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

B. A. Chukin, CEO, Candidate of Engineering Sciences, bchukin@yandex.com
R. B. Chukin, Deputy CEO of Research and Development

Based on the real data from engineering–geological investigation and laboratory tests, probability assessment of pit wall stability was performed for pit wall at the Kumtor gold ore deposit in the Kyrgyz Republic. The sensitivity analysis was carried out determine influence of random variables on the pit wall stability based on the computational experiment according to the scheme of the central composite plan. The chosen variable factors were the parameters of the Hoek–Brown criteria, the general angle of the pit wall slope and the ground water level in the pit. The computational experiment was implemented out using the test example of pit wall 400 m heigh. According to the sensitivity analysis, the dominant factor is GSI—geological strength index, which describes how heavily rock mass is jointed and is determined by the results of engineering–geological investigation. With regard to the influence on the pitwall stability, the GSI parameter is twice as high as the compressive strength of the undisturbed sample and three times as high as the general slope angle and ground water level. Thus, the distribution law of pitwall FoS is primarily affected by the distribution law of GSI parameter, which, gentically is individual per each pit wall area. For probability assessment of stability, a random variable is selected to be FoS. The reliability of FoS calculation is based on a FLAC program option to perform complex processing of random variables by the Monte Carlo method and to calculate stability of each implemented variant. In total, 100 variants were implementede. The statistical estimation of the mathematical expectation accuracy of FoS was performed by the sample mean FoS based on the determination of confidence intervals with a given reliability γ=0.99. In our case, testing the hypothesis of the normal distribution law for FoS was rejected in favor of the Weibull distribution. The law of distribution of the GSI parameter in each geological zone was different from normal. The calculated probability of failure by the Weibull distribution was Р(FoS<1)=31.7 %. The decision on the stability of the pit wall was made based on the comparison of Р(FoS<1) with the critical probability of pit wall failure Р_crit. We believe that the most acceptable values of Рcrit are withn the range from 5% to 0%. The comparison of Р(FoS<1) with Р_crit, in our case, indicates the need to carry out measures aimed to increase overall stability of the pit wall in the study section. The reli ability of probability assessment of pit wall stability is based on the implementation of the rules and methods of statistical processing and analysis of both source and calculated data on the specialized program Statistica.

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