CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2011/001616, filed Mar. 31, 2011, which designated the United States and has been published as International Publication No. WO 2011/120696 and which claims the priority of German Patent Application, Serial No. 10 2010 013 723.5, filed Mar. 31, 2010, pursuant to 35 U.S.C. 119(a)-(d). 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for operating a horizontal drilling device and a horizontal drilling device. 
     Horizontal drilling devices are used to introduce supply and disposal lines into the ground in trenchless construction or to exchange already installed old lines in a trenchless manner. 
     There are many different horizontal drilling devices. Common are horizontal drilling devices in which a drill head is initially advanced angled into the ground by means of a drill rod assembly and starting from a drill boom positioned above ground until the drill head has reached the desired drilling depth. Then, the drill head is redirected into the horizontal position in order to carry out the horizontal drilling. The target point of such a horizontal drilling can for example be located in a target excavation pit which is excavated for this particular purpose or in a basement room or it can also be located above ground i.e., like the starting point, so that the drill head after a defined drilling progress is redirected into a diagonally upwards pointing direction, to let the drill head reemerge above ground. 
     After the drill head has reached the target point, it is often replaced for a widening device for example a conical widening body, to widen the previously generated (pilot) bore by means of the drill boom when retracting the drill rod assembly. This may involve attaching a new line to be drawn into the widening device, to draw the new line into the ground simultaneous with the widening of the pilot bore. 
     Horizontal drilling devices are also used to replace old lines in the ground in a trenchless manner. For this, in a first step the drill rod assembly is pushed by the drill boom along the old line (and in particular through an old line) and after reaching a target point, which can be located in a maintenance shaft of the sewage system, the front end of the drill rod assembly is connected with a widening device by which the old line is cut or burst when retracting the drill rod assembly, wherein the fragments of the destroyed old line are radially displaced into the soil. At the same time, a new pipe can be drawn into the old pipe. Destroying the old pipe an displacing the fragments of the old pipe allows the new pipe to have an outer diameter which corresponds to the outer diameter of the old pipe or even exceeds this diameter. 
     As an alternative, an adapter can be connected to the front end of the rod assembly which adapter engages on the rear side end of the old line and pulls the old line out of the ground when retracting the drill rod assembly. This allows avoiding that fragments of a destroyed old line remain in the ground which may otherwise cause damage to the new pipe due to sharp-edged brakeage edges and the pressure exerted by the surrounding soil. 
     Horizontal drilling devices usually have a linear drive with which the drill rod assembly can be advanced and retracted within the ground. Further, a rotational drive is usually provided with which the drill rod assembly (and with this the drill head and widening head connected thereto) can be rotated. The rotation of the drill head or the widening device allows improving the advance in the soil. 
     Further, most of the steerable horizontal drilling devices require a rotation of the drill head to steer the drill head into a desired drilling direction. The drill heads of such horizontal drilling devices have an asymmetrically formed (for example slanted) drill head front, which leads to a lateral deflection of the drill head during movement through the soil. When the drill head is simultaneously rotatingly driven when being advanced in the soil, the asymmetric configuration of the drill head has no influence on the straight drilling course, because the lateral deflection evens out over a rotation. On the other hand, when the rotation of the drill head is stopped and the drill head is exclusively advanced by pushing—optionally supported by strokes of a stroke device which is integrated in the drill head or in the drill boom—the asymmetric configuration of the drill head leads to a (constant) lateral deflection. This achieves an arched drilling course and as a result a change of the drilling direction. 
     Horizontal drilling devices which are exclusively intended for replacing old pipes which are already installed in the ground often have no additional rotational drive. 
     Horizontal drilling devices in which the drill boom is intended for positioning above ground often can only be used in non-urban areas because the horizontal drilling devices have to be positioned at a considerable distance to the region in which the bore or the new line is to be introduced into the ground or in which an already existing old pipe is to be exchanged, due to the drilling distance required to reach the desired drilling depth. Oftentimes, corresponding special requirements are not available in built-up areas. A further disadvantage of such horizontal drilling devices is that these drilling devices which are commonly configured as self-propelled drill boom, cause significant crop damage which has to be remedied by a corresponding financial effort. 
     Because of these disadvantages, the trenchless line construction in built-up areas is still largely limited to the trenchless replacement of old pipes because the old pipes always extend between subterranean hollow spaces (in particular supply shafts and basement rooms) which are already present and which can be used for the positioning of the horizontal drilling device. Excavation work and as a result, crop damage can thus mostly be prevented. Horizontal drilling devices have been developed which are configured so that they can be positioned in a supply shaft. Because new supply lines often are not to be installed along existing supply routes these horizontal drilling devices are often not available for newly installing supply lines. 
     From DE 196 33 934 A1 a horizontal drilling device is known which is configured for use in small excavation pits with a square cross section of about 70 cm×40 cm and a depth of about 1 m to 1.5 m. These horizontal drilling devices include a frame whose dimensions roughly correspond to the cross sectional dimensions of the excavation pit and are lowered into the excavation pit. A part of the frame protrudes over the upper edge of the excavation pit. In the section of the frame which is located inside the excavation pit, a combined linear/rotary drive is provided via which a drill rod assembly which is composed of individual rod assembly sections is advanced into the soil. The linear/rotational drive includes a rotational drive which can be moved within the frame in horizontal direction by means of the linear drive which is formed by two hydraulic cylinders. For advancing the drill rod assembly, the last rod assembly section is force fittingly fixed in the rotational drive for which the rotational drive has clamp jaws. The rod assembly sections which are successively screwed to the rear end of the already drilled drill rod assembly are supplied to the linear/rotational drive via a rod assembly lift which transports the rod assembly sections from a rod assembly magazine which is arranged in the upper section of the frame which protrudes over the edge of the excavation pit. The rod assembly lift includes a changer motor whose motor shaft is provided with a threaded pin. The threaded pin is screwed into the rear end of a rod assembly section which is provided for the transport to the linear/rotational drive. By displacing the changer motor along the rod assembly lift, the rod assembly section can then be transported to a position which is axial to the drilling axis. 
