A manhole looks like a simple concrete box with a lid on it, but it is one of the most expensive structures in a collection system to replace, and one of the few pieces of infrastructure that gets attacked from the inside, from the top down, by something that never even touches most of it. Understanding why manholes fail is the first step toward catching the problem while it is still cheap to fix.
Manhole Materials You'll Encounter
Most manholes in a collection system are built from one of a few materials:
- Precast concrete - factory-cast sections (base, barrel rings, cone) trucked to the site and stacked in place. This is the dominant material in modern construction because it is consistent in quality and fast to install.
- Cast-in-place concrete - poured on site inside forms. Common in older systems or custom structures such as large junction manholes and drop structures