A utility vault is a underground room or enclosure used as an access and control point for public utilities. These rooms typically house gas or water control valves or electrical switch gear and may be completely or partially buried. Depending on their size, utility vaults are accessed in various ways including manhole and ladder arrangements or removable lids. They are usually made of prefabricated concrete although fiberglass or steel varieties are fairly common. Utility vaults are available in a wide selection of sizes and designs and often include integral features such as cable trays and access knockouts.
It is common utility distribution practice to group controls and instrumentation in centrally located and easily accessible points. Most utility feeds including gas, water, electrical, and telecommunications lines are routed underground which presents a problem in this regard, however. Installing a utility vault in a central location solves this problem and creates an easy to access, clean, and convenient location to group valves and switchgear. These vaults are typically simple square or rectangular boxes buried in the ground into which the pipes and cables are routed. These boxes are usually large enough to accommodate all the lines and their associated equipment while allowing personnel to stand or move around in them.
Typically constructed from precast concrete, the utility vault is either buried completely or with only a small portion of the walls protruding from the ground. Access is achieved through a manhole and ladder or a removable grate or lid. Some vaults are also made of steel or plastic composites although they tend to be less robust and more inclined to "float out" due to hydraulic forces exerted by ground water accumulation. Utility vault sizes depend on the number of lines they are designed to accommodate with common sizes including 3 x 3 x 3 feet (1 x 1 x 1 meters), 4 x 6 x 8 feet (1.2 x 1.8 x 2.4 meters) and larger 8 x 12 x 8 feet (2.4 x 3.6 x 2.4 meters) variants.
Most utility vault designs are constructed off-site and installed as a completed unit. It is common practice to include a range of integral features during construction which include cable or pipe trays, light fixtures, and knockouts. The latter are grouped around the walls of the vault and consist of circular rings of thinly cast material which allow the center disc to be knocked out to allow cable or pipe entry. All utility vault designs, particularly the larger examples, are considered to be hazardous environments due to their enclosed nature; service personnel should always guard against the possibility of carbon monoxide and fuel gas accumulation.