Manholes date back to the early 19th century when sewer designers determined to access the underground lines was necessary. To ensure the sewage flowed freely, designers installed an invert, which allows the wastewater to run from the inflow pipe through to the outflow.
The invert itself is an elevation which the interior of the incoming pipe sits. Benching, which precipitates the hydraulic flow through the manhole, is an essential part of that design.
As the manholes are access points for sewage repair and rehabilitation, the design of the bench is an important consideration. (Read Trenchless Sewer Repair and Cleaning 101.)
Benching is installed at a pitch and uses a sand and cement mixture to complete. It is essential that the bench is raised for the channel to drain appropriately.
There are two styles of benching: the half bench and the full bench.
With both half and full bench designs, the slope should still provide adequate footing for a worker to stand and space to lay their equipment if needed.
The slope also needs to have a height sufficient to ensure proper drainage. It means they should have at least an 8% or 1-inch per foot slope from the wall to the channel edge.
What Does It Mean to Have a Half-bench?
When construction workers design a manhole with a half-bench design, only the lower half of the pipe extends through the manhole. The horizontal benches in this design extend from the semi-circular channel and into the manhole interior wall.
Pros of Half-Bench Design
Many municipalities choose to use a half-bench design in their manhole construction and rehabilitation projects. The cost of preparing a half-bench is less than that of the full bench design.
It also leaves an area of the channel open should there be an issue which maintaining the channel is necessary.
It makes it easier for workers to address those issues. The slopes are not as steep as ones found in full-bench designs, making it easier for workers to keep their footing and place their tools.
Cons of the Half-Bench Design
Using the half-bench design comes at a cost, however. While it does make maintaining the invert channel easier and is less expensive to create, it doesn’t provide the same hydraulic effect.
With a lower hydraulic result, the system may not reach its peak flow velocity for the sewer to reach a self-cleaning speed needed. Without self-cleaning velocity through the pipes, deposits build up to obstruct the sewage lines.
What Does It Mean to Have a Full-Bench?
As the name implies, a full bench has an entirely formed full-depth cannel. Instead of having only part of the channel benched, the entire area has the sand concrete mix infill between the manhole walls and the inflow pipe.
Pros of the Full-Bench Design
Having a bench over the full depth of the channel is optimal in areas willing to build one. With this design, you see a higher hydraulic flow. The heightened flow helps to ensure the sewage lines connected through the manholes attain their self-cleaning flow velocity at least once during the day to keep debris from obstructing the path.
Cons of the Full-Bench Design
It is costlier to create a full-bench design. The slope between the channel and the walls are higher to cover the full depth of the circuit. It also makes it a little more difficult to reach into the channel to clear debris if needed.
As there needs to be an adequate slope to provide drainage, it may be too steep for workers to stand safely while maintaining the system.
Why You Need Quality Invert Benching
The hydrodynamic forces from the flow of sewage through the channel can cause it to scour or degrade the material. Concrete is most resistant to the scouring from stormwater and wastewater.
However, substantial solids flowing through the channel can cause a process better known as “shock loading” which may cause scouring to occur. If the channel and benching are poorly formed, then the solids may become trapped. With a continuous smooth surface between the benching and the invert, you can reduce the scour and “shock loading” events.
Necessary Equipment for Building a Bench
Building either a full or half bench does not take many pieces of equipment. You do need concrete poured into place in the manhole. The concrete pour takes place after all the pipes are connected to the manhole.
The channel should either be straight or have a continuous curve to connect the inlet and outlet pipes. It is vital that all dimensions and details are included with the manhole installation specifications to ensure a durable invert and benching pour occurs.
Once inside the opening, workers can use shovels to adequately slope the bench from the wall to the channel opening. Using shovels and moving the cement by hand before it sets allows them to slope the sides as per their design instructions sufficiently.
In some cases, the invert and benching pour happens at a precast plant. They accomplish this by forming the manhole walls, base, and channel in one casting upside down. When the flip the product it leaves the appropriate channeling in place.
If the precast plant creates the invert channel in a secondary pour, they do so after the holes for the inlet and outlet are created and cored. Often these inverts and benches are formed by hand.
Using a full-bench in place of a half-bench can give you better flow through the invert channel. However, it can be more difficult for workers to stand to access the lines if there is a maintenance issue.