were split into two excavations. For each excavation, we planned to use slide rail as our shoring system. This excavation shoring method is very common for its modular design employing specialized posts and interconnected panels that act very similarly to a soldier pile system while allowing for easier installation and reuse.

 

With the larger of the two pits, our team was faced with several challenges that seemed daunting as a new engineer in the excavation world. We were tasked with supporting a 52’x52’x30’-deep excavation pushing the slide rail system to its limits. Our main challenge was how to brace the posts. To support the posts while still allowing room for the construction operations, we used waler rings. Even with 24” deep steel beams, the walers were being pushed past their capacity. Adjustments were made to the plan, and support was added using corner struts. These smaller beams are connected to the larger wales only a few feet from the corner in each direction. While small in size, their impact is large on the capacity of the wales. Employing the use of corner struts, the capacity of the wales increased by almost 2.5 times. These new additions solved the problem of the wale capacity but created a new problem of its own. These added members encroached on the new structure that was to be built. This required us to outline phasing for the contractor to adjust the location or remove supports in a way that matched the construction phasing of the new structure. In the end, we were able to find a system design that was safely engineered, while creating enough flexibility for the contractor to see the project through.

 

Throughout my time on this project, the work continued to push limits. This project stretched me and taught me valuable lessons about myself as an engineer and the nature of our work. Reminiscing on this project, I look back with pride on the work we provided and the impressive scale of work that was achieved. The lessons learned from this project may fade from the forefront of my attention, but it will be imperative to the success of my career just as the utilities will be crucial to the community they serve.

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