The Space Below

May 7, 2009

Ever get the feeling, as you’re walking down the sidewalk, that you’re no longer on solid ground? Maybe you should. In many major cities, the sidewalks are built above hollow “vault” spaces. These were once used as a type of early loading dock. Building supplies were dropped down into the basement through a sidewalk hatch. In some places, you can still see the hatches, however, it’s more concerning when these vaults are covered up and forgotten.

The City of Chicago requires annual inspections of sidewalk vaults located in the central business district. During these inspections, I’ve had a chance to explore below some of the city’s landmarks. The vaults are frequently larger than you might expect and extend out to the edge of the roadway. Construction types vary greatly, but they usually match the methods used for the adjacent building. Sometimes new buildings are built on the site, but in order to keep the sidewalks open during construction, the vaults are never filled or updated.

The most frequent problems observed in the vaults are related to water infiltration. Old construction methods rarely incorporated waterproof membranes. Water also tends to seep through metal access covers or glass-block-filled iron grating. In addition, most sidewalk vaults were never designed for the heavy trucks allowed on the roads today. What if a fire truck pulls onto the sidewalk? You might think of the sidewalk at a bridge deck in these cases.

Fortunately, many building owners, sensitive to liability for any accidents on their property, proactively shore or reinforce their vault spaces. A long term solution usually involves completely demolishing the sidewalk and building a new deck or simply filling the vault with soil or concrete.

Exploring the vaults below the city’s streets adds a small sense of adventure to my job. It’s not quite like the show on the History Channel, but it does provide a link to a bygone era. If building owners and engineers continue to meet their responsibility, the public can continue to walk the city’s sidewalks without worrying about what exists below.

For more on sidewalk vault construction see http://www.structuremag.org/article.aspx?articleID=885

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