The Grundodrill in Slovenia, On The Trail of Karl May

Carola Schmidt, Tracto-Technik GmbH ,
special collaboration

The secret, underground chalky world in Postojna is an area, which used to form the sea in the south-west regions of Slovenia. The cave labyrinth, stretching over 27 km, with its stalactites and sintering pillars, is known as one of the most impressive natural wonders in Europe and a main tourist attraction in Slovenia. The cave offers the ideal environment for the cave lurch, an endangered species of amphibian.

Even the producers of movies based on the works of Karl May could not withstand the charm of this scenery and decided to film some scenes for the German film classic "Winnetou" in the sixties.

The cave tour starts by entering the cave by rail, followed by a round tour through the rows of lit-up cave halls and underground pathways. The battery operated cave paths are regularly reloaded at a central loading station.

After years of operation the sanitation of the loading station was to be carried out. For practical reasons the cave administration team decided to relocate the loading station from the inside to outside the cave. In order to do this a connection line had to be produced. The open-trench method could not be taken into consideration, due to the installation length, the awkward installation paths and the extremely stony ground. The responsible planners therefore decided on a direct connection. This required two rock drillings through the cave wall, each at 30 m lengths at a distance of 1m from the outside to the inside. It was also planned to install steel pipes as protection pipes into the expanded rock bore hole in order to retain the power cables.


Vilkograd, the contracting company, was handed the project, who have specialized in horizontal bores and trenchless pipe renewals for 10 years. The company has 60 employees and is not only well known for its exceptional quality in Slovenia, but also in Croatia, Serbia and Montenegro.

The horizontal bore unit type Grundodrill 13X was applied for this task. "We bought this bore rig in 2002 and it is the best we have," according to company owner Viljem Kolar.

It was the first large-scale rock drilling task for Vilkograd and they therefore called for René Schrinner, the bore tool specialist from Tracto-Technik, to support their bore team.

To determine the optimal starting point, the direction and the inclination percentage were measured beforehand with a laser-theodolite.

A Grundorock-Mudmotor was applied to accomplish the pilot bore through the extremely hard chalkstone, which took approximatively 30 - 40 min for the 3 m long drill rods. The advantage of the Grundorock-Mudmotor is basically the fact that only a relatively small quantity of drilling fluid is required for the operation. The relevant drilling fluid was lead out and later disposed of. The roll and pitch of the mud motor was controlled precisely with the cable guided measuring system.

The expanding bore then followed with a 10" hole-opener (equals a diameter of 250 mm). Then the steel pipe ND 219 was built into the bore hole with the support of an Olymp rammer. Due to the surroundings, only short steel pipe lengths could be rammed in at a time, which had to be welded, one after the other. Last, but not least, the power cable was installed and connected to the new loading station.

Already, after 6 days of work, the construction work around the rock drilling was completed without too much fuss, as one has become expect, by the Vilkograd company..

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