A number of great challenges
Despite the success of the grand opening, no project is without challenges, and this installation was no exception.
For instance, the main challenge was integrating the new expansion as architecturally seamless as possible without disrupting any of the ongoing activities or diminishing the community’s enjoyment of the facility. This meant the facility’s existing 65-m pool, complete with bulkheads, dive tower, and a water slide—which transitions from indoor to outdoor—needed to remain open during construction.
The existing sanitary sewer system was also a consideration in constructing the new pools as the backwash volume and flow rate would exceed the capacity of the system if sand filters were installed. Therefore, to reduce the amount of backwash water, the new pools were designed with regenerative diatomaceous earth (DE) filter systems, which provide superior filtration and water clarity while using a fraction of the backwash water required for sand filters. These filters, along with the implementation of a backwash water holding tank, allow the discharged water outflow to be regulated, minimizing the load on the existing sanitary sewer system, and negating the need to install a larger sanitary system.
Further, as soil conditions are always a major consideration for pool designers/builders, soil engineers were consulted by the project’s general contractor, Ball Construction (Kitchener, Ont.), at the onset of the project to perform bores for a snapshot of the site’s average conditions. The resulting reports indicated very fine saturated sand and a high water table, making it clear from the get-go this was not going to be a typical, straightforward build.
After obtaining a dewatering permit, site wells were installed below the excavation elevation at intervals of approximately 2 m (6.5 ft) around the perimeter of the excavation. Dewatering pumps operated 24-7 to evacuate the area to allow construction to progress. Upon excavation, the soils investigation proved accurate regarding the nature of the material as the sand substrate had no cohesion and flowed like sand in an hourglass. With any rainfall, it would flow like lava to the deepest parts of the pools.
Ball Construction performed the excavation, which required the footprint of the dig to be expanded to compensate for the low angle of repose established by the sand. In order to construct the pool floor, which had areas of slope up to 27.5 per cent, Acapulco Pools needed to install a mud bed of lean fill over the excavated slopes to allow workers to move around the excavation effectively. This also prevented the pool’s sloped areas from eroding when it rained. A footing also had to be poured to provide a suitable base to form the pool walls as the loose sand substrate would not support any formwork, making it impossible to retain a set elevation in such a ‘fluid’ material.
Finally, the lap pool’s design called for an accessible tunnel to be installed around the entire perimeter, which would allow all of the pool/drain piping and heating, ventilation and air conditioning (HVAC) ducts to be inspected. It would also eliminate the possibility of moving seals, which can damage buried pool piping.
Therefore, the design needed to account for the high water table around the pool deck. Both the pool and the accessible tunnel needed to be protected from leaking and hydraulic lift. To do this, hydrostatic pressure relief valves and collection tubes were installed in the main drains to allow excessive external water pressure to be relieved into the pool.
Hydrostats work great, but the truth is they are not an absolute assurance the pool will never float. This was a real concern for the new pools being installed given the known water table condition.
To combat this, Halcrow Yolles designed portions of the pool floor to be sufficiently massive (almost 1 m [3.3 ft] thick), making the pool shell heavier than the water it would displace. Further, with this ballast in place, it is impossible for the empty pool to become buoyant in the surrounding ground water even if the hydrostatic relief valves became plugged.
When there is so much water around a structure, corrosion of the reinforcing steel is also a concern; therefore, a C-1 concrete mix was provided to offer corrosion resistance. Sufficient and well-placed water stops and concrete coverage over the steel was also particularly important in this scenario. The specified C-1 concrete mix design for the pool tank included a liquid corrosion inhibitor admixture, which is added during the batching process. It chemically inhibits the corrosive action of chlorides on reinforcing steel in concrete. The corrosion inhibitor is also known to act as a concrete set accelerator, which aggravated the slump and made it more difficult to place the concrete. A transit time of 45 minutes to an hour from the batching plant to the site made the concrete difficult to pump, place, and finish, thus providing no forgiveness for mistakes or moving too slowly.
Further, as the concrete swimming pool vessels were constructed during the hot summer months, Acapulco Pools provided continuous water curing of the concrete to prevent it from cracking. To do this, the concrete was covered with burlap, which soaked up the water to keep the pool’s walls and floors constantly wet. In some cases, the concrete mix design needed to be changed and the concrete and substrate had to be protected via use of tarps and water to keep some moisture on the structural steel to prevent it from overheating. Finally, as the concrete’s set time was much shorter, more labour was required to pour a slab than originally anticipated.
Wind was another factor as it can dry out the concrete’s surface along with wreaking havoc on layout strings and tarps. Snow removal also presented a challenge as the concrete needed to be insulated to allow it to cure properly.
As is often the case with tunnel pools, a surface-applied crystalline waterproofing was specified for the project. The chemicals present in the penetrating cementitious material react with moisture and free lime in the concrete, creating insoluble crystalline complexes, which seal the capillaries and minor shrinkage cracks in the concrete. Once the crystalline is cured, it must be completely removed from the surface for thin set tile applications as it acts as a release agent causing tile delamination. Typically, the surface is blasted to a concrete surface profile (CSP) between three to five (i.e. a light to medium-heavy shotblast) for tile installation. This is a fairly aggressive texture where the aggregate is significantly exposed. The corrosion inhibitor added to the concrete is also known to promote strength development in the concrete. The specified C-1, 35 MPa mix design has 28-day cylinder test strength results of up to 68.9 MPa. This remarkable high strength made concrete blasting more difficult as well as more time consuming than it would normally take to achieve the proper CSP. To speed up this process, Acapulco Pools used a 40,000 PSI hydro blaster.
