Proceedings of the Tomsk Polytechnic Institute Named after S. M. Kirov, Holder of the Order of the October Revolution and the Order of the Red Banner of Labor

Vol. 177
1971

On the Issue of Work Safety in the Extraction of Burnt Rocks from Waste Heaps of Kuzbass Mines

P. A. Leonov

(Presented by the scientific seminar of the Department of Safety Engineering and Mining Technology)

Burnt rocks from waste heaps (mine rock dumps) are a good construction material. They are used for road construction, concrete preparation, and other purposes. In addition, burnt rocks are used as a stowing material.

The number of waste heaps in Kuzbass reaches hundreds. Large rock dumps in the indicated basin have a volume of 1–1.5 million m³.

For more than ten years, at a number of mines, work has been carried out on waste heaps to extract burnt rocks. Recently, at some mines, rock has begun to be stored in flat dumps. However, the existing inactive and active waste heaps still contain a large volume—several tens of millions of cubic meters—of burnt rocks. For most production workers, mine rock dumps appeared to be safe large piles of rock. Therefore, the extraction of burnt rocks from waste heaps was carried out by the simplest method—removal at the base along the perimeter of the waste heap of a triangular layer by means of a single-bucket excavator up to a bench height of 4–5 m (Fig. 1). In this process, part of the rocks located higher on the slope of the dump slid and collapsed downward. These collapses sometimes yielded small volumes of rock, but there were cases of the collapse of large masses of burnt rocks on the order of hundreds and thousands of cubic meters. The working space at the base of the waste heap was buried. Under analogous conditions, several years ago, a landslide-collapse occurred in the Karaganda basin. Studies of the factual material on major collapses established the following. The height of the waste heaps varied greatly—from 40 to 70 m. The quantity of collapsed rock (rounded, according to approximate estimates) ranged from 100 to 1000 m³. The duration of collapses in some cases was not measured, while in others it was determined as 40 min., 1 hour, 3 hours 30 min., and 5 hours 30 min. Each collapse consisted of several separate displacements (landslides) of rock from the slopes of the dump—

Fig. 1.

shaft. The time intervals between individual movements were from 10–15 min to 5 h. The working area was covered with lump material over a distance of 15–30 m from the edge of the cleaning bench at the base of the dump. The scattering of dust during collapses depends on the mass of rock sliding down the slope and on the strength and direction of the wind. In calm weather, dust is carried 40–80 m from the waste heap. Wind moves it and rock fines from the base of the dump to 200 m and more. There are known cases (Mine No. 5–7 of the Anzherougol trust) of dust being carried 1 km from a waste heap. A layer of hot dust during one collapse was observed 40 m from the bench near the base.

During collapses, a considerable quantity of incandescent rock dust and hot coal dust rises into the air. They form clouds of black and brown color. The coal dust ignites, producing tongues of flame reaching up to 20 m, both horizontally and vertically. These dust clouds form continuously during the collapse process. It should be noted that in collapses of all types, no sound phenomena of an explosive character, nor any rumbling, were observed.

In clear weather without precipitation, 91% of all collapses occurred; during rain and snowfall, 6%; and during blasting operations (undercutting the overhanging part of the rock in the bench when excavating at the base of the waste heap), 3%. Precipitation is not of decisive importance for the occurrence of collapse. It probably serves as the initial impulse for deformation.

Collapses on waste heaps present a significant danger both to people and to production equipment, buildings, structures, and residential houses.

1. The danger from rock that has collapsed to the base of the dump. This rock usually fills the entire excavation near the bench and is partially displaced beyond its limits; it produces mechanical and thermal effects, since it contains many lumps and fines heated to a high temperature. Consequently, the entire excavation area at the base must be regarded as dangerous. The width of the hazardous zone may reach 30 m.

2. The danger from hot coal dust and incandescent rock dust suspended in the air is considerable. In clouds and billows of the indicated composition, tongues of flame are formed in vertical, inclined, and horizontal directions. The length of the tongues may reach a large value. Unfortunately, there are not sufficiently complete data on the distance traveled by hot rock dust. According to some observations, at a distance of 140 m from the bench down at the base, dust and rock fines still retain such a high temperature that they can cause burns to people and set fire to wooden structures (a booth, fences), and even to a coal storage yard.

The two principal dangers indicated above during collapses resulting from excavation below, at the base of the waste heap, were the cause of accidents (minor and serious) and breakdowns in which cleaning faces (benches) on dumps, excavators, and other equipment were buried.

