A frequent question NPCA Technical Services engineers receive is, “What manhole diameter is used for a specific pipe diameter and configuration?” Though this question appears easy, it’s actually complex.
For instance, unlike pipe penetrations into a flat surface, where the required opening is circular with a diameter slightly greater than the pipe’s outside diameter, a circular pipe entered in a circular manhole creates the intersection of two cylinders. This forms an elliptical opening with skewed cuts within the manhole barrel section. In combination with the specific geometry of the manhole and pipes, this can create challenges. Another consideration to account for is when the structure/pipe connections are grouted or use a resilient connector. Lastly – though not directly affecting manhole sizing, but still important in the design considerations – is when the pipe opening intersects a manhole riser joint.
Keeping the above in mind, it’s important to answer these three questions when sizing a manhole with pipe openings:
- Will the manhole physically accept the pipe and be structurally sound?
- Will the manhole be feasible during construction?
- Will the installed manhole provide the soil or water resistance per design expectation?
Will the manhole physically accept the pipe and be structurally sound?
This typically is a primary concern for the designer. It comes into play when attempting to provide enough manhole wall between pipe openings to offer assurance that the riser barrel will have the structural capacity to carry and distribute the installed soil and live loads. Most design engineers are focused on the finished loading condition of the pipe and structure.
Some owner detail sheets show the minimum required dimensions between manhole wall openings, while others may provide a table that shows manhole diameter sizes based on incoming pipe diameters and deflection angles. Yet for many precast manholes with large pipe openings, severe loading conditions occur during handling and installation, which is typically the contractor’s responsibility. However, if the specified manhole size will not work for the pipe installation, the precaster must inform the design engineer in advance of the bid date for further review or an addendum.
Will the manhole be feasible during construction?
The precaster’s responsibility is to furnish a structure that is constructible for the contractor. This may require additional longitudinal steel within the wall area or temporary struts to hold the structure in shape prior to pipe placement and grouting. All manhole sections with large openings require enough peripheral wall to hold the structure together during shipping, handling and installation. Depending on the available height between the pipe invert and casting elevation, the needed wall may be placed below the pipe openings. If this is the case, good communication is required between the contractor and precaster regarding the additional excavation required and potential additional concrete needed to fill the interior sump.
Will the installed manhole provide soil or water resistance per the design expectation?
Most manhole sizing tables provided with project plans and the National Precast Concrete Association’s manhole sizing guide assume the pipe-to-manhole connection will be grouted. Grouted connections, when done with quality materials and installed per the manufacturer’s requirements following ASTM C1821, “Standard Practice for Installation of Underground Circular Precast Concrete Manhole Structures,” will provide a watertight connection. However, these connections are typically used for soil-tight applications. If project specifications require the use of resilient rubber connectors between the manhole and pipe, the manhole size will be dictated by the connector manufacturer’s requirements. For this connector type, and depending on the connector style, it is critical to obtain the required compression forces against the expanding steel bands on the manhole wall or against the incoming pipe.
A science and an art
Regarding the respective geometric configurations and calculations, manhole sizing is a science. However, when the numerous variables of available formwork, regional preferences, type of connections, etc., are included, it is also an art. The best means to ensure the correct sized manhole is specified for a sewer project is for the designer to contact the local precast concrete manhole manufacturer and determine the most economical structure for the project conditions.