In the HDD method, a pilot borehole is first drilled along the planned drilling route with the drilling rig using a steerable drill head with attached drill rods. A drilling fluid made of water and the clay mineral bentonite supports the process. It is applied via nozzles in the drill head during the drilling process to flush the soil out of the drill channel and stabilise it and to cool the workings.

Via a transmitter in the drill head, the drilling progress is permanently monitored by a locating system and can be corrected if necessary. This controllability of the pilot bore allows obstacles such as existing pipelines to be bypassed or undercut. When the drill pipe is retracted, the drilling channel is enlarged by an expansion head and the attached pipe is pulled into the route. In this way, pipelines with large diameters can also be laid along flexibly plannable routes under or parallel to roads, bodies of water, railway tracks or buildings without breaking the surface in all geologies up to rock.