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Garden railway mark III: digging the pond

Last modified: Fri Aug 3 08:52:26 2007

Well, I say `pond', but perhaps `puddle' would be a better word. However, I was determined to have a pond large enough to hold water without it all evaporating away on hot days, and large enough for a train to cross it on a bridge.

Max and Bryony stamping the bottom of the pond smooth, in preparation for laying the liner. The liner itself is warming up on the lawn

So I settled for a pond/puddle of about 3 feet by 5 feet, and about one foot deep, lined with 4'' x 4'' paving blocks. Bigger blocks would have been better, in that they would have been able to stand a person's weight without moving, but there wasn't room.
      As I learned from our previous pond-building efforts, there's no point trying to save money on the pond liner. Even sturdy-looking plastic materials degrade in strong sunlight, and eventually leak. So I had to clench my teeth and pay for a proprietary liner. Ouch.
      I wanted to have the edging stones flush with the top of the soil, not sitting on the soil; otherwise this would have required the track to be raised by the height of the edging stones. So I started by marking out the shape of the pond, then digging out a channel in that shape just deep enough to sink the edging stones. With all the edging stones sunk in the soil, I then dug out the rest of the pond shape inside the edging stones. Received wisdom is that a pond should slope down gradually, rather than having straight sides and a flat bottom like a bathtup. The reason for this is that the bathtub shape causes high pressures to be exerted on the sides of the pond when the water freezes. However, if you intend to support a railway bridge on the bottom of the pond, you'll have to compromise a bit on this.
      So I now had a hole, lined with edging stones whose tops were flush with the top of the soil. Of course, the liner has to go under the edging stones, not over it, so the next step was to remove the stones and stack them neatly out of the way. This left a shelf around the pond the same height as the stones. It is absolutely crucial that the shelf on which the edging stones will set is exactly, perfectly level. If it isn't, you're going to see the plastic liner under the edging stones (or you'll have to increase the water level until some of the edging stones are underwater).
      A good quality pond liner is very rigid, and it's easier to lay if it's warmed up a bit, rather than taken straight out of its packing and thrown in the hole. After half an hour to warm up, I put the liner over the hole, and gradually filled it with water. As the pond shape fills up, it gradually pulls the liner into the correct shape. I was then able to push the edging stones down into their recess, on top of the liner, and mortar them together.

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