penetrating unit 22 during introduction in the ground deviates from the predetermined direction then the articulating arm 22 is corrected in a direction perpendicular to the entry direction so long until the desired penetration direction is adjusted thereby the penetrating unit 22 is subjected to a bending load or force which brings about torsional forces on the connecting piece 14 or transverse forces acting perpendicular to the connecting surface of the connecting piece 14 and the penetrating unit 22 these forces which can lead to a damaging of the connection or the articulated arm 2 3 4 are detected by the force sensors the sensor signals provided by the force sensors are supplied to the control unit of the articulated arm 2 3 4 whereby the sensor signals during the movement of the articulated arm 2 3 4 are taken into consideration and wherein the articulated arm 2 3 4 is subjected to correction for correcting the penetration direction only so long as the forces occurring are below an acceptable maximal value if the occurring urging forces exceed the acceptable value the penetration process is interrupted the deviation of the penetration unit 22 from the ideal line can result from an impediment occurring in the ground which may necessitate a renewed penetration process at an adjacent position besides monitoring the forces occurring during the directional correction of the arm 2 3 4.