R&D Laboratory for Geotechnical Stability, Bishkek, Kyrgyzstan:

B. A. Chukin, Director, Associate Professor, Candidate of Engineering Sciences, bchukin@yandex.ru
R. B. Chukin, Deputy Director of R&D, Candidate of Engineering Sciences

In 2002 and 2006, pit walls collapsed in Kumtor Mine. The collapse of 2002 was accompanied by negative consequences. The volume of the collapsed rock mass was about 3 Mm3. The height of the collapse was 280 meters, and the width of the collapse at the base was about 270 meters. It took a significant amount of cost and time to resume the work of the open pit. In all cases of pit wall collapses, it is important to perform a cause analysis in order to reduce the risk of re-collapse in the course of further mining. The investigation analyzed data from monitoring of pit wall displacements, structural mapping, rock properties, meteorological and seismic data, groundwater level data, mining sequence prior to collapse, etc. Explanations of eyewitnesses of the collapse were heard. Invited experts from various consulting firms considered various versions of the possible causes and processes that led to the collapse, but so far the final opinion among experts about the reasons for the collapse in 2002 has not been formed. It is believed that the main reason for the collapse was the unfavorable three-dimensional orientation of the fault, which was not identified during the period of engineering and geological surveys. In support of this version, the results of numerical simulations in a 2D formulation were presented. Numerical modeling showed that a similar picture of pit wall collapse is possible in the presence of a gently dipping fault, which cuts the exposed steeply dipping fault. At the same time, the gently dipping fault did not have an outlet to the surface of the wall and extended below the bottom of the open pit. To improve the safety of mining operations, it was decided to clarify the structural-tectonic and geological model of the open pit and update the field observation program by use a laser monitoring system. Thanks to the updated monitoring system, it was possible to avoid the tragic consequences of the collapse of the pit wall in 2006. At the same time, the collapse occurred and it was necessary to analyze the causes of the collapse. The main direction of the analysis of the causes of collapse is the search for structural-tectonic faults, along which the collapse surface was formed. The search for the failure surface, taking into account the exposed surfaces of structural-tectonic faults, indicates that the failure surface is not always formed according to the classical schemes adopted for fractured rock masses. The pit wall collapses that occurred in Kumtor Mine in 2002 and 2006 indicate that the failure surface was formed by several intersecting structural-tectonic faults. The analysis of the causes of pit wall failure should be performed taking into account the three-dimensional orientation of structural faults on the basis of numerical modeling. 3D numerical modeling was carried out in FLAC 3D. The numerical simulation results for the collapse confirmed that the 2006 failure surface was formed from several structural faults. The obtained results allowed the development of recommendations for safe mining operations for 15 years.

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