     The horizontal drilling device known from DE 196 33 934 A1 enables introducing bores into the ground starting from any desired starting positions. Because only a relatively small excavation pit is required for the positioning of the horizontal drilling device, and the horizontal drilling device can also be transported easily due to the compact design, its use is associated with relatively minor crop damages. 
     A disadvantage of the horizontal drilling device known from DE 196 33 934 A1 is that due to the coaxial orientation of the changer motor, the new rod assembly section and the drill rod assembly, only relatively (compared to the length of the frame) short rod assembly sections can be used. The shorter the individual rod assembly sections, the more frequently new rod assembly sections have to be attached to the rod assembly in order to introduce the bore with the desired length into the ground. The attachment or release of the rod assembly section is associated with significant time consumption. 
     Because of the force fitting fixation of the rod assembly in the rotational drive, the amount of the forces which are transferable to the rod assembly is limited. In addition, the force fitting connection of the rod assembly necessitates the use of high powered and expensive hydraulic cylinders which increases the costs for the drilling device. 
     SUMMARY OF THE INVENTION 
     Proceeding from this state of the art the invention is based on the object to provide an improved method for operating a horizontal drilling device and a horizontal drilling device. In particular, a method for operating a horizontal drilling device is proposed which enables the use of rod assembly sections which are as long as possible and a horizontal drilling device, which inter alia is suitable for implementing this method. 
     The invention is based on the idea to provide rod assembly sections for the drill rod assembly which are as long as possible, in order to minimize the time required for a rod assembly change (i.e., the attachment or release of a rod assembly section to/from the rod assembly). In drilling devices which—as is known from DE 196 33 934 A1—are arranged in an excavation pit with small dimensions the maximal length of the rod assembly sections is limited by the dimensions of the excavation pit in the direction of the drilling axis. Such drilling devices further involve the problem of handling of the rod assembly sections during the rod assembly change. In the drilling device of DE 196 33 934 A1 the rod assembly sections are held by a changer motor during rod assembly change. Because this changer motor is positioned in coaxial position behind the rod assembly section, the maximal length of the rod assembly sections is decreased by at least the length of the changer motor. 
     The present invention is therefore based on not including a changer motor as in the drilling device of DE 196 33 934 A1, to carry out the rod assembly change of the rod assembly sections by means of the rotational drive (via which the rod assembly is then rotationally driven when operating the drilling device). Because according to the invention a rotational drive is used which does not engage on the front or rear end of the rod assembly sections but embraces the rod assembly sections (so that it engages on the sheath of the respective rod assembly section), a loss in length for the rod assembly sections due to constructive reasons is avoided. These can thus be configured as long as possible. 
     A method according to the invention for operating a drilling device which has a linear drive, a rotational drive which is displaceable by means of the linear drive, and a drill rod assembly in the form of a rod assembly string composed of a plurality of interconnected rod assembly sections, wherein the rotational drive forms a through opening into which the rod assembly is insertable, and wherein force transmission means for transmitting compressive and/or pulling forces and/or a rotational torque to the drill rod assembly are provided, is characterized according to the invention in that a rod assembly section is fixed within the through-opening of the drill rod assembly and is connected to or released from the rear end of the rod assembly by a linear and/or rotational movement of the rotational drive. 
     In a preferred embodiment of the method according to the invention, the rod assembly section intended for the rod assembly change can be form fittingly fixed within the through-opening of the rotational drive. Compared to the force fitting fixation known from DE 196 33 934 A1 a form fitting fixation of the rod assembly sections has the advantage that the risk of a sliding through of the drill rod assembly at high stresses is avoided and in addition the constructive effort which is required by the use of hydraulic cylinders for forming a clamping device, can be avoided. 
     A corresponding horizontal drilling device which is in particular suited for operation according to a method according to the invention, has a linear drive, a rotational drive which is displaceable by means of the linear drive, and a drill rod assembly, wherein the rotational drive forms a through-opening into which the drill rod assembly is insertable. According to the invention, the rotational drive has force transmission means within the through-opening for form fittingly transmitting compressive and/or pulling forces and/or a rotational torque to the drill rod assembly. 
     In a preferred embodiment of the horizontal drilling device according to the invention, the force transmission means are configured so that the drill rod assembly is insertable into the rotational drive in a first angular position (of the force transmission means relative to the rod assembly) or removable from the rotational drive, while in a second angular position a locking of the drill rod assembly is established in which according to the invention compressive and/or pulling forces and/or a rotational torque can be transmitted. A locking of the rod assembly within the rotational drive is thus achieved via a relative rotation of the rod assembly section relative to the force transmission means of the rotational drive. 
     A locking of the rod assembly in the rotational drive by a relative rotation can preferably be achieved in that the force transmission means have a force transmission ring which forms an opening through which the rod assembly can be passed. In addition, the rod assembly has a first cross section in at least a first section, which cross section corresponds to the cross section of the opening to the degree that the rod assembly can only be passed through the opening of the force transmission ring in the first angular position. The drill rod assembly further has a second cross section in at least a second section which is formed as a partial section of the first section, with the second cross section being different from the first cross section, and being configured so that the drill rod assembly is only rotatable into the second angular position when the second section is located within the opening of the force transmission ring. 
