The Importance of Geological Features in Designing Open-Pit Lignite Exploitations

Summary

The world oil crisis obliged all countries to intensify the exploitation of solid fuels. These are mined either in open pits or underground. The high productivity of open pits is the main reason for their preference to underground collieries. The oil crisis will have as a consequence the opening of new open pits under more and more unfavourable conditions. The reason is that the more favourable have been either exploited or nearly exhausted.

The new open pits require huge investments and high energy consumption. A correct planning os such open-pit exploitation must seriously concern all the countries that have no other alternative.

Greece covers today 50 per cent of its electric power generation by burning low grade lignite extracted from open pits. Greece or more correctly the Greek Public Power Corporation is planning to increase this percentage to TO by the year 1990. To achieve it, new open pits will be opened and the depths of the exploitations will increase stepwise from 80 m to 240 m and the average exploitation ratio will vary from 2.5 + 1 to 6 + 1 (m³/ton), and in extreme cases 10 + 1.

The design of such exploitations should not have as only target the selection of suitable mining equipment. Designing should undoubtedly match the geological parameters of the seam. This calls for identification and evaluation of these parameters. Each seam has its own characteristics which only in very rare cases can be compared with the others. Although tremendous investments are made for the procurement of equipment, relatively small amounts are spent for exploration, and study. We believe that no country has ever spent the necessary amount to investigate all the geological parameters that characterize a seam. This is mainly due to the hurry for mining solid fuels after the world oil crisis. In those cases risks for invested capitals are high and profits may be limited. Under these conditions design and exploitation studies are based on theoretically calculated models usually not coinciding with reality.

Greece has over 20 years of experience in open-pit mining. The first open pit started in Ptolemais in the mid 1950s, foday the lignite production is almost 25 million tons per annum and the quantity of rock handled yearly amounts to 60 million cubic metres. By 1990 lignite production is expected to reach 60 milliontons and the quantity of rock to 300 million m approximately.

Greece wishes to communicate its experience to and exchange it with other countries, especially on geological problems that affect the exploitation of open pits. Greek open-pit exploitations have been based on bucket-wheel excavators, belt conveyors and spreaders, the so-called continuous system or “German method”. Initially electric trains have been used for transportation. Today some amounts are transported by trucks. The major part is transported by conveyors to the crushers of the power plants.

Theoretically a system consisting of a bucket-wheel excavator, belt conveyor and spreader assures optimum efficiency.

In practice, however, there is a serious discrepancy between theoretical and achieved targets. For exploiting the new open pits bigger and bigger machines are designed. The discrepancy in the case of big machines will be disproportionate to their size. The discrepancy is due mainly to geological factors of the seam that generally cannot be correctly evaluated and affect negatively output. Experience in the Greek open-pit mines allows us to classify these parameters as follows:

• (a) stratigraphy and thickness of the layers;

• (b) quality of overburden;

• (c) hardness of the layers and their diggability in relation to the cutting force of the bucket-wheel excavator;

• (d) inclination of the seam;

• (e) total depth of the open pit and height of the spoil bank;

• (f) tectonism;

• (g) safety of benches both in coal and overburden and of spoil banks;

• (h) behaviour of the materials when transported by belt conveyors (thixotropy)

• (i) water problems.

The purpose of the paper is to put emphasis on the so-called “non-appreciable” factors of a seam. These factors decrease substantially the expected output, and increase the exploitation cost noticeably.