Newsletter - July 1999
Trunk sewer and pumping station improvements begin with new Stoneybrook Trunk Sanitary
Sewer
The impact on the immediate and surrounding ecosystem when construction is proposed
near a waterway, is always in question, especially where construction of a new sanitary
trunk sewer is planned.
The Stoneybrook Pumping Station in the City of London was originally constructed in the
1960s, and upgraded in the late 1980s. Existing development in the drainage area put the
City in the position of having to consider development freezes on large parcels because of
limited pumping station capacity. Further expansion of the system was neither a
cost-effective nor an environmentally acceptable option.
The preferred solution was to construct a new pumping station with adequate capacity
for all future developments in the watershed. The proposed pumping station would be
located on the south side of the Thames River, within the confines of the existing
Adelaide Pollution Control Plant. Flows from the existing pumping station would be
directed to the new location by means of a new gravity sewer crossing under the North
Branch of the Thames River. The need to transmit sewage under the river in a pressurized
asbestos cement pipe some thirty years old, would be eliminated.
The City of London appointed Proctor & Redfern Limited as consulting engineer for
the pre-design of the Adelaide Pumping Station, and the pre-design, design and
construction supervision of the Stoneybrook Trunk Sanitary Sewer. The project was funded
under the Canada Ontario Infrastructure Works (COIW) program.
The Upper Thames River Conservation Authority (UTRCA) and the Ministry of Natural
Resources had imposed restrictions on the actual river crossing. The construction work in
the river would only be allowed in the winter months or during the month of July. An
assessment was made of the costs and benefits of undertaking winter work considering
anticipated hardships and difficulties usually encountered during these months. It was
decided that an early spring tender call indicating the time restrictions for the river
crossing would be the most cost-effective approach, and provide the opportunity for a
higher quality installation.
Various pipe materials were considered before and during the tendering period. The City
expressed a firm requirement that the new gravity sewer should be free from both
infiltration and exfiltration. Since the new pipe would be located below the water table
and under the river, the City's concern with protecting the environment was justified.
Reinforced concrete pipe was specified and installed, based on assurances that the
existing specifications for this pipe, and its past performance history, made it quite
capable of meeting the required performance specifications. The additional cost associated
with alternate pipe systems could not be justified.
Elgin Construction commenced the work on the south side of the river early in June with
the stripping of topsoil, while Con Cast Pipe began delivery of 632 m of 1,500 mm 100-D
pipe. Because of the high water table, installation of a single line of wellpoints by
Double Diamond Construction was completed to the west of the centre line of construction.
This de-watering system was augmented with sumps and submersible pumps which provided the
required conditions to allow for proper pipe installation.
Before pipe laying reached the river crossing, it was apparent that the required
approvals from the UTRCA would not be in place to permit continuous laying of the pipe
across the river. Elgin Construction decided to move construction to the north side of the
river and continue laying pipe to Windermere Road. To facilitate this change in schedule,
a 2,400-mm diameter precast maintenance hole was substituted for the maintenance hole tee
on the north side of the river. This allowed the contractor to proceed without incurring
any downtime. Construction of the 1,500 mm sanitary pipe to be laid beneath the river bed
would commence when approval was granted, and then be connected to the maintenance hole.
Installation of the sewer continued to Windermere Road where a precast concrete chamber
had been proposed by Elgin as an alternative to the specified cast-in-place structure. The
design and construction of a 4,500 mm x 3,000 mm precast concrete chamber was reviewed and
approved by Proctor and Redfern. Consequently, traffic disruption was minimized and the
length of time required to bypass sewage flows was substantially reduced. It was estimated
that the installation period for the chamber took half of the time of a cast-in-place
structure.
Near the end of July, verbal approval was granted by the Ministry of Natural Resources
for the river crossing. The coffer dam on the south side of the river was initiated with
the installation of sheet piling, followed by a coffer dam on the north side. The final
connection to the north section of pipe was made at the maintenance hole installed earlier
in the month.
Final testing showed that the installation was sound as there are no visible
indications of infiltration. Following overflows at the existing pumping station, a small
submersible pump is activated in the most southerly downstream maintenance hole on the
grounds of the Adelaide Pollution Control Plant. A manual control panel for this pump was
also included in the contract.
The new 1,500 mm diameter gravity sewer pipe will serve as a storage tank for any
overflow events that may occur during the operation of the old pumping station. Should
this pipe become full then the former overflow to the Medway Creek sewer system will
continue to function.
This approach to the Adelaide Trunk Sewer and Pumping station improvements had several
benefits. The contract allowed the City of London to take advantage of the COIW funding.
It created jobs in 1998 and provided for the installation of the Stoneybrook Trunk
Sanitary Sewer well ahead of the completion of the new pumping station.
By John Orr, C.E.T., City of London, and Adam Polski, C.E.T., Con Cast Pipe
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