     Preferably, the force transmission ring is arranged in the rotational drive so as to be exchangeable. This keeps the effort for maintenance of the horizontal drilling device according to the invention low because the force transmission ring which, due to the locking contact with the rod assembly, is subject to relatively great wear, can be exchanged without having to exchange the entire rotational drive. 
     In a further preferred embodiment, the drill rod assembly can have a circular basic cross section, wherein in the first section this circular basic cross section is flattened at least on one side (and preferably on two sides with opposing parallel flat portions). Further preferably the drill rod assembly can have at least one (preferably two) arch-shaped groove(s) in the second section, which groove(s) run out into the lateral flat portion(s). 
     In a preferred embodiment of the horizontal drilling device according to the invention, the drill rod assembly is configured as rod assembly string composed of a plurality of interconnected rod assembly sections, wherein each rod assembly section has at least two second sections. This allows ensuring that the rotational drive or the force transmission means of the rotational drive can engage at two (spaced apart in longitudinal direction of the rod assembly section) positions of each rod assembly section. 
     Engagement of the rotational drive on at least two positions of each rod assembly section allows for the rod assembly section—in a preferred embodiment of the method according to the invention—to be fixed at a first position for changing the rod assembly, and to be fixed at a second position for drilling the rod assembly. This makes it possible to configure the rod assembly section longer than the maximal lift of the linear drive, which for constructive reasons normally always has to be significantly shorter than the length of the excavation pit (in the direction of the drilling axis). 
     Preferably a respective first and a respective second section are located in the region of the two ends of each rod assembly section, wherein at least a third section is arranged between these ends, which has a lower bending stiffness than the two first sections. Via a defined bending stiffness of this third section the overall bending stiffness of the individual rod assembly section and with this the entire drill rod assembly can be adjusted. 
     The rotational drive of the horizontal drilling device according to the invention preferably has a hollow gear with a differential gear wheel which is driven via a motor, wherein the differential gear wheel meshes with a gear ring which in turn is connected to the force transmission means in a rotatively fixed manner. This configuration makes it possible to arrange the motor of the rotational drive in vertical direction (i.e., perpendicular to the drilling axis) within the excavation pit, which allows keeping the rotational drive overall as short as possible (in the direction of the drilling axis). Under conditions which only provide a narrow space, a rotational drive which is as short as possible allows realizing a stroke for the linear drive which moves the rotational drive and which is as great as possible. 
     The linear drive of the horizontal drilling device according to the invention is preferably configured in the form of one or multiple drive cylinder(s) (preferably driven hydraulically or pneumatically). The rotational drive can then be connected with the linear drive, by virtue of being connected with the cylinder tube(s) of the drive cylinder(s). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the following, the invention is explained in more detail by way of an exemplary embodiment shown in the drawings. 
         FIG. 1  shows a horizontal drilling device according to the invention in a perspective view; 
         FIG. 2  shows the horizontal drilling device of  FIG. 1  in a second perspective view; 
         FIG. 3  shows an enlarged section of the representation according to  FIG. 2 ; 
         FIG. 4  shows the lower section of the horizontal drilling device according to  FIGS. 1 to 3  in a perspective view; 
         FIG. 5  shows the representation according to  FIG. 4  in another operating position of the horizontal drilling device; 
         FIG. 6  shows an isolated representation of the rotational drive of the horizontal drilling device in a perspective view; 
         FIG. 7   a  shows an isolated representation of the rod assembly receiver of the horizontal drilling device in a first operating position in a perspective view; 
         FIG. 7   b  shows an isolated representation of the rod assembly receiver of the horizontal drilling device in a first operating position in a sectional view; 
         FIG. 8   a  shows an isolated representation of the rod assembly receiver of the horizontal drilling device in a second operating position in a perspective view; 
         FIG. 8   b  shows an isolated representation of the rod assembly receiver of the horizontal drilling device in a second operating position in a sectional view; 
         FIG. 9   a  shows an isolated representation of the catch ring of the rotational drive including a rod assembly section in a first operating position in an isometric view; 
         FIG. 9   b  shows a front view of the catch ring and the rod assembly section shown in  FIG. 9   a;    
         FIG. 10   a  shows an isolated representation of the catch ring of the rotational drive including a rod assembly section in a second operating position in an isometric view; 
         FIG. 10   b  shows a front view of the catch ring and the rod assembly section shown in  FIG. 10   a ; and 
         FIG. 11  shows an isolated representation of the rod assembly receiver and the lower section of the rod assembly lift in an isometric view. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows in an isometric view a horizontal drilling device according to the invention  1  during the introduction of a pilot bore into the soil. 
     The horizontal drilling device includes a cylindrical housing  2 , which is partially closed via a cylindrical sheath  3 . Functionally, the horizontal drilling device  1  or respectively, the housing  2  of the horizontal drilling device  1  is divided into two sections, namely a lower section referred to as “pit section”, which is located within an excavation pit  4  which was excavated especially for receiving the horizontal drilling device  1 . In the pit section of the horizontal drilling device  1  the housing  2  is essentially completely closed by the sheath  3 . This prevents that soil which becomes dislodged from the walling of the excavation pit  4  falls into the hollow space which is formed in the housing  2  where further functional elements of the horizontal drilling device  1  and in particular a combined linear/rotational drive  5  are located. Soil which falls into the hollow space might otherwise contaminate these functional elements thereby impairing the function of the horizontal drilling device  1 . 
