The basic grouting procedure included: drilling 25 mm diameter holes through the floor slab at a regular spacing of 2.5 m; pumping grout under minimal pressure into one hole; tracking from which adjacent hole, if any, grout emerged after it had travelled laterally through a void and filled it up; and, plugging holes with wood dowels once clean grout emerged.
Each hole was filled systematically and volumes of pumped grout were recorded. Grout refusal at any hole indicated a lack of voids at that specific location.
The primary focus during the grouting operation was to prevent the floor slab from heaving while the grout was being pumped. The floor is a two-way slab on grade with a 5 m span and a thickness of 225 mm. As-built drawings and historical construction photos indicated that only minimal “temperature” reinforcement exists within the slab. This means that there is virtually no upward bending capacity in the slab.
Pressure was limited to 5 kPa, which is equal to the weight of the slab. This pressure constraint posed significant challenges for the sub-contractor to accurately measure low pressures and to utilize a grout mix with sufficient fluidity to be pumped an average distance of 2.5 m under 5 kPa.
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Throughout the grouting operation, the sub-contractor would continually read and throttle the flow rate and pressure as required, to prevent the floor from heaving and cracking. A safe flow rate and uplift pressure had been calculated and validated in the field prior to initiating full mobilization. With the grout return line, the volume of wasted grout was minimized and the risk of over-pressurizing the slab was significantly reduced.
The recommended grout mix consisted of 120 litres of water, 80 kg of Portland cement, and 5 kg of Bentonite. The target Marsh Funnel time was 35 to 45 seconds, the target specific gravity was 1.4, and the target compressive strength at 28 days was 5 MPa. These properties ensured a dense yet fluid grout that was able to displace water and flow under low pressures. Field testing included Marsh Cone tests, Mud Balance tests, and taking multiple grout cubes to ensure that the targeted properties were achieved.
Ultimately, the majority of voids were located and filled by “chasing” them, like a detective following clues. Over a period of three weeks, a total of 14 m³ of grout was used, with 11 m³ directly pumped into voids and 3 m³ used for quality control, pressure line loss tests, or waste at the end of each day.
Long-term repair plan
The scope of work completed to date is considered short-term, as additional long-term repairs are required to further extend the lifespan of the 1920s reservoir. Long-term repairs will be completed once the new reservoir is in service and the old reservoir is completely decommissioned and dewatered. At that point, a final visual condition assessment of the entire reservoir, including the interiors of the influent and effluent chambers, will be undertaken.
Some long-term repairs include rehabilitating deteriorated walls and columns. This work will include high pressure water blasting, followed by finishing with a suitable patching repair mortar and cementitious waterproofing. Some columns will be jacketed with concrete to ensure that chlorinated water can no longer attack the segregated concrete or the exposed reinforcing steel.
Conclusions
Due to the quick thinking of the Peterborough Utilities Commission, and the collective experience of the operators, contractors and the consultant, the short-term rehabilitation efforts were successful. The old reservoir was recommissioned and is currently in operation while the new reservoir is nearing completion.
Although not all of the leaks were eliminated, structural stability has been restored to the old reservoir and there is no longer a threat of lost time on the new construction site due to flooding. It is anticipated that, after long-term repairs are completed, the 1920s reservoir will continue to successfully service the needs of PUC and the citizens of Peterborough for at least another 30 years.
Mark Bruder, P.Eng., and Matthew McGrath, EIT, are with R.V. Anderson Associates Limited. This article appeared in ES&E’s July/August 2015 issue.
