Uphill 250 m with GRUNDOROCK 375-6.5

08.04.19 - The Rothenfels am Main barrage (Main-Spessart district, Lower Franconia) with a lock for shipping, barrage system and hydropower plant dates back to the 1930s. In 2016, the operator (Uniper Kraftwerke) commissioned the building of a second hydropower plant with a horizontal Kaplan tube turbine next to the existing two-turbine plant to exploit the potential of the water power to a greater extent. All operating facilities of the new hydropower plant are installed underground. In order to starting trialling and going into regular service with the new turbine, in the summer of 2018 construction work began for the channelling of the energy into a feed point and laying of the new 20 KV power cable to feed into the existing grid.


Besides installing the greater part of the 20 KV system in protective tubing on the surface between the hydropower plant and grid, on four sections the cable had to be laid underground. On account of the geological (rock fissures) and topographical conditions (steep banks), these four sections called for an exceptional degree of experience, expertise, know-how and, not least a sure feel for the job. For at this point, the job involved the underground installation of protective tubes (PE 100, DA 225 mm) using HDD wash drilling on a section of around 250 m – but, and here's the crunch, downhill all the way. For PFAFFINGER-BEERMANN PB Drill- & Energietechnik GmbH or rather their experienced and well-practised drilling team it was just the sort of challenge they thrive on. They soon settled on what equipment to use for the job: a GRUNDODRILL 28Nplus with the new mud motor V2 and attached GRUNDOROCK 375-6.5 rock drilling unit from TRACTO-TECHNIK.


Opting for this drilling equipment was well-advised, as the new combination of mud motor and rock drilling unit (in contrast to its predecessor) offers not one, but several benefits:

- locating the drill head couldn't be easier as the transmitter housing with built-in transmitter is located just 0.9 m behind it
- the bore heads have a cutting diameter of 165 mm (6.5"), i. e. the 6" Hole Opener can be used directly after the pilot bore
- torque has been increased by 60 % for guaranteed success even in the most adverse conditions thanks to the heavy-duty rock drilling unit mounting, surface pressures of up to 160 kN can be withstood
- minimum bore radius when the mud motor is used: 75 m
- particularly compact design compared to conventional mud motors. Total length without drill head: 3,950 mm

The main advantage for the operator is being able to react precisely to changes in direction. Thanks to the positioning of the transmitter at the front, the bore head can be located much more precisely. It was not only a thrilling experience for him, but for all concerned, given that it was one of the first times the mud motor with attached GRUNDOROCK 375-6.5 rock drilling unit had been used.


Once planning was completed, the drilling fluid mixed and drilling equipment selected, the GRUNDODRILL 28Nplus was positioned on the narrow cycle path along the banks of the Main, and drilling could begin. The pilot bore, measuring 165 mm in diameter, was started from the foot of the embankment next to the power plant at a piercing angle of 10 % and ended according to plan with an exit angle of 62 % after 252 m at a distance of around 18 m from the top edge of the embankment. In just 22 hours, exactly 98 metres in height were tunnelled – through hard unyielding rock.
The 165 mm bore then had to be widened to accommodate the PE 100 protective tubing with a diameter of 225 mm. In a pit excavated for the purpose, the drill head was exchanged for a 14" Hole Opener (355 mm) and the borehole widened appropriately. The PE 100 conduits for the new power cable were then welded on site and installed without a hitch in around three hours.


Even after nigh on 90 years, the Rothenfels am Main hydropower plant, now extended by a tube turbine, still stands for reliable, clean and environmentally friendly energy generation. Efficiency and eco balance sheet have been optimised and existing facilities used in line with the principle of sustainability, not to mention climate protection as electricity generated by water power leaves a zero carbon footprint. The HDD wash drilling method used here for the trenchless installation of new power cables on various sections leading to the grid is the perfect environmentally friendly complement.



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