     In the upper section of the horizontal drilling device  1  according to the invention, also referred to as “surface section”, the housing  2  is partially configured open in order to provide access for operating personnel to a rod assembly lift  6  which extends as far as into this region. 
     The horizontal drilling device  1  is positioned “suspended” within the excavation pit i.e., the horizontal drilling device  1  is supported not on the floor of the excavation pit  4 , but rather via a support device with a total of three support legs  7  which are fastened in the region of the surface section of the horizontal drilling device  1  on longitudinal supports  8  of the housing  2 . Each of the support legs  7  can be fastened to a total of five different points on the respective longitudinal support  8 . This allows for a height adjustment of the horizontal drilling device  1  which is suspended in the excavation pit  4 . This height adjustment is important, for example for positioning the linear/rotational drive  5  which is located in the pit section, at the correct height for introducing the pilot bore into the soil. A fixing of the support legs  7  at the different points along the longitudinal supports  8  occurs via a respective transverse bolt  9 , which is inserted through a through-bore in a transverse support  10  of the respective support leg  7  and the respective longitudinal support  8  of the housing  2 , and is then fixed. 
     Each of the support legs  7  further has a spindle support which is connected to the transverse support  10  of the respective support leg  7  via a pivot joint. The spindle support includes a threaded rod  11  which has a support foot  12  on its foot end. A handle  13  is provided on the end of the threaded rod  11  which is opposite the support foot  12  via which handle  13  the threaded rod  11  can be rotated about its longitudinal axis, thereby achieving a longitudinal displacement relative to the spindle housing  14  which surrounds the threaded rod. The spindle supports serve for accurately orienting the horizontal drilling device  1  within the excavation pit  4  after a first height adjustment was already achieved by the fastening of the support legs  7  on the longitudinal supports  8  of the housing  2 . 
     It can be recognized in  FIG. 1  that the excavation pit  4 —like the housing  2  of the horizontal drilling device  1 —has a (substantially) cylindrical shape whose inner diameter essentially corresponds to the outer diameter of the housing  2  of the horizontal drilling device  1 . The sheath  3  of the horizontal drilling device  1  in the region of the pit section rests thus more or less directly against the wall of the excavation pit  4 . The fact that the inner diameter of the excavation pit and the outer diameter of the housing largely correspond to one another not only allows limiting the size of the excavation pit to be excavated to a minimum but also to achieve a most even support of the horizontal drilling device on a largest possible surface within the excavation pit  4 . The circular cross section of the excavation pit  4  and the housing further render the support independent of the respective rotational orientation (about the longitudinal axis of the horizontal drilling device). 
     The excavation pit  4  was excavated by first introducing a ring-shaped groove having the required (outer) diameter into the surface sealing (asphalt cover) with a core drill (not shown), removing the thus exposed disc-shaped asphalt cover and subsequently sucking away the soil located underneath with a suction dredger (not shown). The suction dredger which was used for this purpose includes a suction nozzle which also has a circular cross section. The excavation pit  4  is excavated somewhat deeper than necessary to allow for height adjustment of the suspensory supported horizontal drilling device  1  inside the excavation pit  4 , without causing an unintended touch down of the lower end of the horizontal drilling device  1  onto the pit bottom. 
     After excavation of the excavation pit  4 , the horizontal drilling device  1  was lowered into the excavation pit  4  by means of a crane (not shown) until the support legs  7  which where previously fastened to the longitudinal supports  8  of the housing  2  come into contact with the ground surface. The horizontal drilling device  1  was then rotatively oriented by means of the crane within the excavation pit  4  by rotating the horizontal drilling device  1  about its longitudinal axis until the bore axis which is defined by the linear/rotational drive which is arranged inside the pit section of the horizontal drilling device  1  points into the desired starting direction for the pilot bore. A fine adjustment of the working height of the horizontal drilling device  1 , and to a limited degree also the tilt of the horizontal drilling device  1  relative to the vertical, was then achieved via the spindle supports. 
     Because the wall of the excavation pit  4 —in particular in the case when it was excavated by means of a suction dredger—commonly is not configured evenly cylindrical, the horizontal drilling device  1  according to the invention has overall four support elements  15  in the region of the pit section which are evenly distributed across the circumference. These support elements  15  include support plates  16  which in a retracted position each form a section of the cylindrical sheath  3  of the horizontal drilling device. The support plates  16  can each be extended outward in radial direction by means of a hydraulic cylinder  17  to generate a direct contact of the horizontal drilling device  1  with the wall of the excavation pit  4  to securely support the horizontal drilling device  1  inside the excavation pit  4 . 
     The individual components of these support elements  15  are well recognizable in  FIG. 3 . Each of the support plates  16  is connected to a first end of an extension lever  19  via a first pivot joint  18 , with the extension lever  19  being in turn rotatingly supported on the housing  2  of the horizontal drilling device  1  by means of a second pivot joint  21 . A second end of the extension lever  19  is connected to the head of a piston rod  20  of the hydraulic cylinder  17 . An extension or retraction of the hydraulic cylinder  17  thus causes a partial rotation of the extension lever  19  about the pivot joint  21 , whereby the respective support plate  16  can be radially extended or retracted again. End stops  22  prevent that the support plate  16  enters the inner space defined by the sheath of the housing when retracting the hydraulic cylinder  17 . 
       FIG. 2  shows a representation of the entire horizontal drilling device  1  which corresponds to the representation of  FIG. 1  in which, however, a part of the sheath  3  in the excavation pit is removed to show the functional elements arranged therein. 
