| Most plenum failures I know of, were not attributed to the gravel being to large.
More the other and to fine was used, clogging down the system. Remember most of the older methods, used courser on the bottom and finer on the top, something not practiced by Jaubert. Also the usual failures because of the plenum design itself and in other cases, the bed being smothered by rock.
Bu two screen layers, I thought it was on the plenum. Your correct in the two screens, one on the plenum and one half way up. However a problem with that method, is as the gravel dissolves, more is added. Would not the gravel between the screens dissolving, drop the top screen down towards the bottom screen and further from the surface, as new gravel is added.
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Doug
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