Water was also found to be leaking into the reservoir from the influent and effluent chambers via the slab-wall interface, due to the absence of a waterstop in the joint. Although these leaks were not a water quality or safety concern, they resulted in 100 mm of standing water across the CCT#1 and CW#3. In addition, a thin layer of alum residue was present across the floor and within cracks. This made it challenging to identify the size and location of cracks during the visual condition assessment.
Despite the noted deficiencies, the reservoir was deemed to be in repairable condition. Some of the deficiencies required immediate, short-term repairs prior to recommissioning. Others were considered to be long-term repairs that could be postponed until after the new reservoir was commissioned.
Short-term repair plan
Due to the presence of voids underneath the floor slab, the structural integrity of the reservoir could be reduced. Therefore, short-term repairs included sealing floor cracks with polyurethane injection and filling voids with concrete grout. This would strengthen the foundations, improve stability, help to reduce leakage, and permit temporary recommissioning of the reservoir.
Due to the extremely cold temperatures in January and February 2015, sub-contractors were constantly exposed to blistering winds and temperatures well below -20ºC. This was coupled with the threat of frazzle ice in the Otonabee River blocking up the raw water intake and reducing flows into the plant. Thus, it was critical to rapidly complete the short-term repairs while maintaining operation of the water treatment plant and ensuring the health and safety of the workforce. This was accomplished with confined space procedures, constant communication between all parties, and strong quality assurance coordination.
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Staging of repair efforts was necessary, since crack injection required removal of all standing water. Floor cracks had to be sealed to keep the grout within the voids. Although local standing water could be cleared with squeegees, pressurized water leaking from the influent chamber necessitated the construction of sandbag walls wrapped with watertight polyethylene sheets. Once contained, this water was then pumped to a sump inside the reservoir.
Potable water dive
A potable water dive in the influent and effluent chambers was completed to help locate and repair (on the positive pressure side) any cracks through which water was leaking into the reservoir. This was done primarily to help reduce dewatering efforts inside the reservoir prior to crack injection.
The patching material was hydraulic cement, a one-component, cement-based, NSF certified, fast-setting waterstop mortar. The procedure involved wedging hand-sized balls of cement into large cracks. The high density of the cement would inhibit washout and its hydraulic properties initiate curing immediately upon contact with water.
Plant operators arranged a four-hour window each day where the flow through the influent and effluent chambers would be stopped for the safety of the diver. Diving and logistical operations were contained in a small truck parked nearby. Clean surfaces were laid out across the ground and all equipment was disinfected prior to the dive. The thick neoprene diving suit consisted of lifelines, a safety harness, and audio/video recording equipment. The diver also brought along a ruler for measuring crack widths, and a ribbon to track flows through the floor slab or wall-slab interface.
Despite the repair efforts by the dive team over two days, cracks inside the influent and effluent chambers were not fully sealed by the hydraulic cement. This was due to large flows at the crack interface and visual impairment of the diver during patching, as the water became murky upon first contact with the hydraulic cement. Ultimately, in the short-term, patching these cracks was purely to reduce dewatering efforts in the CCT#1 and CW#3. It was not critical to recommissioning the reservoir.