       FIGS. 3 to 5  show different views of this section of the horizontal drilling device  1  in enlarged representations. It can be seen that the combined linear/rotational drive  5  at the lower end of he horizontal drilling device  1  is arranged within the housing  2 . The linear/rotary drive  5  serves for rotatingly advancing a drill rod assembly which is composed of individual rod assembly sections  23 , into the ground. 
       FIG. 6  shows a partial section through the linear/rotational drive  5  in a representation in which the linear/rotational drive  5  is isolated from the remaining elements of the horizontal drilling device  1 . The linear/rotational drive  5  is formed by two hydraulic cylinders  25 . The piston rods  26  of the two hydraulic cylinders  25  traverse the respective cylinder tube  27  completely and are connected with their two ends to the housing  2  of the horizontal drilling device  1 . The piston rods  26  each have a centrally arranged piston (not shown) which divides the ring space which is respectively formed between the cylinder tube  27  and the piston rod  26 , into two working chambers, which can each be supplied with hydraulic oil via a hydraulic line  66 . Depending on the pressure of the hydraulic oil which is supplied to the individual working chambers, a movement of the respective cylinder tube  27  on the piston rod  26  in one or the other direction is achieved. The movement of the two hydraulic cylinders  25  of the linear drive is synchronized. 
     A rotational drive is arranged between the two cylinder tubes  27  of the hydraulic cylinders  25  which form the linear dive, and fastened to the two cylinder tubes  27 . The rotational drive includes a motor  29  (in particular a hydraulic or electromotor) which is flange-mounted to a hollow gear  28 . A drive shaft  30  of the motor  29  is connected with a differential gear wheel  31 , which in turn meshes with a gear ring  32  which in turn is connected to a drive sleeve  34  via screw connections  33 . The drive sleeve  34  is rotatingly supported within a housing  36  of the hollow gear  28  via two rolling bearings  35 . A rotation of the drive shaft  30  of the motor  29  thus causes a rotation of the drive sleeve  34  about its longitudinal axis. This longitudinal axis corresponds essentially to the drill rod assembly  24  held therein and therefore also the drilling axis i.e., the starting direction of a pilot bore to be introduced or the longitudinal axis of a bore or an old pipe extending in the wall of the excavation pit  4 . 
     For transmitting the rotational movement of the drive shaft  34  and the longitudinal movement which is generated by the hydraulic cylinders  25  of the linear drive to the drill rod assembly  24  which is held in the drive sleeve  34 , a catch ring  37  is used which—in an operating position of the drill rod assembly  24  within the catch ring  37 —fixes the drill rod assembly  24  in a form fitting manner. The catch ring  37  is form fittingly supported within the drive sleeve  34  and can be easily exchanged in case of wear, by first removing a retaining ring  63  from a corresponding groove in the inside of the drive sleeve  34  and then pulling out a spacer ring  64  from the drive sleeve. The catch ring  37  can then be easily pulled out of the drive sleeve  34 . 
       FIGS. 9   a  and  9   b  as well as  10   a  and  10   b  each show two views of the two operating positions of the drill rod assembly  24  within the catch ring  37  which are relevant for the operation of the horizontal drilling device  1 . These two operating positions differ in a 90° relative rotation of the catch ring  37  about its longitudinal axis relative to the drill rod assembly  24 . In the operating position shown in  FIGS. 9   a  and  9   b  the drill rod assembly  24  is locked in the catch ring. This locking is achieved by the particular sheath shape of the rod assembly sections  23  of the drill rod assembly  24 , and a shape of the central opening of the catch ring  37  which is adjusted thereto. 
     Each rod assembly section  23  of the drill rod assembly  24  has a cylindrical basic shape with a middle section  38  with a relatively small diameter and two end sections  39   a ,  39   b , with a relatively large diameter. In each of the end sections  39   a ,  39   b  of a rod assembly section  23  two parallel flat portions  40  are provided, thereby resulting in a cross section with two parallel straight sides and two opposing arched-shaped sides. The catch ring  37  forms a through-opening which corresponds to this cross section so that it is possible to insert the rod assembly section  23  into the through opening of the catch ring  37  and to freely move it (in longitudinal direction) therein, when the catch ring  37  and the rod assembly sections  23  guided therein are arranged in the rotational orientation relative to one another shown in  FIGS. 10   a  and  10   b.    
     For locking the rod assembly section  23  in the catch ring  37 , the catch ring  37  is moved inside the through-opening until two arched-shaped locking grooves  41  which are formed in each of the end sections  39   a ,  39   b  of the rod assembly section  23 , are located within the catch ring  37 . These locking grooves enable a relative clockwise rotation of the catch ring  37  by 90° into the operating position shown in  FIGS. 9   a  and  9   b  (locking position). A rotation by more than 90° is also prevented by the fact that the two locking grooves  41  which are arranged offset to one another by 180° about the longitudinal axis of the rod assembly section  23 , are only arch-shaped within an angular section of 90° and then extend straight. As a result of this, two cams  42  are formed whose distance is greater than the narrow width (corresponds to the two straight edges of the through-opening of the catch ring) of the through-opening for the catch ring  37 . These cams  42  abut on the edges of the catch ring  37  in the locking position shown in  FIGS. 9   a  and  9   b  and thus prevent a further (clockwise) rotation. 
     In the locking position of the rod assembly section  23  in the catch ring  37 , longitudinal forces (in longitudinal direction of the rod assembly section axes) and a rotational torque (in  FIGS. 9   a  to  10   b  clockwise) can be transferred to the entire drill rod assembly via the catch ring  37 . 
