For a long time, the standard in New York for excavating foundation pits for new-building construction has been sheeting and shoring. The pit is dug out, by hand or by machine, vertical piles installed at the perimeter and braced with diagonal rakers, and wood plank run between the piles to retain the adjacent earth. It’s an ancient system, updated for the use of modern steel piles and braces. Since OSE generally doesn’t work on new buildings, we see this most often when we’re working on buildings adjacent to new construction. Because of the current building boom in New York, we’ve been spending a lot of time investigating old buildings that have been affected by adjacent construction.
In the last few years, we’ve been seeing secant-wall foundations, which are a more recent way of building a new-building foundation. In this system, a single element – the concrete secant wall, constructed before the bulk excavation is performed – takes the place of both the temporary sheeting and the concrete foundation wall in the old system. That’s good for the people building the new building, as it saves them time, money, and often some room. Unfortunately, the secant walls tend to move a bit during the excavation process.
The picture above is an abandoned chimney on an old industrial building in Brooklyn, adjacent to a party-wall parapet. That’s a brand-new crack running across it: the upper portion of the chimney stayed with the parapet, while the lower portion was pulled to the right as most of the building moved during excavation with an adjacent secant-wall foundation. The movement at the foundation level was small, maybe 3/8 of an inch, but it was magnified at the roof level as the downward motion of one side of the building becomes rotation of the building as a whole.
It’s a good thing that chimney was not in use.