Stability Assessment of the High Safety Pillars in Slovenian Natural Stone Mines: Fid-Bau Portal

Stability Assessment of the High Safety Pillars in Slovenian Natural Stone Mines

Kortnik, Joze

For the first time in Slovenia, the underground excavation of natural stone blocks was introduced on a trial basis at the Hotavlje I colourful limestone quarry in 1993, in 2002 at the Lipica II limestone quarry, in 2008 at the Lipica I limestone quarry and in 2009 also at the Doline limestone quarry. This was primarily because of the geological structure of the site, the quarry’s condition, the potentially large amounts of the overburden in the event of an expansion of the surface part of the quarry, and the increasing needs for this raw material, i.e. natural stone. The underground excavation of natural stone in all locations are done using a modified room-and-pillar excavation method that is adjusted to each site’s characteristics, with regularly or irregularly distributed high safety pillars. Since the underground excavation of natural stone blocks is performed at a relatively shallow level under the surface, i.e., at a depth of only 10-40 m, the value of the primary vertical stress state is also relatively low (less than 1.0 MPa). This significantly increases the risk of wedge-shaped pieces or blocks falling out of the ceiling in open underground spaces. In previous years, special attention was paid to the installation of stress-strain systems for controlling the planned dimensions (width and height) of large, open, underground spaces (rooms) and the dimensions of the high safety pillars, along with continual monitoring and identification of the instability phenomena in the ceiling and sides of the large open spaces (rooms). The paper presents the methods and devices used for the optimization and the safety monitoring of high safety pillars for the underground excavation of natural stone blocks in Slovenian natural stone mines.