     The center section  38  of each rod assembly section  23  has a reduced outer diameter in order to achieve a smaller (defined) bending stiffness relative to the end sections  39   a ,  39   b . This is intended to enable the use of a controllable slanted drill head. By redirecting the drill head  43  in the soil, a drilling course which is arched in sections is achieved. The drill rod assembly  24  has to adjust to this arched drilling course which leads to a corresponding bending stress. The center section  38  of each rod assembly section  23  which has a reduced diameter and is thus relatively bending soft compared to the end sections  39   a ,  39   b , serves for maintaining the rod assembly section  23  overall bending soft, however, at the same time serves for configuring the end sections  39   a ,  39   b  stiff which, due to the threads are particularly at risk of breaking. 
     Due to the arrangement of the combined linear/rotational drive  5  at the lower end of the pit section of the horizontal drilling device  1 , and due to the smaller dimensions of the horizontal drilling device  1  (the housing  2  has a maximal diameter of about 60 cm) the individual rod assembly sections  23  cannot be manually fed to the linear/rotational drive  5 . Rather, an automated rod assembly feed is provided for this purpose which is formed by a rod assembly receiver  44 , which is arranged at the height of the linear/rotational drive  5  and the rod assembly lift  6 . 
     The rod assembly receiver  44  is shown in the overall representation of  FIGS. 4 and 5  and by itself in the representations of  FIGS. 7   a ,  7   b ,  8   a  and  8   b . The central element of the rod assembly receiver  44  is a receiving mandrel  45  which is supported in a bridge  46  which is connected to the two cylinder tubes  47  of two further hydraulic cylinders  48 . The hydraulic cylinders  48  are also of the kind in which the piston rod  49  protrudes out of the cylinder tube  47  on both sides. The two free ends of the two piston rods  49  are connected to the housing  2  of the horizontal drilling device  1  so that by a corresponding impingement of the hydraulic cylinders  28  with hydraulic oil, the cylinder tubes  47  and thus the rod assembly receiver  44  can be displaced on the stationary piston rods  49  in horizontal direction. 
     The receiving mandrel  45  of the rod assembly receiver  44  is supported within the bridge  46  for pivoting about a horizontal axis, wherein a pivoting between the two end positions shown on one hand in  FIGS. 7   a ,  7   b  and on the other hand  8   a ,  8   b  is possible. The pivoting is achieved via a further hydraulic cylinder  50  which is supplied with hydraulic oil via corresponding hydraulic connections  65 . 
     In the orientation shown in  FIGS. 7   a ,  7   b , the longitudinal axis of the receiving mandrel  45  and a rod assembly section  23  attached onto the receiving mandrel  45  is coaxial to the drive sleeve  34  of the rotational drive and thus points in the drilling direction of the horizontal drilling device  1 . In the vertical operating position shown in  FIGS. 8   a ,  8   b  which is thus pivoted by 90° relative to the operating position according to  FIGS. 7   a  and  7   b , the receiving mandrel  45  and the rod assembly section  23  attached onto it are positioned within a guiding track  51  of the rod assembly lift  6 . In this operating position of the receiving mandrel  45 , a rod assembly section  23  can be attached onto the receiving mandrel  45  from the rod assembly lift  6  or removed from the latter. 
     Within the guiding track  51  of the rod assembly lift  6 , a receiving sled  52  which can receive a rod assembly section  23 , is movably guided, wherein the receiving sled  52  is fastened at a trumm of a drive belt  53  which extends outside of the guiding rail  51  and parallel to the latter. An upper driving roller of the driving belt  53  is connected to the motor (not shown) in order to drive the latter. A lower deflection roller  54  is supported on an axle  55  which is guided at both its ends on a threaded rod  56 . By rotating the threaded rods  56 , the vertical position of the lower deflection roller  54  can be changed so as to tension the driving belt  53 . By means of the driving belt  53  the receiving sled  52  can be moved up and down in the guiding track  51 . In this way a rod assembly section  23  which is inserted into a loading station  58  in the surface section of the horizontal drilling device  1  by operating personnel, can be transported to the rod assembly receiver  44  in the pit section—and vice versa. 
       FIG. 11  shows in an isolated representation of the rod assembly receiver  44  and the lower part of the rod assembly lift  6  including the receiving sled  52  in which a rod assembly section  23  is held. The receiving sled  52  forms a through-opening in which the rod assembly section  23  can be inserted from the side by the operating personnel in the region of the loading station  58 . In the receiving sled  52  the inserted rod assembly section is supported suspensory, i.e., two pairs of projections  59  each form a free space which is only slightly broader than the diameter of the center section  38  and narrower than the broader side of the end sections  39   a ,  39   b  of the rod assembly section  23 . One of the projection pairs engages into the locking grooves  41  of the front end section  39   a , while the second projection pair engages in the center section  38  of the rod assembly section  23 . Via the two projection pairs of the receiving sled  52 , the rod assembly section  23  fixed therein is form fittingly held (in vertical and lateral direction). Of course it is also possible to use only one projection pair or only one single projection to hold the rod assembly section  23  within the receiving sled  52 . 
     By lowering the receiving sled  52  within the guiding track  51  of the rod assembly lift  6 , the rod assembly section  23  which is held in the receiving sled  52  is attached onto the vertically oriented receiving mandrel  45  (compare  FIG. 5  [receiving sled not shown] and  8   a ,  8   b ). The receiving mandrel is then pivoted by 90° into the horizontal operating position shown in  FIGS. 4 and 7   a ,  7   b , whereby the rod assembly section  23  is pivoted in lateral direction out of the receiving sled  52 . The receiving sled  52  can then be moved to the loading station  58  again so that a further rod assembly section  23  can be inserted. 
