Nebojša Gojković’s research while affiliated with University of Belgrade and other places

55 INTRODUCTION e cement marls deposit "Filijala" is the main source of the basic raw material for the cement factory today. It has been known since 1838, and its exploitation works began in 1869 where the open-pit mine "Filijala" was developed. is deposit consists of three mining areas of unequal size and degree of exploitation called: "Severno polje", "Srednje polje" or "Međupolje" i "Južno polje"(Ganić M. et al., 2012). The cement marl basic technology of exp-loatation is continuous with the use of a bucket wheel excavator. Due to the conditions in the deposit, especially due to the quality of marl, it is not possible to apply exclusively continuous technology, so the discountinu-ous exploitation technology is used as well. Discontinuous technology is used for the exploitation in parts of deposits where the bucket wheel excavator can't operate, as well as for simultaneous excavation from both technologies in order to homogenize the quality of the Abstract: Since 1976 cement marl excavation at open-pit mine "Filijala" is carried out by continuous exploitation technology using bucket wheel excavator SH 400 manufactured by Orenstein und Koppel (O & K) company. Due to the bucket wheel excavator time-worn and high operating cost it was necessary to replace it with new one. In order to test the new bucket wheel excavator and prove its capacity a testing zone was defined as test block for excavation, in which the bucket wheel excavator testing will be conducted. In the test block area detailed investigations were carried out which included laboratory testing in order to determine the value of material cutting resistance.

Lack of geomechanical parameters for the digging force in underwater mining conditions was noticed during analyzing the available technical documentation at preparatory phase for the revitalization of the vessel bucket wheel excavator "Kovin" vital parts. This fact has caused the need for the implementation of relevant research program that will determine the water pressure effect on the coal mining process. The research program included certain geomechanical laboratory tests which are adapted to the specific conditions and needs of underwater coal mining. The program had included coal laboratory testing of the linear and surface digging force in underwater conditions at different hydrostatic pressures of the water column. Intention of this laboratory testing program was to simulate underwater coal mining conditions at different depths of the excavation-digging according to the "in situ" principle.

Wells conditions are different in wide range of temperature and depth. Cement slurries in such conditions have to remain pumpable and after setting to stay homogenous in production well life. For cementing operations in wide range of temperature and pressure and to accommodate cement slurries for individual well requirements more than 50 additives are now used for various API classes of cement. Laboratory in NIS – National oil company of Serbia, provides cement analysis of dry cement, tests on cement slurries and cement stone in compliance with methods prescribed by API standard, whereby quality and type of equipment fully meet the requirements. Adaptation of the local cement to the well conditions had already been done by these additives and by numerous tests of pure cement and cement mixtures we developed a palette of typical cement mixtures for cementing oil wells in the south eastern region of the Pannonian basin.

The development on the exploitation at the marl open pit "Filijala" was accompanied with an appearance of slopes instability in the zone of northern and western open pit final slopes. According to the planned direction of works development it is necessary to fully stabilize the perceived instability and prevent the possible appearance of new ones. Stabilization of the unstable zone is carried out by appropriate measures for stabilization, and then the proper system for monitoring the slope stability (monitoring system) was defined to reduce the risk of major open pit slope deformation and thus ensure the marl exploitation process.

