that object is attained in that the drill string is surrounded by a hollow outer string enclosing an annular space which surrounds the drill string and through which the drive medium is passed out of the borehole the invention further concerns a method of enlarging an existing borehole using a drilling head of that kind application_number US 25272702 A description referring to fig1 the mode of operation involved in directional drilling can be seen therein using a forward drive machine 1 to produce a pilot bore a directional drilling head 2 is driven into the ground at an angle by means of a directional drill string 3 the directional drill string 3 is carried on a rail guided sliding carriage of the forward drive machine 1 and is driven into the ground by a linear drive after a forward drive movement by a given distance a fresh section of the directional drill string 3 is attached thereto and the sliding carriage is withdrawn in order further to advance the directional drill string 3 which has been increased in length arranged in the proximity of the directional drilling head 2 is a usually magnetic probe 4 which makes it possible to ascertain the respective precise position of the directional drilling head 2 by way of a navigation system and a monitor unit the forward drive machine 1 also has a rotary drive.