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