In open pit mine design, geostatical calculations of slope stability hold the key importance in defining the construction parameters of the Mine, with an immediate reflection on defining the boundaries of exploitation grasp, exploitation economics, safety and security of operations. Today, during the analyses and calculations of slope stability, a whole line of contemporary methods is used. Our experiences pinpoint the need for caution in geostatical analysis, because of the need of continuity from the method selection, through definition of input physical-mechanical data, to reading, interpretation and acceptance of the calculation results. The paper is analysing the problem in question on a practical problem, and demonstrating the covered traps and risks, originated from the errors noted. KEYWORDS Geostatics, Slope stability, Open pit mines 1 STARTING CONSIDERATIONS It is well known that during the open pit mine design and operative management of the opencast mining of mineral-raw materials, one of the most important problems is statics, or the slope stability. Emphasizing the importance of slope statics is related with the multidimensionality and sensibility of this problem, with its solution closely related with the work safety, security, efficiency and economy production effects at open pit mines. In real-time circumstances, this is not an easy task, and the solution should fulfil the two opposed criteria: one to achieve mining operations with maximum safety, and the other to minimise the quantity of material to be excavated during the slope formation, or, in other words to minimise the production costs. The problem is not always the same in all circumstances, it may vary in complexity and demanding level by deposits, depending on the geological structure of working environment and a number of features from the general area of engineering geology, geomechanics, hydrogeology, geomorphology, tectonics, seismicity, etc. This fact is making the problem even more complex. The level of exposure and the sensitivity of the problem in focus depend also on the constructive geometry of the open pit mine, the depth of the deposit, location and way of overburden disposal, advancement dynamics and direction of mining operations, and, up to a certain level, the machinery used and the applied technology of mining at open pit mines. The principle "the lesser the slope angle-the higher the stability and safety" sounds simple enough, and it would be applicable without limitations if there is no consequence of increasing the mining area grasp or the quantity of overburden along with the negative ecology and economy effects. We will present the connection between the change of general slope angle and the increase in overburden quantity following the example of the copper open pit mine "Juzni revir"of the Majdanpek Copper Mine (Serbia). The "Juzni revir" open pit mine is classified as the deep open pit mine, its depth at the moment is approximately 500 m, and the general slope angle 39 °. If the general slope angle is decreased by 1 ° (to 38 °), an additional amount of 41×10 6 m 3 of overburden should be excavated at this depth, thus increasing the mining cost by approximately 80 million of euros. From the environmental protection viewpoint, a negative consequence of the general slope angle decrease by 1 ° reflects in increasing the degraded terrain surface by 1.63 ×10 5 m2. The additional comments are unnecessary, the effects are very clear. The previous example cannot be considered as a general standard, because for certain open pit mines, in particular shallow non-metallic mineral raw-materials open pit mines, such as stone, clay, gravel and sand the change of the general slope angle bears no influence to the quantity of overburden. All these facts certify the gravity, the complexity, and the level of demands lying before the engineering task of solving the slope statics problem at open pit mines. To solve this task is critically important in opencast mining, and it is obvious that in the process of looking for the most suitable solution, that fulfils both criteria the maximum safety and minimal costs, all the influential factors, conditions and limitations are to be considered. One of the ways for finding the task solution is the adaptive algorithm approach. The adaptive algorithm approach consists of the geostatical analysis of slope stability, adjusted for a real-life situation (problem) where the integration is made of: process and assessment of the starting data available, development of the working environment model; selection of an adequate mathematical model geostatical

This paper is dedicated to the problem of stability prediction of the terrain above underground mining operations. After the initial introduction to the problem, then the short analysis of the model approaches used to solve it, and giving the algorithm for rock massif stability prediction, we describe the concept of the multiattirbute terrain stability prediction method. The application of the multiattribute prediction method for stability of the terrain above underground mining operations is presented on the example of the Brown Coal Mine Aleksinac. The used method is original, essentially different from the other methods of mathematical modeling, because its prognosis of the rock massif stability under the influence of underground mining operations is based on the balance of the stability indicators. Our comparative analysis of the results obtained by multiattribute prediction and the data obtained by measurements of real deformations and terrain settling in multiple mines shows high mutual correlation, with an average deviation of less than ±10%. These results are confirmed entirely on the example of the Brown Coal Mine Aleksinac.

A theoretical concept and an illustrated practical example of the limited interval dynamic model, aimed for application in optimizing the exploitation life of the mining machinery with shorter life cycle such as bulldozers, scrapers, haul trucks, excavators with one working element and smaller capacity, etc., is presented in this paper.

Analysis covers results of tangential stress and deformations at the point of contact between casing-cement stone, because both values reach their maximum at this point. Analysis has been conducted for the previously described models, where calculation has been done for each model for both cement stone Class G and Cem. Sistem, therefore calculation for 10 series of most usually used well designs has been done. Such models are easily customized to a well design, whether for analysis of cemented annulus for intermediate or production casing string in various well geometries.

Usage of cement slurry with additives for expansion and flexibility is not always reasonable. In the wells, which shall not undergo cyclic stress or changes in well conditions, it is required to analyze, through application of a model, whether compression strength of cement stone is sufficient to sustain radial σr, tangential σq stress and deformations. If that would be the case, than standard types of cement slurry, with corresponding compression strength of cement stone, assure stability of cemented annulus during production well life.