Major dewatering and tunnelling project

A four kilometre expansion of a trunk sewer in Barrie, Ontario, requiring both tunnelling and open cut methods, is nearing completion; sewer diameters range from 825 mm to 1200 mm. The trunk sewer will service new development, while enabling existing subdivisions to get off septic systems. Hewitt's Creek Management Inc. and the City of Barrie shared costs on the $5 million project, with no federal or provincial funding.

Environmental considerations were a major factor in the project which required tunnelling and open cut. Photo - Mark Munro

The Atlas Corporation was the dewatering contractor and a well point system was used to draw down to six metres, with a sewer range between 10-11 metres in depth. Typically, there were 2-3 metres between well points in a staged system (stepped). Due to native soil conditions, well points were run on both sides of the valley to drain off water in some areas.

An Atlas Corporation eductor system was also used to dewater a tunnel running under the road, 70 feet below grade. The whole project had to be dewatered due to the depth of the valley and the close proximity to Lake Simcoe. Andy Ingriselli, The Atlas Corporation, said this was the most challenging project he had seen in years.

Filter sand was installed with well points to prevent removal of sand and sediment with the water, which was further pumped through enviro-bags to filter again before discharging into the existing creek.

Significant efforts to protect existing environmental conditions were applied throughout the project. Trees were replanted in finished areas. Seals on pumps were redesigned and oiled with purity oil (a food oil made by Petro Canada) to avoid any contamination.

Bryan Richardson, P.Eng., President of Richardson Engineering Ltd. was a consultant on the project, and Guy Acri, of Tectonic Infrastructure Inc., was the contractor.

While tunnelling under a creek, soil difficulties were encountered, so the project required an open cut section. When tunnelling under the road at approximately 70 feet deep, conditions made pipe jacking impossible so the contractor had to tunnel the remaining length by rib and lag method. Since 22 m of 1200 mm jacking pipe had already been installed, 825 mm pipe with a reduced bell diameter was manufactured to fit inside the existing pipe and the remainder of the run was installed after tunnelling was completed. Munro Concrete Products responded by constructing a special pipe insert and suppling all of the concrete pipe which ranged from 825 mm to 1200 mm with strengths from 100D to 220D.