Closed-bottom fiberglass manholes (also referred to as “watertight manholes”) are lightweight, watertight, and corrosion resistant. Their appearance is similar to a conventional manhole with a base, bench and flow channel, riser, reducer cone, and manway opening. Depending on design preference, the overall construction can be monolithic or modular, and have a straight sidewall profile or a transitional diameter profile. A wide variety of pipe connections are available, depending on the pipe material selected.
Monolithic closed-bottom manholes arrive on the jobsite as a single structure from top to bottom. This type of construction provides a “jointless” structure where the only connection points are at the pipe and ring & cover interface. Modular closed-bottom manholes are constructed in two (or more) sections, typically a base section and riser section. The modular sections are joined with two styles of joints: alignment ring and tongue & groove, depending on preference. Each provides a permanently-bonded watertight joint when completed. The alignment ring method is constructed with an internal ring (or internal bell) so that the riser section can be placed upon the base section with the alignment ring overlapping. The sole purpose of the alignment ring is to prevent lateral movement prior to a sealing. The joint is sealed using a fiberglass laminate field kit (provided by LFM).
The tongue & groove epoxy joint incorporates a groove joint at the top of the base section. The groove joint is designed to perform as a reservoir to hold a predetermined quantity of epoxy adhesive (provided by LFM). The epoxy adhesive is injected into the bottom of the groove joint in equal amounts around the joint. The riser section (plain end) is then lowered into the groove joint. As the riser section is pressed toward the bottom of the groove joint, the epoxy adhesive is displaced and rises along the sidewall of the riser section toward the upper edges of the joint. Once cured, the epoxy adhesive provides a structural bond between the base section and riser section. In cold weather, an external heat source may be required to either heat the manhole’s interior or exterior surface to promote curing of the epoxy.
Due to the lightweight construction of fiberglass manholes, concrete ballast may be required to prevent buoyancy or uplift where groundwater is present. Modular manhole construction allows for steel-reinforced concrete bases to be precast onto the fiberglass base section by LFM. With few exceptions, when a monolithic design is selected, concrete bases are required to be precast at the jobsite or poured-in-place. Whether monolithic or modular, both designs include an external base flange along the circumference and external C-channel reinforcement underneath the base that anchors into the concrete ballast. The quantity of concrete required to prevent buoyancy or uplift is dependent on a variety of factors.