Underground Infrastructure - August 2023 - 32

Force Main Rehabilitation
By NASSCO Member Dr. Mark Knight, PhD, FIAM M. ASCE, Associate
Professor, Department of Civil and Environmental Engineering, University of
Waterloo, Canada.
Force mains, also called rising mains in the United
Kingdom, are low pressure pipelines that lift the
sewage from one location to another by pumping
the sewage under positive pressure (Figure 1).
Force mains are constructed with a limited finite
length and access, only when a gravity sewer
does not have sufficient elevation grade to make
gravity sewers flow.
Unlike gravity sewers, force mains flow full with
sewage at internal pressures that can vary from 20
to 60 psi. They also are circular pipes, 4 to 36
inches in diameter, that have no lateral
connections. Pressure release valves (PVRs) are
often installed at high points to release entrapped
air and to limit surge pressures when the pump
starts. Frequent pump starts and stops also
occur to maintain the sewage level within the well.
Force mains should be designed to withstand the
normal working pressures, maximum surge
pressures, and the frequent turning on and off the
pumps that can cause fatigue failures. Pipe
materials such as PVC can be susceptible to
fatigue failures in force mains.
Force mains are critical components in a
successful operating sewage network. Figure 2
provides common failure modes observed in force
mains.
Figure 1 Gravity flow sewage is collected into a pump station well, then pumped under pressure through a
force main pipeline.

Underground Infrastructure - August 2023

Table of Contents for the Digital Edition of Underground Infrastructure - August 2023

Underground Infrastructure - August 2023 - 1
Underground Infrastructure - August 2023 - 2
Underground Infrastructure - August 2023 - 3
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