Definition - What does Benching mean?
Benching as it relates to trenchless operations is defined as the smoothed infill concrete placed between the channel pipes and the chamber walls of a manhole. Benching influences the hydraulic flow through the manhole during sanitary sewer and drainage events. The shape and elevation of the benching prevent rodents from leaving the system and avoid the buildup of waste in the manhole that results in blockages.
The finished surface of the benching is smooth so that it is self-cleaning and allows for the unimpeded flow of sewage or drainage. Benching is typically formed from the channel’s edge, at the minimum, the pipe’s crown to allow for smooth flow. There should be no gaps, as water should not be allowed to penetrate through the benching.
Benching requires a fine-grade concrete such as granolithic mortar as it provides the necessary, thin, and resilient surface that manholes warrant. For benching needs higher than 30 mm, a semi-strong dry concrete is placed before the application of the granolithic screed. The desired gradient of the finished screed lies between 1:10 to 1:30.
Benching is also referred to as a bench.
Trenchlesspedia explains Benching
Types of Benching Designs
Half bench and full bench are the most common types of benching configurations utilized. A half bench design indicates that the pipe’s lower half extends through the manhole. In the half bench configuration, the horizontal benches extend from the semi-circular channel to the interior manhole wall. A full bench design is one that has a full-depth channel.
Comparisons of the half bench and full bench designs show that the full bench design is associated with improved hydraulics. Studies have illustrated that there is little reduction in head loss when an unbenched manhole is compared to that of a half-benched manhole. However, the decline in head loss is greatly improved where a full bench design is employed.
Figure 2: Most Common Benching Configurations (source)
Key Points to Consider in Benching Design
For benching designs to be effective, it should always be installed at the required minimum pitch so that any overspill of water of waste always makes its way back to the channel. When exposed to solids contained in stormwater, channels, although benched with concrete, can experience scouring from shock loading. If there are flaws in the channel, abrasive material can be trapped and washed away during high-velocity events. Scour abrasion of the invert and benched flooring can happen during these events.
Benching is formed with smooth and continuous surfaces that are correctly aligned to avoid scouring. Surfaces should also have a non-skid finish and be able to accommodate maintenance crews.
Repairing Damaged Benching
Repairs of damaged sections of benching are ideally carried out when the system is not in use. This act avoids liquid coming into contact with the concrete before setting. There are many products available that will expedite the concrete curing process, thus rendering the system fit for use in a shorter period.
Depending on the magnitude of disrepair, benching should be completely removed and replaced to ensure continuity and smoothness in the surface. If this is not possible, new sections are carefully tied into the existing for a seamless finish.