outer column as shown in the plan view of fig7 optionally the heads of the second shorter columns 3 may be provided with additionnal and individual elevating means such as jack 26 shown in fig2 for a more specific adjustment of the level of the tiers supported by the second columns through said jack so as to obtain a further slope profile adapted to specific needs it is emphasized that the difference of altitude for the same tier between the supporting head of the second shorter columns 3 and the head of the first longer columns 2 in the inactive position of the second columns increases from the lowest tier towards the upper one i e from left to right in fig5 this difference being a consequence of the difference in the slope profile determined by the first columns and by the second columns when they are successively put in active position when the elevating means are actuated the head of each second column 3 of the lowest tier comes first into contact with the said tier and accordingly the weight of the said tier alone is added to the raising effort of the elevating means and only after a moment the second columns of the adjacent tier arrives in its turn into contact and.