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Closed-Bottom Manholes


Closed-Bottom Fiberglass Manholes (also referred to as Watertight Manholes) are lightweight, watertight and corrosion resistant.  Their appearance is similar to a conventional manhole configuration with a base, bench & 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 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 providing 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 groove 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 require to either heat the manholes interior and/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 preventing buoyancy or uplift is dependent on a variety of factors.

Fiberglass Provides Economic Savings

As an economic benefit, closed-bottom fiberglass manholes are available in a “transitional diameter” configuration.  As manholes increase in diameter and depth, the cost of a manhole can be minimized by reducing the diameter of the riser section relative to the base section.  Typically, the diameter of the base section is determined by connecting pipe diameters, number of pipe connections and directional changes in the flow line.  These factors may require a minimum base section diameter in excess of 4-feet while a transition to a 4-foot diameter riser section may be perfectly acceptable.  Greater savings will be realized as the difference between base diameter and riser diameter increases.  The amount of savings will depend on the required minimum base diameter, desired minimum riser diameter and total depth.  Unless the manhole is greater than 20-ft deep, a “straight sidewall” manhole is usually the most economical design.

Sealed and Water-tight

The base of a closed-bottom manhole consists of an integral fiberglass plate to prevent infiltration of ground water.  Closed-bottom manholes may be constructed with or without a fiberglass bench and flow channel.  If corrosion of the bench and invert is not a concern, brick & mortar or concrete flow channels may be formed in place after the manhole has been installed.  Pre-fabricated fiberglass flow channels can be built as ½, ¾ or full depth flow channels.  For optimal hydraulic performance, fiberglass flow channels offer a Mannings smoothness coefficient of 0.009 and directional changes include smooth-sweep elbows.

Pipe connections for closed-bottom manholes include fiberglass laminated pipe stub-outs, flexible rubber boots (i.e. Kor-N-Seal©), segmented rubber seals (i.e. LinkSeal©) and flanges.  Internal drops are typically pre-fabricated at LFM’s manufacturing facility and incorporate a drop tee or bowl at the top, with brackets anchoring the downcomer to the sidewall and a bottom elbow to direct flow in the desired direction.  External drops are typically pre-fabricated with an external PVC pipe stub at the desired elevations.  The external drop is then assembled and connected in the field by the installer.