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Rock mass mechanical behavior in deep mines: in situ monitoring and numerical modelling for improving seismic hazard assessment

PhD thesis defense of Ineris

 

Date : February 05, 2019.
Place : Mines Nancy, Campus ARTEM, France.
Candidat : Francesca De-Santis

 

Thesis of Francesca De Santis defended on 05 February 2019 in Nancy Mines, before the jury composed of:

  • Beata ORLECKA-SIKORA, Associate Professor, Institute of Geophysics, Polish Academy of Sciences – Rapporteur
  • Martin GRENON, Professor, Université Laval – Rapporteur
  • Evelyne Foerster, Cadre scientifique d’EPIC, CEA – Examinatrice
  • Katshidikaya Tshibangu, Professor, Université de Mons – Examinateur
  • Yann GUNZBURGER, Maitre de conférences, Ecole Nationale Supérieure des Mines de Nancy – Directeur de thèse
  • Pascal BERNARD, Physicien du Globe , IPGP – Co-directeur de thèse
  • Isabelle CONTRUCCI, Cadre scientifique d’EPIC, Ineris – Encadrante
  • Savka DINEVA, Professor, Luleå University of Technology – Invité
  • Anders NYSTRÖM, Engineer, BOLIDEN – Invité


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Seismicity in underground mines is the result of stress perturbations and rock fractures induced by excavations. In the last decades, the problem of mining-induced seismicity has become more and more important due to the increasing depth of underground mines. Understanding mining-induced seismicity is, then, a fundamental purpose for improving seismic hazard assessment in deep mining operations, for mine infrastructure stability and mine workers safety.

 

With the aim of better understanding interactions between stress modifications induced by mining and the generation of seismic activity, a deep area of Garpenberg mine (Sweden) was instrumented by Ineris with microseismic probes and geotechnical cells. The main objective of this thesis is to realize a comparative analysis of the recorded seismic and geotechnical data, along with a 3D numerical model, taking into account the mining sequence and the geological conditions.

As a first major contribution of this thesis, recorded microseismic activity between 2015 and 2016 (~700 events) has been analyzed and interpreted. Results show a clear dependence between blasts and microseismic events, even if the rock mass response to mining appears to be very variable across space and time. Two seismic clusters are observed: one located in the major production area (Central Cluster) and another (Right Cluster) located at some distance from the excavations, in a zone characterized by a heterogeneous distribution of weak (talc) and stiff (limestone, polymetallic ore) rocks. Seismic source parameters analysis demonstrates that the two clusters are characterized by different dynamics, with Right Cluster events being mainly controlled by geological heterogeneities. Indeed, weak rocks impose high stress concentrations in the stiff rock masses, inducing a mechanism of stress transfer from the exploitation area toward the weak zone.

Geotechnical data analysis shows important strain changes throughout the study period, whose intensity seems to be more correlated with mining sequence and the proximity of weak geological zones than to the amount of extracted rock mass. Geotechnical measurements also show that mechanisms of differed strains take place at Garpenberg mine, and that seismic activity decays proportional to the decaying rate of measured strains. This latter observation implies that, in addition to the immediate stress change induced by blasting, aseismic creep may be another mechanism driving seismicity at Garpenberg mine.

A 3D elasto-plastic geo-mechanical numerical model was run considering a precise reconstruction of the geology, the virgin stress state and the mine sequence. The model simulates 70 excavation steps. The results show how the mining sequence, with one column of stopes being exploited upward and downward simultaneously, leads to high stress concentrations in the remaining pillar and to strong plastic deformation within the weak rocks.

In the last part of this thesis, model results have been compared with seismic data analysis to investigate whether correlations exist. Results shows that mine-wide numerical models can be suitable for the analysis of mining induced seismicity at large-scale. However, there are some aspects of the induced seismicity that the model cannot fully explain. This is particularly true for remote seismicity occurring at a distance from excavations, while better correlations are found when considering seismicity close to production areas. Further analysis will be needed in future works to deeper characterize remote seismicity and found appropriate constitutive law able to represent it within the model.