4 Things You Need to Know About Pipe-to-Manhole Connections
By Alan Siebenthaler
HK Marketing/Territory Manager
While we have used a Legos analogy before to describe the ease of piecing together gasketed box culverts, installing a pipe-to-manhole connection isn’t quite that simple.
The seal at the connection point is integral to preventing water leakage, which can erode the fines around your structure. A watertight seal also prevents any sewage or stormwater from leaking out and contaminating surrounding groundwater. Needless to say, that seal is critical. That’s why we’ve compiled a list of things to know about pipe-to-manhole connections to ensure that your structure stays watertight.
1. Which types of connectors are there?
Mortar: A combination of cement or similar material, fine aggregate, water and sometimes other biological fibers like glass or wool to increase watertightness. Mortars are mixed and placed on-site.
Cast-in connections: A variety of connectors that are cast into the manhole during production. These are affixed using a mandrel inside the manhole form prior to pouring the concrete.
Cored-hole/mechanically installed connectors: This involves a cored or formed hole and a separate, mechanically installed connector.
Boot seals: Available either as a cast-in or mechanically installed, these flexible rubber seal connectors protrude from the manhole wall and allow for the insertion of a pipe. This type of seal will often have a steel band that is tightened around the perimeter to create a watertight seal with the pipe.
2. Which materials matter?
Most connectors are made from EPDM Rubber. This rubber offers resistance to acid, alkali, ozone and UV rays. Frequently, connectors are shipped to job sites and sit in the elements for days, weeks or months before finally being installed. The UV resistance prevents premature aging due to excessive exposure and maintains the required physical properties.
Nitrile Rubber should be used if the drainage system is going to convey any type of petroleum or oil product. This type of rubber withstands deterioration due to runoff from gas stations, car washes, parking garages, parking lots, and airports.
Mortar is still used frequently across the United States for stormwater projects. Sanitary projects, however, usually require watertight rubber connectors, since mortar is not as resistant to movement of the pipe or manhole, and more likely to allow leakage over time. In terms of lifespan and labor costs, rubber is much more resistant to differential settlement, and takes less installation time because there is no need to mix and apply mortar and then wait for it to set before filling the trench.
3. Which connector should I choose?
We generally recommend against using mortar joints because they are less watertight and will inevitably lead to infiltration into your conveyance systems.
For smaller pipes, the option to specify cast-in connections may be based on your confidence in the precaster’s ability to precisely install the mandrels containing the connectors inside the forms. Once those connectors are placed and the concrete is poured, there is no changing their location. If the precaster has a coring machine, they will likely prefer to provide cored connections, since the coring can be done on demand to meet the requirements of the project. Also, in terms of quality control, some producers prefer coring because they feel more confident that the pipe connection will be exactly where it needs to be during installation.
For larger pipes, we recommend cast-in connectors due to the curvature of the manhole. The larger the hole, the more defined the curve, and the more difficult it becomes to get a watertight seal between the connector and the hole around the perimeter of the rubber seal.
For plastic pipes, we strongly suggest using a rubber connector regardless of the conveyance system because mortar does not bond well to plastic, leading to leaks through the connection from the get-go. Additionally, plastic pipe is flexible. A typical soil load on most types of plastic will cause the pipe to deflect even just slightly. Rigid mortar cannot accommodate this type of dimensional change, thus increasing the likelihood of a leaking connection. Resilient rubber connectors are by far the best way to seal a plastic pipe to concrete structure connection. Just make note that if you use corrugated plastic pipe on your job site, you will also need a sleeve to go over the corrugated surface and convert it to a smooth surface within the connector. Consider this sleeve when ordering your manholes so the correct outside diameter can be used for installation of the proper connector at the precast plant.
4. Before installation
Prepare the pipe: Depending on the connector type, this could mean beveling the end of the pipe, de-burring the pipe to knock off any sharp edges that could tear the rubber, and/or lubricating the connector and the pipe. Use care when preparing and installing the pipe. This is important, especially with cast-in connections. Jagged edges could ultimately tear the connector that is embedded into the concrete, and the only viable solution would be to cut out the connector, core a larger hole and then find a new mechanical connector that will fit both the hole and the pipe. The other, less-effective option would be to mortar the pipe into the new hole.
Pipe alignment: Make sure that your pipe is aligned with your connector before insertion. We occasionally hear of a contractor who has installed their pipeline out of alignment with the manhole. Sometimes the alignment is not severe, so they are able to deflect the connector slightly and make it work. ASTM C923 states that connectors must be capable of holding 10 psi of water pressure when deflected 7 degrees in order to meet this standard, so deflection to this amount is likely going to be acceptable. But, if the alignment is going to be more severe, they might be required to core a new or a bigger hole in the manhole and then try to use a mechanical connector to seal the connection. Contractors should be careful to maintain a precise direction of the pipeline to avoid these field repairs.
Prevent differential loading and settlement: There are instances where varying loads can be applied to the top of the pipe or the manhole due to construction equipment or vehicular traffic. If the base has not been prepared properly beneath the manhole and pipes entering into it, then this loading could result in the displacement of the pipe in relation to the hole and connector. A resilient rubber connector will allow for pipe movement up to 7 degrees to meet ASTM C923 requirements.