This technical note has been retracted by the authors under invitation of the editor in chief. Following a detailed note from a reader, pointing out substantial duplicating parts with previous articles published in other journals by the same authors, a thorough analysis of the published material was undertaken, involving the authors in the process. The conclusion reached is that this article is to be considered as a case of over-publication. While it is acceptable to repeat some information on methodology and test equipment, this should be limited. There is nothing wrong with publishing results on different rock materials, but the manuscripts should be significantly different and innovative. The online version of this article contains the full text of the retracted article as electronic supplementary material.