BACK-ANALYSES OF FAILURES AND SUPPORT RE-DESIGN OF A MAGNESITE ROOMAND-PILLAR MINE EXCAVATED IN STRATIFIED ROCK
*J. Arzúa and L.R. Alejano
Department of Natural Resources & Environmental Engineering
University of Vigo, Spain
(*Corresponding author: jarzua@uvigo.es)
P. Rodríguez
Magnesitas de Rubián, S.A. Mining company
Lugo, Spain
ABSTRACT
After six years of successful excavation with a support pattern, some roof failures have happened in a roomand-pillar underground mine excavated in a stratified rock mass. In this paper the authors give explanation
to the failure mechanisms and redesign the support in order to solve the recent problems of roof collapse
(Figure 1).
The magnesite bed is approximately 14 m thick, runs in the E-W direction with an average dip of 18º. Due
to mine constraints and the inexpensiveness of the ore, the excavation is performed in the mineral itself,
avoiding the need for mining dumps. Rooms and drifts are oriented in the mineral bed to form an angle of
19º to the direction of the strata, reducing the road gradients to less than 10%. Rooms are 11 m wide and
pillars are 7 m wide. They are designed to leave a 1 meter thick magnesite bed in the roof of the rooms acting
as a supporting beam. More often than not there is a layer of very poor quality marly shale of up to 1 m thick
resting on the roof magnesite. Overlying the poor quality marly shale there is a better quality, self-supporting
sandy shale.
In first place, a characterization of the involved rock masses (magnesite, marly shale and sandy shale) is
performed, collecting the geotechnical data from previous studies and performing new in situ geotechnical
surveys to check the soundness of the previous data. Also a distinction is made between different quality
zones inside the mine by means of the RMR classification, associated to the occurrence of faults.
Back-analysis of some of the failed rooms and data from exploration boreholes were used to determine the
failure mechanisms (associated to voussoir type failure or occurrence of sub-horizontal joints crossing the
magnesite beam), to fine-tune the methods used to design the support and to analyse the stability of the
different possible thickness of the magnesite beams of the rooms and galleries of the mine.
Numerical models of the most relevant conditions found in the mine were performed in order to better
understand the behaviour of the roofs and to simulate the supporting solution obtained in the previous
analyses (Figure 2).
Most relevant conclusions regard to the main elements causing the instability of the roof, like the appearance
of sub-horizontal discontinuities crossing the magnesite bed, which causes total instability of the roof and
usually leads to collapse; excessive thinness of the magnesite bed in the problematic rooms; and the
decompression of the deepest (around 270 m in depth) zone of the mine because of existing faults.
Figure 1 – Failed room into the mine. Notice the hanging support and the thin beam.
Figure 2 – Numerical model of the roof showing the major principal stress in a beam 0.5 m thick.
KEYWORDS
Stratified rocks, Voussoir beam, Roof collapse, Support design, Stability