Case Studies

Main Street Pumping Station

Town of Halton Hills (Georgetown)

Dealing with Ground Water

This project, which incorporated external sewer and forcemain work, in addition to a state of the art pumping station, was an attractive project for Con-Drain. The Region of Halton awarded the project to Con-Drain in April of 1999 and construction soon began under the supervision of MacViro Consultants.

The construction was separated into two parts external sewers and the pumping station structure. The external sewer work was comprised of a 525 mm overflow sewer which ran approximately 300 m from the station to the existing Silver Creek Truck Sewer, a 400 mm forcemain and 150 mm watermain approximately 170 m in length as well as a 250 mm temporary bypass sewer. All three sections were to be done at different intervals. The pumping station structure was comprised of a wet/dry well approximately 11.0 m x 11.0 m x 10.0 m deep with a series of buried columns to support the control building above which had a footprint of approximately 13.5 m x 15.5 m. The total value of the project was 2.3 million.

Construction of the structure and the external work was scheduled to commence in early May but was delayed due to a MOE moratorium on dewatering. During this period the site was cleared and all rough grading was completed until the moratorium was lifted in July. At this point two deep wells were constructed in order to conduct a drawdown test to establish the total of amount of dewatering required to open cut the wet/dry well excavation. Work on the external sewers commenced on the east side of Main Street and Black Creek, which divided the site.

Based on the soils information and the ground condition encountered Atlas Dewatering was employed to install a shallow dewatering system around the proposed jacking pit for the creek crossing as well as a portion of the 525 mm overflow sewer. James Elliot Construction was employed to install the 900 mm steel casing under the Main Street and Black Creek over a distance of 40 meters. The casing was originally to be installed by conventional jacking and boring machine but was changed to jacking the casing using a pneumatic hammer (brought in from Texas including a Texan to run the machine) over the entire length. The casing was then to be excavated by hand and backpressure in which the soil inside the casing was pushed out using air pressure. The reason for the change in methodology was the concern that due to the poor ground conditions and the high water table, the creek bottom might have ended up in our jacking shaft. This was our first encounter with what we were to discover was a very wet site.

During the tunnel operation a small crew proceeded with the installation of the 525 mm overflow sewer. The alignment went through an old landfill site to the existing Silver Creek Truck Sewer where we were to add a control manhole and various slide gates to control the flow for future diversion to the pumping station. All the work was to be done with out interruption of flow to the truck sewer. In order to accomplish this, a pressure by-pass was installed in which a 375 mm forcemain was connected adjacent to the existing 600 mm sewer at spring line, upstream of our connection point and 3 m downstream of the new manhole. The 600 mm sewer was plugged and the sewage backed up and around the bypass the surcharge was contained in a cast in place structure around the pipe. The system worked with no interruption to the sewer and we were able to work in a dry clean environment.

During this time our drawdown tests on the west side of the site were completed and a report was prepared by Morrison Environmental Limited which identified the volume of water required to be pumped and the influence on the surrounding water table. The report identified that groundwater was much worse than estimated in the previous soils reports. It was estimated that approximately 2.0 million gallons a day would have to be pumped in order for the open cut excavation to take place. The amount of pumping was to have significant impact on the surrounding water table in particular Black Creek and its fisheries habitat. We now had to re-evaluate the construction methodology of the wet/dry well, with time also becoming a factor since the Ministry would not allow dewatering past September into the fall spawning period.

The solution presented to, and accepted by the Region was to construct a sea wall around the perimeter of the wet/dry well to a depth of 13 m using interlocking sheet piles, which were installed by Shore-Cais Construction. The interior of the sheet pile enclosure was then to be excavated to a depth of 3 m below the underside of the slab. This lower section of the excavation was to be filled with concrete to balance the water pressure on the other side of the wall. Despite the above, dewatering was still required to ensure that the uplifting forces did not affect the construction of the base and the walls. Five deep well in total were used around the perimeter of the station to each pump removing approximately 100 gallons per min for a total of 720 thousand gallons a day. The ground water was collected and transported along a pipe system to a settling pool to control sediment and warm the water before discharging to Black Creek, all of which was done under the monitoring of Morrison Environmental Limited and Dance Environmental Inc. We also took advantage of the dewatering to install all the deep sewers around the pumping station.

After the September cut off, the pumps were shut down and the ground water level around the wet/dry well returned to its static level of 3m below finish grade or in this case 1 m below our current pre-cut level. At this point the water pressure on the sea wall was evident by the leaks springing through most of the joints. The problem was that the soil did not have enough fine silty material to migrate into the joints to stop the leaks. Construction of the cast in place structure, by Toronto Zenith, could not continue under these conditions. A special meeting was called with the Department of Fisheries and Oceans and the Region to allow us special permission to extend our pumping period into October. Upon review of our dewatering system and monitoring in place a special permit was granted and the construction of the wet/dry well was completed. Construction proceeded into the winter with little incident compared to the chaos the groundwater caused.
Completion of the internal walls and suspended floor for the HVAC room proved to be more fun than was expected which was evident by the spider web of rebar and supports required to hang the forms.

Work on the above grade building by Rumber Construction began in early January, which comprised of a concrete block building with exterior brick and architectural block finish. The ultimate goal was to create a structure that would fit into the surrounding future residential development with out being an eye soar to the residents.

Completion of the internal mechanical by Leader Plumbing and electrical components by Lexsan Electrical went well with the normal number of glitches due to faulty equipment and delayed deliveries. Once the pumps and the discharge header were installed, the station could truly be appreciated for its size.

Despite the delays and the troubles encounter all parties were pleased with the final product, the result of the teamwork of The Region of Halton, MacViro and Con-Drain.