Abstract: A vertical drainage element is constructed and laid in position while at the same time forming a narrow vertical channel in the ground by means of an excavating machine or trencher by performing the following successive operations:a strip of filtering material is unwound from a rotating roll of strip behind the trencher;the strip is introdued within the channel formed in the ground while being shaped in the form of a trough, the bottom of the trough being placed at the bottom of the channel while the side walls of the trough are placed against the channel walls;the top edges of the trough are held in position one against the other while filling the trough with bulk material in granular form which subsequently constitutes a vertical filtration block;the top edges of the trough of filtering material are closed on top of the completed block and are fastened together so as to form a closed jacket around the block.