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Grundodrill bores through mountainous terrain in Germany


A Grundodrill Type 25 N HDD rig from Tracto-Technik was used to install a new bore in the German town of Sollstedt.

The bore was part of a project to renew an ageing gas pipe, which was to run for 2km through mountainous terrain, from a plateau on ‘Sollstedter Hölle’ mountain to the gas-pressure regulation unit in the town of Sollstedt. Boring contractor Beermann used an open-trench method for much of the installation.
The Grundodrill HDD unit was chosen to drill a 150m parallel, underground bore in a steep slope with a 78% gradient. The bore was to replace a section of pipe that had previously run over ground.

The distance between the HDD rig and the start of the steep slope was approximately 70m. To enable the continuous straight-line bore to meet the planned bore exit point at the foot of the slope, an acute entrance angle of 42% was necessary.

Ground conditions on the bore path comprised layers of partially weathered and partially solid limestone rock, up to soil classification 7, which was often clearly visible on the surface of the steep terrain.

The pilot bore had to be prepared taking into account these ground conditions, the very tight working conditions and the inclination of the bore. A mud motor could not be applied, so the use of an aggressive boring head with special, hard-metal bore tips was the only solution.

A depth sonde (with 28m depth capacity), from specialist steering systems manufacturer DCI, was fitted for the monitoring and steering.

The cover on the steep slope was almost 25m deep in parts, causing the signal to be relatively weak.  

“This was a real challenge. However, with our many years of experience and the application of the most modern boring technology, we were confident of carrying the bore out as planned.  Also, the signal got stronger after the critical phase of the bore, confirming that we were always on course”, said Kai Winkler of boring contractor Beerman.

The pilot bore was completed precisely on target.  

Two expansion bores were made with 250mm and 350mm-diameter reamers, with the runs taking just five minutes per drill-rod length to pull through. After each expansion bore, the backreamer was pushed back through the borehole in order to compress and rinse out the bore spoil from inside the bore at its deepest point.

This cleaning process was vital in order to prevent the product pipe getting stuck when it was finally pulled in. After the final cleaning process, the bore was cleaned thoroughly and made ready for installing the product pipe.

Some 12 steel pipes with an outer diameter of 200mm, and a PE and reinforced fibre-glass coating, were welded together, x-rayed and checked for tightness with water at 100bar.

Then the PE and reinforced fibre-glass coating was reinstated at the welding point. Four HDPE pipes of 50mm diameter and 4.6mm wall thickness were also installed to host the control cable and fill the annular space.

A Bagela cable winch pulled the pipe string uphill to the borehole. The pipe installation process began as soon as the final backreamer had been connected.

The annular space along the HDPE pipe was filled using Drillmix to give the gas pipe a secure position and bedding, preventing corrosion of the outer pipe surface and stopping the penetration of water into the annular space. 

Courtesy Geo Drilling International


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