Sidewalk Vaults

By Stephen McDermott

In Boston, Massachusetts and other older established cities, some vintage structures were built with basement spaces that extended below public sidewalks.  These basement vaults, or typically described as hollow sidewalks, were used to store goods and receive coal delivery.

The vault width is usually 10 ft. wide and can extend around the building.  The roof of the vault (sidewalk) can be constructed with slabs of granite stone, brick masonry arches supplemented with steel beams, or reinforced concrete slab.  The walking surface can also consist of a concrete topping slab, granite slab, asphalt, or brick paver.

Some vaults contained small diameter (about 5 inch) glass inserts built into the sidewalk in order to provide minimal natural lighting into the vault space.  Similar lenses may be found in cast iron entry stairs to the first floor.

Over time these vaults may be abandoned by the building owner, filled with earth, or retained as storage space.  A common problem with vault spaces is keeping them dry.  Due to the exterior exposure above the vault, the vault is exposed to moisture.  In most cases the construction of the original vault did not provide for waterproofing.

Depending on the construction elements that support the vault ceiling, deterioration often occurs over time due to moisture.  If steel beams and columns are part of the vault structure they are subjected to moisture and over time corrosion occurs.  The oxidation or corrosion of the steel will create an expansion of the steel which results in rust jacking.  This rust jacking exerts forces on the masonry or concrete, which then creates cracks and spalls.  The steel oxidation may also reduce the capacity of the steel to support the vault ceiling and live loads.

Responsibility for the vaults and sidewalk usually has the owner charged with maintaining the integrity of the structural aspects of the vault and the city is charged with the walking surface of the vault, although responsibility may vary depending on contractual agreements made during or after construction.

Currently, there are usually no periodic inspections required by the building department by the owners of these vaults for a structural review.  At times, abandoned and structurally deficient vaults are discovered when a failure in the sidewalk occurs.

Existing vaults can be repaired, re-supported, or in-filled with soil depending upon the condition of the vault structure and the parent structure.  A structural review would need to be performed to provide the owner with options for treating a vintage vault structure.