Lined reinforced concrete pipe is a first for London, Ontario
By Neil Wyatt
Welding pipe joint at pre-lined MH connection.
To improve the environment of
a London, Ontario, neighbourhood
and solve major structural
problems caused by sulphuric
acid (H2SO4), a 675 mm diameter
concrete sanitary sewer commissioned
in the late 1960s was replaced
with a specially designed 590 metre
precast concrete system. Residents
along Gordon Avenue brought the
problem to the attention of City officials
when they began reporting a
strong odour of rotten eggs, associated
with the presence of hydrogen sulphide
gas (H2S). Evidence of a major
problem with the sanitary sewer line
mounted when homeowners complained
about deteriorating piping in
their houses that needed repair by
licensed plumbers. Inspections of a
major maintenance hole at the intersection
of Commissioners Road and
Gordon Avenue followed, along with
video inspections of sections of the
sanitary trunk sewer. It was clearly evident
that the sanitary sewer had been
aggressively attacked by H2SO4 and
sections were in need of replacement.
The Gordon Avenue trunk sewer
begins near Hooks Restaurant
(Wharncliffe Road and Southdale
Road) where it receives the flow from
the Dingman Pumping Station. From
there, the sewer weaves through a combination
of streets and easements as it
directs flows to the Greenway treatment
plant by the Thames River. Overall, the
length is approximately 4,500 metres,
with pipe sizes from 675 mm to 1200
mm diameter. The 590-metre section of
this project is just the first problem area
that will be addressed by the City over
the coming years.
The line of reinforced concrete sanitary
sewer, installed around 1967, had
been designed to receive effluent from
the Dingman pumping station.
Composition of the effluent was undetermined,
and even today with a sewer
use by-law in place, there are concerns
that aggressive chemicals are entering
the effluent from unknown sources.
Although the concrete pipe had significant
damage to the crown of the pipe,
it was still structurally sound, but too
costly, and almost impossible technically,
to repair. The new sewer was
designed with an increased diameter to
reduce the velocity of the effluent and
the turbulence that contributes to the
production of H2SO4. Designers decided
to replace an aging waterline and
also a storm sewer while Gordon
Avenue was under construction.
The maintenance hole at the intersection
of Commissioners Road and
Gordon Avenue contributed significantly
to the premature deterioration of the sanitary sewer in two ways. First,
the 90-degree bend in flow for the
sewage created a very turbulent situation,
which combined with other environmental
conditions in the sewer to
generate H2SO4. Flows into the structure
were expected to be in the range of
2.5 to 3.5 million gallons per day. In
fact, the flows were found to be in
excess of 6 million gallons per day.
Secondly, the inspections of the
system discovered a 2-inch x 4-inch
timber, about 8 feet long, that appeared
more like a railway tie because of the
build-up of debris on it. The timber
had lodged in the flow-through maintenance
hole which was also creating
turbulence and helping generate
H2SO4. These two factors contributed
to the corrosion in the maintenance
hole and left an unstable situation
which the City discovered as they
entered the maintenance hole. When
lowered into the maintenance hole, the
inspector reached for the ladder rungs
and discovered the reinforcing steel
was more than accessible to his hand.
Considering the level of corrosion, the
City took action to cover the flow with
some piping, and then fill in the maintenance
hole with concrete. One can
speculate that conditions in this maintenance
hole had contributed significantly
to the premature deterioration of
the sanitary sewers in the area.
The solution that engineers devised
to protect the new reinforced concrete
sanitary sewer was to enhance its structural
performance with a liner. This
solution will extend the service life of
the system well into the 22nd century,
barring any further unforeseen
hydraulic or effluent composition
changes to the sewage from the catchment
area or internal environment of
the system. Production of the lined pipe
and installation by the contractor were
carefully planned, engineered, and documented
according to the best available
knowledge. The installation of this
product is a first for the City of London,
and one of only a few in all of Canada.
Lined reinforced concrete pipe is
available in Ontario, and may be used
where special circumstances warrant.
Small diameter reinforced concrete
pipe does not need to be lined to collect
and carry common sanitary sewage.
Hanson worked with the installation
crew of Elgin Construction to devise a
technique for joining the liner in the field with 10-inch flaps covering the
joints, after the sealed joints of the
concrete pipe had been properly
homed. Hanson produced 260 units of
lined 900 mm and 1200 mm diameter
pipe in its Cambridge plant. Liners
were incorporated into the pipe as it
was being produced, and seams welded
in-plant before shipment to the site.
The seams were located at the crown
of each pipe, and Swift Lift anchors
located on the barrel so that when pipe
was lowered into the trench, the seam
location would be consistent along the
crown. Installation crews were then
able to enter the pipe and weld liner
flaps to the liner of the homed pipe to
provide a completely sealed interior
space with little impact on the
hydraulics of the system.
Delcan, the consulting engineering
firm for the project, had to design the
project so that the flows would not be
interrupted until it was time to decommission
the old line and connect the
new. Engineering called for the construction
of a larger diameter parallel
line, complete with lined maintenance
holes, to a location where the new line
would cross over the trench to connect
with the existing sewer, approximately
590 metres upstream from the maintenance
hole at the intersection of
Gordon Avenue and Commissioners
Road. As the new system was being
constructed, a 300 mm reinforced concrete
storm sewer was also installed.
Construction was required along
the existing paved portion of Commissioners
Road at a depth of approximately
7 metres, while maintaining the
flow of through-traffic. A trench box
was required, along with a series of
steel I-beams placed across the top of
the open trench to support the exposed
watermain and utility conduits during
the installation of the sanitary sewer.
On Gordon Avenue, the engineers
designed and placed the new sanitary
sewer along the line of the old storm
sewer, and placed the new storm sewer
along the line of the old sanitary sewer,
ending with a complex crossover of the
sanitary and storm sewers at the
upstream end of the project. The sanitary
sewer maintenance hole at the
crossover has a drop structure built
into the invert to eliminate or reduce
turbulence in the line. The contractor
used a series of two Godwin Pumps to
maintain the flow of the sanitary
sewage as construction proceeded
between maintenance holes.
Engineers redesigned the maintenance
hole at Commissioners Road and
Gordon Avenue so that the benching
directs the inflow to the outflow at a
wider angle, thereby significantly
reducing the opportunity for turbulence
of the sewage to occur. The maintenance
hole is also lined to enhance the
performance of the structure.
The final phase of the project may
be the construction of a biofilter to
pull any H2S gas from the air in the
sewer. The City has decided to monitor
gas emissions in the new line to determine
if a biofilter is necessary.
The $1.7 million contract began in
late April and concluded by the end of
July 2004.
Neil Wyatt is with Hanson Pipe &
Products Canada, Inc., Cambridge,
Ontario.
Contact e-mail: neil.wyatt@hansonamerica.com
See our home page on how to order your subscription. We regret we can
only accept orders from Canada.
