Sewers perform the simple function of moving water or waste streams from a source to a destination, but sewer design is more complicated than you may think. Most sewers work on a gravity system, meaning that they rely on a downhill slope from the source to the destination to ensure the product moves in the right direction. But, flow is affected by other factors too:
- Grade of the slope.
- Volume of liquid in the pipe.
- Distance from the source to the destination.
- Friction against the pipe walls affects the flow.
- Presence and design of maintenance or utility holes in the system.
It is in the design and construction of maintenance holes that benching plays a significant role.
The Need for Maintenance Holes
Considering that the presence and construction of utility holes can play such a prominent role in sewer hydraulics, it might be worth considering leaving them out altogether. However, they have other functions, which make them a necessary component of sewer systems.
Maintenance holes allow maintenance and service personnel to inspect sewers and identify blockages. Sewer networks can be broken down into segments between utility holes so that the problem area can be identified and resolved without compromising the entire sewer network.
Airflow through maintenance holes causes sewer ventilation which prevents the build-up of toxic gases. They play an essential role in the safety of workers involved in sewer maintenance. (Read also: A Brief History of Manholes and Why We Need Them.)
Utility holes also play a critical role in sewer connections. Multiple sewer branches can connect to one utility hole where they combine into the sewer main. Connections can even be accommodated at different heights due to their vertical construction, which makes provision for this. A maintenance hole is a perfect location to change the direction of a sewer line.
Maintenance holes are an essential part of sewer networks, but how can we design and construct them to minimize their negative impact on sewer hydraulics?
What is Benching and How Does it Work?
Benching is the concrete base of a maintenance hole that connects the incoming and outgoing sewer pipes. It offers an open view of the sewer flow because of the exposed channels connecting the inlet and outlet.
Benching is always sloped downwards from the vertical utility hole wall to the top of the product channel. This slope ensures that any liquid build-up in the utility hole will drain into the sewer pipes when the incoming flow subsides.
The two main types of benching - half and full - have to do with how deep the channel is and where the benching concrete meets the channel. There are advantages and disadvantages to each type, and design criteria may dictate which one is the most appropriate for a particular application. (Read also: Half Benching vs Full Benching: Key Differences.)
Half benching is where the concrete base meets the channel at the height of half a pipe width.
The concrete should slope down from the wall to the piping channel between a minimum slope of 1:30 and a maximum of 1:10. The advantage of half benching is that the flow of product is highly exposed, making it easy to inspect and easy to clean if there is a build-up of debris
However, the hydraulic performance of half benching is not as good as that of full benching. There is a substantial head loss when using half benching, which reduces the flow rate and fluid velocity.
Full benching is when the concrete floor of the maintenance hole is higher than the apex of the inlet and outlet pipe.
The same slope applies for the horizontal section of the concrete utility hole base (i.e. 1:30 to 1:10). However, with full benching, the channel for carrying the product from inlet to outlet is the same height as the pipes. Full bench maintenance holes are more challenging to inspect and clean as the channels are much deeper. However, hydraulic performance is much better than that of half benching. This design results in a much lower head loss and therefore, the preservation of product flow and velocity.
Benching Design Considerations
There are several design factors to take into account when planning sewer utility holes. Here are some of the primary considerations to bear in mind:
Designing maintenance hole benching must take into account the hydraulic performance of the sewer as a whole. If the product moves too slowly through the pipe, solids present in the waste will drop out of the water and build up on the pipe floor. Friction against the pipe walls may cause this build-up to happen all around the pipe leading to blockages.
Sewer designers compensate for this by ensuring a minimum velocity for self-cleaning. The system must achieve this self-cleaning velocity at least once per day. Designers must choose a utility hole benching strategy that maintains the system's fluid velocity above the self-cleaning limit. (Read also: Hydraulic Radius Explained: Sewer Design, Formulas and Perimeter Calculations.)
Expected Solids Content
Some sewer networks are relatively clean and free from large debris that can clog up pipes. In these cases, full benching can work well because there is seldom a need for workers to dig out blockages in the channels. However, other systems prone to solid objects and debris should be designed to ease access and cleaning. This most likely means half benching.
Pipe or Concrete Scoring
While the minimum velocity of product in the sewers is necessary for self-cleaning, maximum velocity is also significant. A high velocity of a fluid with solid particles has an abrasive effect on concrete and internal pipe walls. In extreme cases, this erosion can lead to leaks from the sewer network. Rough pipe walls also increase the friction coefficient, which means that, in low flow conditions, solids could build up on the pipe walls leading to blockages.
Maintenance holes play a significant role in sewer networks, but designers must take care to prevent sewer problems. Hydraulic calculations will help ensure the product velocity remains in the optimum range for self-cleaning without causing problems through scoring.