     The horizontal drilling device  1  is configured for carrying out flush drillings i.e., a drilling fluid is supplied via the rod assembly  24  to the drill head  43  which is arranged on the front side of the rod assembly  24 , which drilling fluid exits through front side and lateral exit openings. To enable the supply of drilling fluid to the drill head  43 , the individual rod assembly sections  23  of the drill rod assembly  24  are configured continuously hollow. The drilling fluid is supplied to the drill rod assembly  24  via the receiving mandrel  45  which for this purpose is also configured continuously hollow. Only on the rear side end i.e., the end which protrudes out of the attached rod assembly section  23 , the receiving mandrel is closed by means of a screw cap  60 . The drilling fluid is supplied to the inner space which is formed by the hollow receiving mandrel  45  via a shaft which is also configured hollow and on which the receiving mandrel is rotatingly supported. Two sealing rings on the outside of the receiving mandrel  45  prevent a leaking of the drilling fluid through the gap between the receiving mandrel  45  and the rod assembly section  23 . This allows easily achieving a secure and constructively simple connection of the pivotal receiving mandrel  45  to the source of the drilling fluid. In contrast, a connection to the drilling fluid source while at the same time maintaining the pivotability of the receiving mandrel via flexible supply tubes requires more constructive effort, because the high pressure with which the drilling fluid is supplied to such a rod assembly  24  necessitates the use of extremely pressure resistant and with this poorly elastic supply tubes, which in turn would impede the pivoting movement of the receiving mandrel  45 , which would require a greater and higher powered hydraulic cylinder  50  for the pivoting. 
     For generating a pilot bore, the horizontal drilling device  1  is used as follows. 
     Before lowering of the horizontal drilling device  1  into the excavation pit  4 , the drill head  43  shown in  FIG. 1  is inserted into the drive sleeve  34  of the rotational drive through a through-opening  61  for the drill rod assembly which through-opening  61  is formed in the housing  2 . This is necessary because the drill head has an integrated transmitter for localization by means of a so called walk-over-receiver and is therefore longer than the rod assembly sections  23 . The drill head has a (rear) end section  62  which corresponds to the end sections  39   a ,  39   b  of the rod assembly sections  23  with regard to the geometric shape: Two arch-shaped locking grooves are introduced into the end section  62  with a cylindrical basic shape which is provided with parallel flat portions on two opposing sides, into which grooves the catch ring  37  can be rotated by a 90° clockwise rotation, whereby the drill head  43  is locked in the rotational drive. The rotational drive is located in the rear most position in which the latter can be driven as far as possible away from the through-opening  61  by means of the linear drive. 
     The horizontal drilling device  1  is then lowered into the excavation pit  4 , oriented and supported, as already described. 
     By using the linear/rotational drive  5  the drill head is then drilled into the soil as far as possible. Due to the length of the drill head  43  the drilling occurs with two strokes of the linear drive; in the first stroke the catch ring  37  is located at the front end of the two parallel flat portions so that the pressure forces are transferred over the protrusion formed there, and the rotational torque is transferred via the parallel flat portions which serve as wrench flats. After the first stroke, the linear drive is retracted so that the catch ring  37  can engage in the locking grooves and lock the drill head  43 . After this, the linear drive is moved forward again, whereby the drill head  43  is completely drilled in. The rotational drive is then located in the front most position shown for example in  FIGS. 4 and 5 . A locking fork (not shown) provided in the region of the through-opening is then lowered. The fork width of the locking fork corresponds to the distance of the two parallel flat portions of the drill head  43  and the distance of the two locking grooves. Previously, the drill head  43  was oriented by means of the rotational drive so that the two flat portions of the end section are oriented vertically so that the locking fork can travel over the end section (in a section before the locking grooves) of the drill head  43 , thereby temporarily preventing a rotation of the drill head  43  by means of a form fitting fixing. 
     During the advancement of the drill head  43  into the soil, a first rod assembly section  23  was already inserted into the receiving sled  52  by an operating person and by displacing the rod assembly lift  6  attached onto the receiving mandrel  45 . After pivoting of the receiving mandrel  45  and the rod assembly section attached thereto, by 90° into its horizontal orientation, the rod assembly section  23  is in a predominantly coaxial position relative to the already drilled drill head  43 . By displacing the two hydraulic cylinders  48  of the rod assembly receiver  44 , the front side of the threaded plug of the rod assembly section  23  can be driven to the rear side threaded socket of the drill head  43 . The catch ring  37  is then released from the locking grooves of the drill head  43  and the linear/rotational drive  5  retracted until it is located in a defined region of the front end section  39   a  of the first rod assembly section  23 . By actuating the rotational drive, the first rod assembly section  23  is screwed together with the drill head  43  which is fixed in rotational direction by the locking fork, wherein the rotational torque is transferred via the parallel flat portions  40 . Due to the fact that the catch ring  37  is not yet locked in the locking groove  41 , the rod assembly section can move in axial direction relative to the catch ring  37  during screwing. This allows realizing the longitudinal movement of the rod assembly section  23  which is necessary for the screwing of the rod assembly section  23  without an elaborate length compensation which is realized by the linear drive. 
     The position of the rotational drive during the screwing is chosen so that the locking grooves  41  of the front end section  39   a  are located within the catch ring  43  after the rod assembly section  23  is completely screwed together with the drill head  43  so that the catch ring  37 , after a rotation of 90°, can engage directly i.e., without necessitating a further displacement of the linear drive, in the locking grooves  41  to fix the rod assembly section  23  also in longitudinal direction. The drill rod string is then drilled until the rotational drive reaches its front end-position again. 
