The subsurface pillar-supported mining openings in Mendig extend over an area of about 200,000 m2. They are remaining from former basalt mining activities. In certain parts of the disused mining openings stability problems like pillar failure and surface collapses occurred already. In situ two failure modes of pillars can be observed. These include structural controlled failure along natural discontinuities and strength failure of intact rock. The observed failure modes were confirmed by numerical 3D modeling based on results of laboratory tests including peak and residual strength tests at rock samples and shear tests along discontinuities.
In the city of Mendig, Germany, large pillar-supported mining openings are remaining from mining activities mainly during the 19th and 20th century. The mining openings are located in a depth of 15–20 m below the current ground surface and were formerly used to excavate the so-called "Mendig basalt". Since the shutdown of mining in the 1960s the pillars have to ensure a stable and safe environment for the meanwhile intensely utilized areas on top.
In certain parts of the mining openings the strength of rock mass pillars has been exceeded and consequently the pillars experienced either brittle deformation or structural failure. Locally, also surface collapses occurred already.
To assess the currently given rock mass strength the mechanical and geological properties have to be considered. For that reason samples have been taken from core drillings, which were executed as a part of a currently ongoing research project. Numerous laboratory tests with these samples were executed at the Ruhr- University Bochum. The results are published in Bock et al. (2015).
As part of the research project also a 3D laser scanning survey as well as the geotechnical mapping of known and explored mining areas is under investigation. In addition to that a numerical modeling for stability assessment is ongoing.