Some very good questions here! Firstly on bracing - Andrew there are several possible ways to address this and looking at our plunges it seems over the years that all have been tried and each seems to have been successful. The longer plunges are actually divided into shorter lengths with blocks across the width to tie the two sides together. Others have had metal ties put across, while some have been built slightly sloping inwards so that any pressure is pushing them only back towards vertical. On small plunges of the size most amateurs would build I suspect that the blocks are strong enough not to need any extra bracing, but that's just a personal view.
David asks about lining. We haven't done this - as long as you get the kind of concrete blocks intended for outdoor use they will withstand moisture very well. I suppose you may get some leaching of lime or other salts from the blocks but we haven't observed this affecting the plants at all - I guess that every time we water, any lime etc is washed down into the bottom of the plunge.
Arthur, the sand we use is straightforward washed sharp sand that you can get at any builders merchant. The bottom of the plunges is filled with rubble, broken clay pots and your obligatory kitchen sink. This isn't actually necessary, it just fills some of the large volume more cheaply than using sand all the way down. Having said that, the sand is still pretty deep - our plunges are four blocks high and the rubble only comes up one and a half blocks, the rest is sand. Such deep plunges not only work well from the drainage point of view but mean we can plunge our longest long-tom pots up to the rim with ease. We haven't put a membrane between the rubble and sand but we did take care to wash sand into the voids and compact well at the bottom to avoid subsequent sinking of sand. I guess a membrane would be a good if not better solution.
The new alpine house is now open and I'll reveal all in the next log which should be next Thursday!
