Manholes allow personnel access to buried utilities for inspection and maintenance. Manholes are an integral part of buried sanitary sewer and storm sewer systems, but larger electrical, telecommunications, and steam distribution systems may also be constructed with manhole access.
Manholes are placed in gravity flow (storm and sanitary) sewer systems at changes in pipe size and slope, as a node where sewer lines join, and wherever single lines change direction. Additionally, there are general rules for manhole spacing in sewer systems based on pipe diameter. For sewer pipes of 15 inches or less, manhole spacing is typically 300-400 feet or less. For larger pipe diameters, recommended manhole spacing increases to about 500 feet minimum. The recommended distance may be further increased if the pipes between manholes are of large enough dimensions to allow a person to walk through.
The sidewalls of manhole structures are typically made of brick, cast-in-place concrete, or pre-cast concrete. Sidewalls made of brick are at least 8” thick, and those made with cast-in-place concrete (or concrete masonry units) are at least 6” thick, up to a depth of 12’-0". Deeper manholes manufactured on-site with brick or concrete will have at least 1'-0" thick sidewalls. The minimum recommended diameter for manhole structures is 4'-0", with a minimum covered opening of 21" in diameter to allow entry. In current construction practice, the most common type of manhole structure for sewer systems is pre-cast concrete, which is usually cheaper to manufacture and install than other methods. There are also some manufacturers that offer HDPE (high density polyethylene) elements for manhole construction.
Pre-cast concrete manholes are manufactured off-site in separate castings that can be prepared based upon the depth and dimensions required by construction drawings. They are circular in cross-section and manufactured with embedded reinforcing steel. Pre-cast concrete manhole structures have a few basic elements which are assembled in place and sealed together at the job site. The lowest section is called the base, and is made with pre-cast openings and bottom channels for specifically sized pipes at a planned elevation. The base section is placed at the proper elevation on soils that have been compacted within an excavation. Above the base, depending on the required depth of the entire manhole structure, there may be one or many riser sections installed. Riser sections accommodate the height between the base and the surface elements of the manhole opening. In some cases, manhole structures may be hundreds of feet deep. Standard riser sections are 48” in interior diameter, but riser sections of greater diameter are available in 12” increments. These larger risers are occasionally used to accommodate deeper manhole structures.
At the surface are metal grade rings that are cast into the concrete to support the removable manhole cover. In some shallow applications, a flat concrete slab top (of the same dimensions as the base section) is used to house the grade rings and manhole cover. In most installations, a transition piece will be required between the last riser, or base, and the grade rings. This transition piece is called a concrete cone, which tapers the structure’s dimensions from those of the last riser or base to the grade rings. Concentric cones place the manhole in the center axis of the overall structure. Eccentric cones place the manhole off to one side of the manhole structure, and may allow safer access to ladder rungs installed within the risers, depending on the cone dimensions.