Slope stability in open pit mines is an important issue. Most open pit coal mines are located in Kalimantan in which rainfall is very high and as such so is the probability of slope failure. It has been well accepted that slope failure risk consists of probability times consequence of the slope failure. In order to have a study on the consequences of slope failure as a function of various geotechnical parameters, a laboratory scale investigation was conducted. A centrifuge with a diameter of 3.65 m, a load capacity of 3 kN and a maximum rotation of 70 rpm was designed and built in house by the Laboratory of Geomechanics & Mining Equipment of ITB in Indonesia. Dimensional analysis was used to scale down the model from a real slope condition in an open pit coal mine. The material used in this study is consolidated clayey sand with slope heights of 10 cm, slope angles of 45°, 60° and 75°; the centrifugal acceleration were 0.15g, 0.23g, 0.33g; 0.46g, 0.59g, 0.75g, and 0.93g. By running the centrifuge at different levels of rpm and slope dimensions, the volume and mode of the slope failures can be obtained and can be used for the input data for assessing risk analysis of slope stability in an open pit coal mine. In order to confirm the results of the physical modeling tests, numerical modeling using the limit equilibrium method and direct monitoring in the field are also carried out.

1 Introduction

Slope stability is the one of the most important factors in the surface mining operation. The factors affected slope stability are shear strength, rainfall, ground water, blasting intensity, probability of earthquakes, etc. Those parameters should be carefully considered, otherwise, the risk of slope failure will increase. It is well known that slope failure risk consists of probability times consequence of the slope failure.

To study the impact of a slope failure as a function of geotechnical parameters, then a laboratory scale research was conducted. By using a centrifuge that has been built in the laboratory, gravitational acceleration can be modeled with various scenarios.The mode of failure caused by the gravitational acceleration can be clearly investigated and the volume of collapsed material can also be calculated.

By using the centrifugal experimentation, the scenario can be planned to fit the field prototype condition. Small scale experimentation has a lot of advantages to determine failure mechanisms, namely, easily-controllable test conditions, adjustable slope geometry and material used.

In this experiment, a centrifugal test was conducted to evaluate the slope stability due to gravitational acceleration which was represented as Ng. The slopes, which were made using clayey-sand material was placed inside a plexiglass box which was located at the end of the centrifuge. It was then spun, gradually increasing in speed so the gravitational acceleration went from 1g to Ng.

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