35 with water in 43 the admission duct 45 is connected to a water pump 47 the input of which is connected by a duct 49 to a water tank 51 the water tank can consist of the top part of the trench 13 in which the volume of water 51 lt lt floats gt gt over the mud 15 the output of the air vacuum pump 35 is connected to an expelling duct water air 53 which opens in 55 above the water tank 51 in fig4 the drain tubes means 17 23 also comprise third drain tubes 57 disposed in the embankment 5 at a level higher than the mid level 230 of the second drain tubes 23 the third drain tubes 57 are perforated along at least the major portion of their length and extend in the embankments wherein they are substantially horizontally disposed see fig5 and 6 the drain tubes 57 comprise fluid input holes 59 adapted for entering at least air therein preferably the drain tubes 57 are disposed in the upper portion of the granular embankment above the maximal level 27 of the liquid to be expelled viz in the portion lt lt h 2 gt gt of the body 5 in the immediate vicinity of the lateral edge of the embankment 5 the drain tubes 57 are connected to tubes 61 already used in the prior art for expelling air previously those tubes were only engaged on a short distance through the membrane into the embankment and took off air by their opened free end only the tubes 61 are connected to the pump 11 at the upper part thereof above the level of.