State of Technology Report for Force Main Rehabilitation, Final ...

External Loads
• Soil Cover (trench or embankment)
• Surcharge
• Live Load (e.g., HS-20 wheel load – dual wheel load of 16,000 lb uniformly distributed over
a surface area of 10 inches x 20 inches (250 mm x 500 mm) as recommended by the
American Association of State Highway and Transportation Officials [AASHTO] and Cooper
E-80 rail load – axle load of 80,000 lb on 5 feet (1.5 meters) centers as recommended by the
American Railway Engineering Association [AREA])
• External hydrostatic head from groundwater (only a factor when the main is not under
internal pressure)
Internal Loads
• Working Pressure
• Surge Pressure
• Test Pressure
Other factors that also enter into the design are corrosion, temperature, fatigue and erosion/abrasion
considerations. High points in a sewer force main are potential pockets for gas accumulation, especially
if air release valves are inoperative. These tend to be areas where microbiological corrosion, due to the
bacterial formation of sulfuric acid (H2SO4), takes place. Any new liner must be resistant to these effects.
The pressure rating of most thermoplastics and reinforced thermosetting resin pipes are based on longterm
pressure regression testing at ambient temperature. Similar testing at higher temperature has shown
that derating of the pressure class is necessary. The AWWA standards for PVC and PE pressure pipes
contain derating tables for continuous operations at elevated temperatures of about 73.8 o F. In the case of
glass-reinforced thermosetting pipe, one needs to contact each individual manufacturer for their derating
recommendations. One would expect that the thermoplastic and thermosetting resin liners offered for
rehabilitation would be no different than their pipe cousins.
Due to the presence of solids, force mains are far more abrasive environments than potable water mains.
It has been reported that some force mains with concrete liners (PCCP and asbestos-cement) have had a
channel eroded down the invert (King County, Tel Aviv, Israel). At low velocities of approximately 1.5
feet (0.46 meters) per second or less, large solids tend to tumble along the invert of the pipe until they are
broken up into smaller particles that can be suspended in the waste stream. The erosion appears more
prevalent at the beginning of the force main and gradually tapers off over a long run. There has also been
a report on a steel force main with a bitumastic liner that has lost nearly all of the liner and up to 70% of
the pipe wall in the bottom one-third of the pipe from the abrasive effects of the effluent (Regina, SK in
Canada). Therefore, the design of a renewal liner for a sewer force main must take the potential for
erosion into consideration. Fortunately, the newer materials used for lining sewers generally have good
abrasion resistance. Polyethylene has been shown to have excellent abrasion resistance in tests conducted
at Darnstad University in Germany.
It has been documented that the long-term performance of PVC can be limited by its fatigue resistance to
cyclic pressure loading. This first surfaced on irrigation distribution systems where the buried PVC
distribution piping was subjected to very frequent cyclic loading from valve operations. Pumped sewer
force mains are similar as lift station pump(s) are constantly cycling in and out as the sewage level in the
wetwell triggers the starting and stopping of the pumps. The Unibell PVC Pipe Association has published
a design guideline for the use of PVC pipe in sewer force mains. This guideline provides
recommendations on maximum pressure ratings based on the number of pressure cycles expected in the
55