Open-Bottom Fiberglass Manholes are lightweight, corrosion resistant and, typically, the most economical fiberglass manhole design for small collection systems. Their construction consists of a riser section, reducer cone and manway opening; however, open-bottom manholes arrive on the jobsite with an open-ended bottom. This manhole design requires a concrete base, bench and flow channel to be formed-in-place on the jobsite. Typically, open-bottom manholes are installed using a “dog-house” installation method where U-shaped cut-outs are made at the manhole base where pipe will penetrate the manhole. With few exceptions, open-bottom manholes arrive on the jobsite as a monolithic structure from top to bottom.
Steel Reinforced Concrete Base
Prior to pouring the concrete base, holes are drilled through the fiberglass manhole near the bottom to accommodate a grid pattern of steel reinforcement. This steel reinforcement not only provides a structural benefit to the concrete base, but also anchors the fiberglass manhole into the base. Typically, the manhole will be placed on cinder blocks at the proper elevation and concrete will be poured underneath and around the manhole and over the pipe penetrations. Care is taken to tightly pack the concrete against the manhole and pipe to prevent infiltration of groundwater.
The poured-in-place concrete bottom performs two functions, providing a base and ballast for the manhole. Due to the lightweight construction of fiberglass manholes, concrete ballast is required to prevent buoyancy or uplift where groundwater is present. The quantity of concrete required preventing buoyancy or uplift is dependent on a variety of factors.