I believe it was Henry Ford who said of his early cars: ‘You can have any colour you like so long as it is black’. Until quite recently a similar phrase might well have been used regarding pool-making materials. It was a question of using whatever material you liked so long as it was concrete. But we have moved a long way since then. A number of alternative materials are now used for this purpose, and very efficient most of them are, too.
Making a successful pool depends, after all, on the solution of a simple problem – how to give a hole in the ground a waterproof lining – and it has to be admitted that for this purpose concrete never was very reliable. In spite of its apparent solidity and permanence a concrete pool, however carefully made, all too often demonstrates a fatal defect. It just doesn’t hold water. It may leak away overnight, at the very first filling, simply because the mix was wrong and the walls are porous. Or it may be splendidly watertight at first and then comes a severe winter, a heavy ice layer, expansion, contraction. Expansion and suddenly – no water. It might on the other hand survive the worst winter can do, but one summer the ground dries, cracks and subsides and suddenly – no water. Some concrete pools, massively and carefully constructed, do go on for a long time, but the percentage is not high simply because the rigid concrete shell lacks tensile strength and so readily cracks under the stresses imposed by ice pressure or soil movement.
To make a successful concrete pool requires careful attention to detail, and the first essential is a thoroughly rammed hardcore foundation 3 or 4 in. thick. The excavation must be considerably larger than the size of the finished pool to allow not only for the foundation but for a minimum thickness of 5 in. of concrete and rendering on both walls and floor. Amateurs tend to make the floor thickest, and to taper off the walls as they get to the top. Solid strength – and the firm support of the soil behind it -is even more important at the rim (which bears the brunt of ice pressure) than at the base. The ingredients (3 parts coarse aggregate, 2 parts sand and 1 part cement, by volume) must be measured accurately, and very thoroughly mixed with water to a firm, even consistency.
The easiest pool to make with concrete – and there are a lot of them about – is saucer shaped, because it avoids the use of shuttering. You simply form a shallow depression, line it with chicken wire, and lay-on the concrete all at one go. Simple but not satisfactory because, as we have seen, a saucer is in-herently a bad pool shape. To make a concrete pool of the desirable shape we have specified, with level floor, steeply sloped sides, and marginal shelves, is far from simple. In practice it is necessarily achieved in stages. First, 4 in. of concrete to form the floor: then, with timber and hardboard shuttering to keep the concrete inuntil it has set, the walls and shelves are added. This must be done reasonably quickly to avoid the leaks that will result if one stage dries out too much for the next to key into it. When the 4-in. Floor and walls are set, apply a rendering coat (3 parts sharp sand to 1 part cement) 1 in. thick. Waterproofing powder may be added, although it must be said that if the mix is sound it is unnecessary. And if it isn’t waterproofing powder won’t make up for it.
Concrete exudes free lime, from which plants and fish must be protected. This can be done by filling the pool, leaving it for at least a week, and emptying it. When this process has been repeated not less than three times, the pool should be safe. A quicker method is to paint the concrete with a colourless neutralising agent such as Silglaze. Permanganate of potash, frequently recommended, is useless in this context.
PLASTICS AND OTHER MATERIAL
I have described the essentials for success with concrete because some gardeners feel that it provides a better finish and a more natural appearance than can be achieved with plastics and, undaunted by the back-breaking labour involved, they may wish to persevere with this traditional material in spite of its undoubted drawbacks. Most gardeners, I am sure, will prefer to make use of one of the plastic materials which have revolutionised pool-making techniques and taken so much of the hard work out of water garden construction.
Not all of it, of course. There is still a hole to be dug, but at least it is only the size of the pool required, without the extra allowance for a hardcore foundation and the thickness of walls and floor as in the case of concrete. And when the hole is dug, that’s an end to the hard labour. With plastics the lining of the hole is so quick and so simple that it is no work at all. It can be lined and filled in no time, and stocked immediately because no treatment or seasoning is necessary. Plastic pools have enough resilience to absorb without harm the kind of stresses that are fatally damaging to concrete. Some of them can certainly be damaged by the sort of accidental violence that would make no impression at all on concrete – such as falling into the pool and trying to save yourself with the garden fork you happened to be carrying at the time. But most plastics can be quickly and completely repaired, which is more than can be said for most leaking concrete pools.
I have referred to ‘some’ and ‘most’ plastics, from which it will be clear that there are several kinds and that they have different characteristics. To assess their qualities, and the differences in installation technique, they had better be considered one at a time.
Glassfibre pools are made by bonding glassfibre with resins on a mould. The finished product is rigid, extremely strong, and its shape is, of course, determined by the shape of the mould.
Of the wide variety of shapes on the market a number appear to have been designed with very little understanding of the needs of the plants and fish they are intended to house, having quite inadequate
depth and volume, and no sensible marginal shelves. Of those which have the desirable features of at least 15 in. depth, steeply sloping sides, and some shelf area about 8 in. wide and 9 in. deep, not many have the size to make a garden feature of any significance. However, for intimate corners and wherever the garden scale is on the small side, the better glassfibre designs can be used to good effect. The rectangular and circular shapes are particularly suitable for. Glassfibre pools are extremely durable and have no disadvantages except size limitation and their high cost relative to other materials.
To anyone toying with the idea of arming himself with glassfibre mat, resins, catalyst and pigments for a major do-it-yourself glassfibre project my advice would be- please reconsider. Unless you are already practised in the considerable skills involved in making glassfibre laminates you have very little chance of finishing up with a sound waterproof pool.
To install a glassfibre pool, dig a hole of the required depth and a few inches larger all round than the pool size. Firm the bottom, remove stones, and place the pool in position. Fill in round the sides, packing the soil very firmly, particularly under the shelves, and checking the top of the pool with a spirit level at intervals so that when filling is complete the top is, in all directions, absolutely level. This level checking is vital in all pool making, whatever the material.
There are some small pre-shaped pools on the market which are much lighter and have an altogether flimsier feel than glassfibre. They are moulded from semirigid sheets of such plastics as PVC and polythene, and they are designed to be as cheap as possible. All are deficient in depth and volume, and most are too small to be useful as anything but bird baths. They are certainly inexpensive and could be the answer if the children want a pool of their own for keeping newts and tadpoles. The installation technique is as for glassfibre pools.