     After this, the rotational drive is unlocked by a 90° rotation (in the opposite direction) of the catch ring and retracted by means of the hydraulic cylinder  25  of the linear drive until the catch ring  37  can engage in the locking grooves  41  of the rear end section  39   b  of the first rod assembly section  23 ; there, the catch ring  37  is locked again by a 90° rotation. Then, the drill rod string composed of the drill head  43  and the first rod assembly section  23 , is advanced into the soil by a further working stroke of the linear drive by using the linear/rotational drive. 
     As soon as the rotational drive has reached its front end position, the rod assembly receiver  44  is moved back into the rear position and the receiving mandrel  45  is pivoted into the vertical position where the latter can receive a second rod assembly section  23  which was already inserted into the receiving sled  52  by the operating personnel which receiving sled  52  was moved into the loading station  58 . 
     After finishing the working stroke of the linear drive, the locking grooves of the front end section  39   a  of the first rod assembly section  23  are located below the locking fork which can then be lowered to fix the drill rod string, while the second rod assembly section  23  is screwed to the existing drill rod string. For this, the second rod assembly section  23  is moved to the rear end of the first rod assembly section  23  by means of the rod assembly receiver  44 . At the same time, the rotational drive is released from the first rod assembly section  23  and moved backwards until it can engage on the parallel flat portions  40  in the front end section  39   a  of the second rod assembly section  23 . By using the linear/rotational drive  5 , the second rod assembly section  23  is then screwed to the first rod assembly section  23 , wherein after finishing the screwing, the catch ring  37  locks again in the locking grooves  41  of the front end section  39   a  of the second rod assembly section and the drill rod string is drilled until reaching the front end position (of the linear drive) again. The linear/rotational drive  5  is then released from the second rod assembly section  23  by a 90° relative rotation of the catch ring  37  and moved backwards again to lock the second rod assembly section  23  in the rear end section  39   b  and to advance the drill rod string into the soil again by a further working stroke. 
     In contrast to the drill head  43 , the locking fork engages in the locking grooves  41  of the rod assembly sections  23  to lock the latter not only rotatively but also against a movement in longitudinal direction. This allows preventing the drill rod string from unintentionally becoming displaced due to elastic re-deformation of the compressed soil and the drill rod assembly which has been compressed or stretched by the loads. 
     The attachment and drilling of further rod assembly sections  23  occurs in an identical manner. 
     After the pilot bore is complete, the drill head  43  can be replaced by a widening device (not shown) to widen the bore during retraction of the drill rod assembly. Optionally, a new pipe (not shown) or another supply line (not shown) can be attached to the widening head which is drawn into the bore simultaneous with the widening device. 
     When retracting the drill rod assembly  24 , the latter is shortened step by step by one rod assembly section  23  at a time. This occurs in the following manner. 
     The catch ring  37  of the rotational drive is locked in the locking grooves  41  of the rear end section  39   b  of the last rod assembly section  23 . The rotational drive is moved backwards by displacing the hydraulic cylinders  25  of the linear drive. The locking fork is then lowered and fixes the second to last rod assembly section  23  by engaging of the locking fork in the rear end section  39   b  of this rod assembly section  23 . The linear/rotational drive  5  is then released from the rod assembly section  23  by a 90° rotation of the catch ring and moved forward again until the catch ring  37  can engage in the locking grooves of the front end section  39   a  of the last rod assembly section  23 . By a further working stroke of the linear drive the drill rod assembly  24  is pulled out of the soil as far as to enable the locking fork to lock the second to last rod assembly section  23  in the front end section  39   a . Then, the last rod assembly section  23  can be screwed off from the second to last rod assembly section  23  by a counter clockwise rotation of the drive sleeve  34 . Due to the particular shape of the rod assembly section in the region of the end sections, a rotational torque can be transferred for releasing the threaded connection without the catch ring  37  being fixed in the locking groove  41  also in longitudinal direction. This allows the catch ring  37  to slide over the rod assembly section according to the thread pitch, which allows avoiding a length compensation via the linear drive. Simultaneously, the rod assembly receiver  44  moves forward to receive the unscrewed last rod assembly section  23 . The rod assembly receiver  44  then moves to its rear most position again and the linear/rotational drive  5  moves simultaneously forward so that the latter can engage on the rear end section  39   b  of the then last (before second to last) rod assembly section  23 . The screwed-off rod assembly section  23  is then completely moved out of the drive sleeve  34  and can be inserted into the receiving sled  52  of the rod assembly lift  6  by pivoting of the receiving mandrel  45  into the vertical position. The receiving sled  52  can then be moved upwards to the loading station  58  where the rod assembly section can be retrieved by an operating person. 
     In the same manner, all rod assembly sections are successively released from the horizontal drilling device. 
     The shown horizontal drilling device is appropriate for use in non-urban environments and in particular for the generation of house connections in the supply field (in particular gas, water, electricity, fiber glass, etc). Bores of at least 20 m in length can be introduced which are then used for drawing in pipes or cables with an outer diameter of up to 63 mm.
A method for operating a horizontal drilling device includes the steps of fixing a rod assembly section within a through-opening formed by a rotational drive of the horizontal drilling device, and connecting the rod assembly section to a rear end of a drill rod assembly of the horizontal drilling device or releasing the rod assembly section from the drill rod assembly by a linear and/or rotational movement of the rotational drive, wherein force transmission means for transmitting compressive and/or pulling forces and/or a rotational torque to the drill rod assembly are provided in the through-opening, wherein the rotational drive is displaceable by a linear drive of the horizontal drilling device, and wherein the drill rod assembly is configured as rod assembly string and is composed of a plurality of interconnected rod assembly sections.