Work on the natural swimming pool began in October 2019 under the supervision of Anton Duivestein of Walker Bay Construction. An unusual element of our project was that the area had already been excavated when the old dam was dug out. Normally, when building a swimming pool, one can dig out the shape of the pool with a shallow and deep end and build the base on already compacted ground. For us to create the same, we had to build the walls up from the base and then fill around with the soil from the old dam wall. But this left a challenge of how to best create a shallow and deep end. One option would be to use hardcore sub-base under the shallow end, but if not compacted very well this could result in subsidence and cracking down the line. Therefore, we decided to dig out further under the deep end, with the shallow end then sitting on the current base of the old dam, which was already firm and would not subside.
Once the landscaping of the base had been achieved it was time to mark out the outline of the pool, protect it with a concrete blinding. Construction of the steel frame for the concrete slab and walls could then begin on top of this. This was all going to plan until we received over 100mm of rain in 48 hours. The old dam, which had never been very good at holding water before, was inundated and most of the prep work disappeared beneath very muddy water (an omen of what was to come). It was however helpful to see that the levels had been done accurately. There was fortunately no lasting damage and once the dam was pumped out and the framework finished, the concrete slab was ready to be cast. Two days later the dam floor was dry enough for the concrete lorries to arrive. As mentioned before, this had been a big concern: whether these lorries weighing several tonnes when full of concrete would make it up our hill or into the old dam to get close enough to where the slab was to be cast. But in the end it all went smoothly and 5 lorries came and went in orderly fashion without any hitches.
It was then time to start building the walls. Due to our desire to minimise concrete use, our builder had settled upon creating a double skin wall of bricks and then filling the cavity with concrete to provide the strength. All concrete was mixed with Penetron to improve strength and increase the waterproofing. The brick walls were also coated on the outside with Cemflex to improve adhesion of the concrete screed and waterproofing. Both these were probably beyond what was necessary to ensure the pool held the water but we considered it better to err on the side of caution than trying to fix afterwards. To date there has certainly been no indication of water loss from the swimming pool except for evaporation. The final element of the pool was an aesthetic coloured Hydroseal layer. We chose the colour “Sandstone”, pale brownish yellow, which fitted with the surrounding shale soil. It would also help to disguise any natural coloration of the water due to tannins.
With the main pool area constructed it was then time to fit together the pipework, pump and electrics. When designing the pool, we had envisaged the pump chamber to be connected directly to the filtration pond and to fill naturally with water from a large pipe. A submersible pump would then be used to pump water into the pool. It soon became clear however that our builder and us had been talking at cross-purposes and that he envisaged the pump chamber to contain a normal swimming pool pump and for the water to be drawn into a separate filter box within the pump chamber. By the time we realised that we had been working towards different ends we were too far down the line. So water is drawn into the pump chamber to a separate filter box that contains a series of meshes to remove any coarse organic matter. It is then pumped up into the pool so that the pool level remains constant regardless of how much the filtration pond level drops. This is a key feature of the design, in that it enables us to allow a 30cm difference or more in water level in the filtration pond before any topping up needs to be done. Our set up works very well, but I still wonder if the submersible pump idea would have worked as well. Everyone presumes that an ordinary swimming pool pump is needed but they are generally noisier and more expensive (we use a 1.1KW pump currently running 8 hours a day).
The final step of the construction was the decking around two sides of the pool. This late addition to the design proposed by our builder has proved inspired. It is evident from watching use of the pool that this is the preferred side from which to enter the pool. It also gives a lovely area to sit and dry off while being able to view the pond life in and flying over the filtration pond.
More information on the design and construction of our pool can be found in these posts: