Need for Project
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Existing pipe designed for gravity outfall flow – peak storm flows
raise pressures above design pressure
Single gasket joint every 12’ subject to leaks with movement of
ground
Vertically pulled (angled) joints that pull further apart and leak with
young bay mud movement or vertical loading
Hydrogen sulfide gas from sewage eating away at unlined pipe concrete and steel decaying
Leaks would spill into commercial and residential areas – potential
damage associated with spill is very high
Upstream sewer system improvements (to reduce upstream spills)
will increase flows and therefore pressures in force main
Events that increase likelihood of leaks:
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Earthquakes
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Surge from sudden power outage
Likely scenarios for leaks: Earthquakes
 Earthquake induced young bay mud movement
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Earthquake causes young bay mud to move as earthquake
forces act on loads near pipe
Pipe joints move when young bay mud moves
Single gasket joints open and concrete pipe at joints crack
Sewage then leaks to surface in commercial or residential
area
Standard response when a leak is noticed is to turn on the
SBSA influent pumps to reduce pressures in the pipe
Turning on SBSA influent pumps does not stop the leak
during high flow events when the influent pumps are
already on – the combination of booster pumps and influent
pumps are needed to carry high flows
Likely scenarios for leaks: Power Outage
 Sudden surge caused by power outage during storm event
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During storm events with high flow a sudden power outage may occur
Pumps suddenly turn off when power cut (before generator can come on)
Flow continues to move down pipe with no new flow following creating a
vacuum in the pipe pulling the decaying concrete pipe inward
The vacuum then reverses the sewage flow and sewage flow slams back into
the pump station
Extremely high outward pressure is created which bursts the decaying pipe
resulting in a sewage spill (the pipe in the worst condition has been replaced)
During a storm event (with huge sewage flows) the normal process to
address a leak of turning on the influent pumps at the treatment plant will
not be enough to prevent an ongoing spill (the influent pumps will likely
already be on)
The spill will continue through the break in the pipe or if the pumps are
turned down the spill will occur upstream in the system as flows back up
Proposed Pipe: HDPE
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High density polyethylene pipe
At least 3 inches thick
Continuous pipe with no joints –
pieces are fusion welded to each other
Not affected by corrosion
Pipe sections when accidently crushed
flat go back to original shape with no
cracking or failure
Pipe sections pressure tested to failure
reach five times the design pressure
before getting a small hole (no
catastrophic failures)
Most impacts from digging equipment
to outside glance off pipe
Risk of cutting into pipe very low when
pipe is in a location with minimal
construction activity (like a levee)
Break after surge due to power outage
Leak near Governors Bay Dr. – leaked at joint
which was broken – took weeks to fix
Leak near Governors Bay Dr. – more