Retaining Order

South of Boston, contractor Julian Crane designed a new facility to store and repair their heavy construction equipment. The superstructure is a prefabricated steel frame. They hired me to design the foundation.

I proposed a stem wall with footing below the four exterior walls of the building. Every project is unusual in some way, and for this facility it was a sloping site that dropped off into a wetland. No sitework was allowed on the wetland itself, but my client wanted to build the facility as close as possible on the higher ground. So I proposed that the foundation wall double as a retaining wall on two sides. The wall would have a combined loading of vertical forces from the building itself and horizontal forces from the soil behind it.

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Retaining wall (top) | Frost wall (bottom)

One of the more insidious forces that engineers must consider is live load surcharge. When you stand in a precarious spot, like atop a pile of dirt or a snowy ridgeline, your weight pushes the ground material outward. Too much force and the dirt collapses or the snow avalanches. Your weight is the live load (because it can move around), and surcharge means it’s an extra force that increases the material’s tendency to spread. At the Julian Crane facility, my retaining wall needs to hold back not only the soil itself, but also a surcharge from the weight of any heavy construction equipment inside the building.

I specified an 8-foot retaining wall under the two sides of the facility where the ground slopes down to the wetland, and a 4-foot frost wall with minimum reinforcement under the other two sides for economy. My client decided to go deluxe and had the contractor use the 8-foot wall detail on all four sides, with my approval. I also specified a minimum bearing strength for the soil below the foundation footings, which the contractor achieved by filling and compacting new soil.

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Inside the steel-frame facility.

Construction proceeded over the last year, and (pending some final sitework including guardrails and paving) the facility is substantially complete.

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Outside the new facility.

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