Targeting the Correct Pipe Slope
Why is pipe slope important?
Most wastewater networks rely on gravity to function. The term ‘💩 runs downhill’, wasn’t coined by accident. Interconnected gravity sewer pipes collect flow from customers and transport the wastewater downstream to a treatment plant through a series of gravity pipes, pump stations and pumped mains.
When designing new gravity sewer mains, the ground topography and required grade of the sewer is the driving factor in determining where it will be constructed and how deep it will need to be. A sewer that is too steep will result in fast flow and increased turbulence. A sewer that is flat or has a sag (belly) is problematic and likely to result in ponding water and a build-up of sediments and solids. This gradual build-up of deposits can create blockages and overflows.
Figure 1 – Problematic flat spots in a pipe
Different ways to measure pipe slope and straightness?
Following the construction of a new sewer, a range of tests and checks are carried out to ensure it has been constructed as per design and is fit for purpose. These checks can include an air pressure test, hydrostatic test, ovality test, and pipe grade check.
Utility organisations, who are the owners of the pipes, may use a variety of methods to be satisfied the new pipe has been constructed to design by housing developers or contractors. These tests check to see if the pipes are straight, free from obstruction and will achieve self-cleansing velocity. Tests can include:
A smooth ball is placed at the upstream end of the sewer pipe and should run unobstructed to the downstream end.
Light & Mirror Test
A light and mirror are sometimes used to ensure a new sewer main is straight with no unintended horizontal or vertical bends. The projection of a strong light through a straight sewer can be confirmed with a full circle of light visible at the end.
Many CCTV camera crawler systems come with inclinometers that will display the grade of the pipe directly onto the inspection video. Sections of the pipe that are too flat or have a sag/belly can be identified from this displayed value.
Figure 2 – Digital inclination recorded by a CCTV crawler
An out of the box solution!
On a recent trip to the USA, the VAPAR team discovered an out of the box solution from a pilot project in Oregon. The City of Salem have been using an alternate inclination testing approach for decades. It has worked so well for them; they continue to use this unique method to check new pipe meets their requirements. Just prior to the CCTV pipe acceptance inspection, tracing dye is released into the pipe to be tested. If the pipe has a consistent negative slope the dye will not collect and pool in a sag. If there is a pool of dye, then there is a sag present, but the dye will not be able to quantify the magnitude of sagging.
The method employed by City of Salem is novel in its ability to identify and quantify a sag in a pipe. A target is installed in front of the CCTV camera that clearly identifies if a sag is present. The magnitude of the sag can be observed using a target where the rings correspond to their internal acceptance levels. Any significant sag in the pipe is immediately obvious as the dye level will reach the 4th target ring.
Figure 3 – Crawler camera with dye target mounted out front
When the pipe doesn’t meet the required slope
If there are a number of these instances of this occurring, it may trigger a complete pipe relay. A single instance where the dye reaches the 5th target ring will generally require a dig-up to address the sag and remove the risk of settling deposits and blockages. In-service pipes with sags/ponding often require regular jetting to minimise this risk.
Figure 4 – The City of Salem target
Evolution of ideas
The team at the City of Salem are an excellent example of not limiting the approach to a problem or task with a narrow and set method, but rather to think about the job at hand and include innovative ideas to achieve a desired outcome. Testing alternate equipment, technology, and methodologies is an important part in process evolution throughout the world.
We would love to hear from others who have a unique solution to complete a part of their job!
About the Author
Mark Lee is the Business Development Manager (Aus/Nz) at VAPAR and a former Senior Asset Engineer who has spent more than a decade managing the asset lifecycle of infrastructure. He has extensive experience managing pipeline networks, including design, construction, condition assessment and decommissioning.
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