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How to Install a Fiberglass Manhole

Openchannelflow fiberglass manholes must be handled and installed according to these general instructions.  Installation details may vary depending upon site and application particulars but these general instructions should be followed regardless.

Failure to comply with these installation instructions will void any warranty coverage and may cause damage or failure of the product and potential injury or death of personnel. 

All applicable national, state, and local construction codes should be consulted as they apply to the work involved.

It is important that installing parties familiarize themselves thoroughly with these instructions before proceeding with an installation.

Handling, moving, AND STORING THE MAHOLE

  • Fiberglass Manholes must be lifted using nylon or similar fabric slings or lifting lugs (if furnished on manhole).  Under no circumstances should cable or chain be used as they will serrate the fiberglass. 

  • Proper rigging practices must be observed. 

  • Taglines should be used to control the movement of the structure during lifting. 

  • Manholes should never be dropped, rolled, slid, or impacted.

  • If manhole has to be stored before installation, store it in its original shipping packaging, on padded shipping cradles, or on a smooth, clean, and flat surface. 

  • Chocks and / or tie downs should be used to prevent rolling and movement due to wind.


  • The site excavation must be of adequate size to accommodate the manhole and provide safe working room for the installing personnel. 

  • Use good excavation procedures at all times.

  • Provide a concrete foundation slab.  The slab should extend at minimum of 12 inches [30.48 cm] on all sides beyond the manhole, base mounting flange, connecting pipes / flanges, and any portion of a structure integrated into the manhole (commonly flumes).  The slab can be poured off-site or formed and poured on-site.

  • The slab should be at least 6 inches [15.24 cm] thick, but as local soil conditions may vary, the final design of the slab and anchoring details are the responsibility of the installing party and must be sized so as to prevent shifting and flotation of the manhole structure.  Retain a knowledgeable engineering firm if necessary. 

  • The concrete foundation slab must have a smooth, troweled surface with no exposed aggregate or debris to provide uniform support over the entire base area.  The slab must be level in both directions to within 1/8 inch [0.3175 cm].  

forming the concrete slab for a fiberglas manhole to be installed on

  • In determining the elevation of the concrete slab, refer to the manhole design drawings provided by in the equipment submittal, making sure to allow for the 1/2 inch [1.27 cm] thick foam pad (provided with the manhole) that fits between the manhole barrel and the concrete slab.

a properly sized concrete slab for a fiberglass packaged flow metering manhole

  • If buoyant forces are high due to a high water table, the flume / pipe stubs may be encased in concrete – poured in 6 inch [15.24 cm] lifts – up to the top of the flume. 


  • Verify that that concrete slab is level and free from exposed aggregate and debris. 

  • Place the foam pad on the concrete slab where the manhole barrel will rest. 

  • Place the flexible couplings on the manhole pipe stubs (where provided). 

fernco flexible pvc coupling on the discharge pipe of a fiberglass packaged flow metering manhole

  • Lower the manhole onto the concrete slab – taking care to secure the manhole from overturning during movement and lowering.
  • Loosely secure the manhole to the concrete slab with anchor bolts. 
    • The quantity, type, diameter, and depth of embedment of the anchor bolts – along with type of concrete mix for the pad - should be based upon buoyant forces and should be determined by the engineer (as indicated in Excavating and Preparing the Site above). 
    • The holes provided in the base anchoring flange as shipped from the factory are for shipping purposes. 
    • The quantity / size / location of these holes may or may not be sufficient to secure the manhole properly.
  • Where additional anchoring holes are drilled on-site in the base mounting flange of the manhole, the exposed fiberglass can be sealed with a compatible resin (typically an ortho- or isophthalic polyester – available at most marine supply stores).   

fiberglass Packaged Metering Manhole - PMM - secured to a concrete slab

  • For Packaged Metering (flow) and Grinder manholes, check that the interior structure (flume / weir / grinder channel) is level, adjusting as necessary. 
    • For flumes verify the level at the inlet floor of the flume. 
    • For weirs verify the level at the face of the weir. 
    • Don't level Flume or Weir manholes by the top of the flume / weir – level the floor of the device. 
  • Minor level variations shouldn't affect Diversion / Flow Control, Energy Absorbing, or Storm Water / Screenings manholes.
  • Once the manhole has been leveled, finish tightening the anchor bolts to secure the manhole to the slab. 
  • All open spaces under any structure (typically a flume) that extends outside of the manhole must be filled with a non-shrink grout to provide permanent structural support.


  • Backfill should be placed evenly around the manhole in lifts of no more than 12 inches [30.48 cm].  A common packing standard is 85% proctor.  Make sure that no void areas exist between the concrete slab and any portion of the manhole structure (i.e. under a flume or pipe connection).

backfilling around a completed fiberglass manhole installation

  • Fill material should be imported sand or pea gravel material (naturally round aggregate 1/4-3/8 inches [0.635 cm – 0.9525 cm] in size).  

  • If another material is used, avoid rocks, concrete, or soil clumps larger than 1 inch [2.54 cm] in size. 

  • It's recommended that the advice of a knowledgeable engineering firm be obtained to ensure that proper backfilling materials and procedures be use to match the local soil conditions. 

  • Don't allow groundwater or surface runoff to accumulate in an open excavation around a manhole that hasn't been completely backfilled.


  • For manholes containing flumes or other structures where there is a void space between the inside of the manhole barrel and the structure, that space should be filled with supporting material to the top of the structure so that water doesn't collect.

  • Note that the void space may be on either side of the structure in the manhole and extended under the structure.   

  • Failing to fill the voids may result in the collection of water that may go septic or become the home of aquatic reptiles, insects, or other pests.  

  • Pea gravel or compacted sand with a 6 inches [15.24 cm] cap of sulfate resistant non-shrink grout may be used.  Pours should be in lifts of no more than 4 inches [10.16 cm] at a time.  

  • Don't distort the structure in the manhole.  Temporary internal bracing should be used to insure that the structure stays in dimension while the shoulder is poured.

  • Slope the manhole shoulder so that it drains into the structure and out the manhole.

  • A protective tape is factory applied to the top flange of the structure. This tape is intended to assist the installer in making a cleaner installation when the shoulder is poured.  Once the grout has been poured and smoothed, the tape should be removed.

a nested Parshall flume installed in a fiberglas packaged metering manhole


For Flume Manholes, if bracing is provided on the portion of the flume in the manhole, it should be left on until the installation has been completed.  Once the concrete bench is poured around the flume inside the manhole and it has cured, the bracing may be removed.  If the bracing is removed, verify the dimensional accuracy of the flume after the removal.