1. In an apparatus for taking an undisturbed soil sample and which includes a sampling tube with a sharp cutting mouth and a supporting tube inside of the sampling tube with some clearance in-between, the improvement comprising: a coupling member disposed above the sampling tube coaxially thereto and concentric about the supporting tube, said coupling member and sampling tube being provided with two sets of stops, the stops of each set cooperating in such a manner that a given axial translation of the coupling member in respect of the sampling tube is allowed, Said first and second stops respectively for transmitting downwardly directed thrust and upwardly directed tensile loads respectively from the coupling member to the sampling tube; a set of clamping fingers and a set of cooperating wedges, respectively with bevelled surfaces, one set being connected to the coupling member and the other one to the sampling tube, so that the relative axial position of the fingers and of the wedges is translated into different radial positions of the fingers and in such a position relation to each other that, when the coupling member is loaded by pressure and the first set of stops of the coupling member and the sampling tube are engaging one another for transmitting thrust force from the coupling member to the sampling tube and the clamping fingers remain free of the supporting tube; upon pulling said coupling member, the coupling member being displaced axially relative to the sampling tube until the second set of stops engage one another, whereby the bevelled surfaces of the wedges drives the fingers inwards in order to pinch off the supporting tube.