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| Placing ballast material on the top of the Aqueduct. |
New lease on life for Shoal Lake Aqueduct
By Andrew Philip and Don Grandy, CH2M Gore & Storrie Limited
Since 1919, the City of Winnipeg has secured its drinking water from Shoal Lake, located approximately 155 km east of the city at the Manitoba-Ontario border, through a pipeline system known as the Shoal Lake Aqueduct (Aqueduct).
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| Placing ballast material on the top of the Aqueduct. |
The Aqueduct is constructed of unreinforced cast-in-place concrete over most of its length. The cross-section of the Aqueduct is primarily a horseshoe-shaped arch that varies in size depending on the Aqueduct's slope, with a typical height of about 2.1 metres.
As expected, the structure has gradually deteriorated over its service life. First, the CG&S and UMA Engineering Ltd. project team conducted an extensive program for the City of Winnipeg to assess the Aqueduct's condition and determine the necessary rehabilitation measures to extend the operational life for another 50 years. Following the assessment, the repair work began.
The repair program focused on strategies for rehabilitating defective Aqueduct sections identified in the condition-assessment program. The primary focus was on achieving long-term structural stability and minimizing water exfiltration and infiltration. Exfiltration in particular promotes deterioration from sulphate attack, swelling clay pressures, ice lenses, and freeze-thaw cycles.
Repair work was carried out from 1994 to the fall of 1998, and involved ballasting, internal, and external repairs. Further repair work is planned for the winter of 1998 and beyond.
Ballasting repairs
The potential for floatation, or buoyancy, is greatest when the Aqueduct is dewatered for inspections and repairs. The buoyancy assessment program determined where buoyancy problems existed along the Aqueduct and recommended appropriate remedial measures. Investigation revealed that the most economical and practical way to address the buoyancy problem was to install a gravel ballast layer on top of the Aqueduct.
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| Injecting an arch crack. |
Most of these repairs were near the Manitoba-Ontario border where the groundwater level is typically above the top of the Aqueduct.
One of three critical areas was ballasted during the 1997-98 winter months, with work wrapping up just before spring thaw. The contractor met the challenges of winter work by constructing a winter road over the bogs and swamps. The remaining two areas are scheduled for completion over the next two winters.
Internal repairs
Internal repairs were designed to minimize water exfiltration and infiltration. The 15-km stretch initially identified as the most critical and in need of restoration (because of extensive longitudinal cracking and faulty joints) has been repaired. Repairs consist primarily of injecting cracks and construction joints in the arch and invert slab with a flexible polyurethane resin. Other less critical internal repairs will be carried out in other stretches in future years.
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| Shotcreting the external shell. |
External repairs
Locations exhibiting severe arch cracking with distortions and locations where the arch walls were severely deteriorated from sulphate attack were identified for strengthening. All of the critical areas just less than 1.7 km of the Aqueduct have been strengthened by applying an external reinforced shotcrete shell over the existing deteriorated arch. Following the installation of internal shoring and bracing, deteriorated concrete was removed using high-pressure water blasting equipment. Once sound concrete was exposed, a reinforcing steel cage was installed and the shotcreting was completed.