Every day, you pass over cast iron manhole covers in the road without a thought of what lies beneath the surface. Precast concrete manholes are a part of the vital underground infrastructure system that helps to accommodate our everyday needs, but the only time the general public is typically aware of manholes is when a problem arises with the utility company.
Utility structure events range in severity from a smoking manhole to an explosion that can dislodge the cast iron cover and are caused by an electrical equipment failure, a combustible gas ignition or both. These issues pose a unique and dangerous challenge for the country’s infrastructure – especially the electrical distribution community – but NPCA members are taking part in creating solutions.
Industry professionals are actively studying the occurrences of fires and explosions in manholes and underground utility vaults to find proactive measures for mitigating and eliminating them. But finding a preventive solution is not simple.
The Institute of Electrical and Electronics Engineers has been working on the issue, including conducting tests on vaults built to replicate utility specifications and conditions conducive to explosive environments. While conducting tests is one approach, there are several other proactive measures that can be employed. Oftentimes, precast concrete structures house utility equipment and materials that are susceptible to fires and explosions, but the precast experiences little damage during an event. This is a testament to the durable design and construction of precast structures. Still, steps still need to be taken to alleviate the pressure buildup. Precast concrete absorbs very little pressure, leaving the access point (cover) as the only avenue for escape.
Ensuring the manhole cover does not fly off the structure during an event is one way to mitigate damage. Some covers are designed to stay in the frame but rise to vent pressure or fumes. Once the pressure is relieved, the cover returns to its original place. Relieving this pressure helps to maintain the integrity of the structure and protects the road surface, utility equipment and people. Precasters are also involved in finding solutions with innovations such as developing precast utility vault and manhole designs that include ventilation or energy-absorption features.
While no single method can guarantee an event will never occur, the precast industry and its suppliers will continue to innovate solutions to reduce or eliminate the negative effects of these events.