The mechanism of large collapses during excavation of a rock dump below, at the base, is rather complex and is not the same for waste heaps of different heights. In studying this question it is necessary to take into account a number of structural features of the body of rock dumps.

The granulometric composition of the rocks of a waste heap is heterogeneous. In its lower zone (Figs. 1, 2), 4–6 m high, coarse-lump material predominates; in the middle zone, mainly medium and

small pieces of rock, and in the upper zone, near the summit, rock that is unsorted by size.

The degree of burning of the rocks on burning waste heaps is also not the same. The greatest burning is observed in the lower zone of the dump. Here, owing to the large pieces, there is a good supply of fresh air to the combustion site. The temperature reaches high limits (1500°). As a result, the rocks are not only well calcined but sometimes are also sintered into a monolithic mass. In all zones of the waste heap there are areas where the rocks are compacted and form separate masses; in other areas the rocks remain in the form of separate unbound pieces, fines, and dust. The pressure of the rocks and their combustion change the condition of the indicated areas.

Fig. 2.

Let us consider the mechanism of rock collapse when developing waste heaps with stable rock equilibrium. These include dumps up to 35–40 m high. When development is carried out at the bottom, at the base (Fig. 1), the bench is located in the lower zone. The rocks located above, along the slope of the waste heap, will slide down along a plane at the natural angle of repose. This process is difficult to control. In the best case, the rocks slide and collapse gradually in thin layers and in separate sections 1, 2, 3. The excavator manages to remove this rock and to prevent the working site and itself from being buried. In some cases, as practice has shown, a large mass of rock from the slope of the waste heap, in the form of a powerful layer ABCD, quickly slides down, collapses, and buries the bench, people, and machines.

On waste heaps with unstable rock equilibrium (more than 40 m high) the collapse mechanism is more complex.

According to Bernatskii [2], the rocks of the slope are in equilibrium when the sum of the friction force, due to the weight of the resisting prism ABO (Fig. 2), and the cohesion force along the surface OC is equal to or greater than the force determined by the weight of the active-action prism BOC. When the waste heap is developed at the bottom near the base, part ADE is cut away from the resisting prism, and the bearing area AO is reduced to EO. As a result, the ratio of forces changes, the equilibrium is disturbed, and collapse occurs. In the general case, the sliding surface AOC is a circular-cylindrical surface.

The volume of rock bounded by the slope and the indicated sliding surface collapses downward. In moving toward the base it acquires kinetic energy and fills the excavation in front of the bench for 15–30 m, not at the natural angle of repose but much flatter. The collapse does not occur all at once as an entire mass, but in successive parts in the direction toward the summit of the dump. At the same time, a considerable quantity of incandescent rock and coal dust rises into the air. They form entire clouds covering the waste heap.

It should be especially noted that when burnt rocks are extracted at the bottom, at the base of the waste heap, in addition to large collapses, large deformations of the following types may occur on the slopes of the head part of the dump: 1) dry thermal landslide-collapses and 2) rain-induced thermal landslide-collapses. In the tail part of the dump the indicated deformations were not observed.

To prevent accidents and injuries on waste heaps during their development, the following safety measures are recommended:

1. The working of waste heaps to obtain burnt rock must be carried out only according to a special design. In this design it is necessary to take into account the laws of equilibrium of the rocks and to specify concrete safety measures.

2. The working of waste heaps with unstable rock equilibrium (more than 40 m high) must be conducted with allowance for the possibility of other major deformations: dry thermal slide-collapses and rain-induced thermal slide-collapses.

3. The extraction of burnt rock on mine waste dumps at their base by means of excavators must be prohibited, in view of the high probability of major collapses dangerous to people and equipment.

4. When working high waste heaps (more than 40 m) in order to obtain a more stable equilibrium of the rocks, it is necessary first to cut down the upper part of the dumps by means of scraper installations, the winches of which are mounted on the ground surface at such a distance from the base of the waste heaps where there is no danger arising from major deformations [4].

5. After the height of the waste heaps has been reduced to 35—40 m, they can safely be worked by excavators in horizontal layers from top to bottom, with preliminary cooling of the burnt rock with water.

Conclusions

1. Major collapses during the working of waste heaps below, at their base, are not accidental, spontaneous events, but entirely regular phenomena. They sometimes cause major accidents with human casualties. Therefore, the indicated deformations, together with others—dry and rain-induced thermal slide-collapses—must be taken into account first and foremost when preparing designs for the extraction of burnt rock on waste dumps.

2. It is necessary to put an end to artisanal practices in the matter of working waste heaps. The extraction of burnt rock may be carried out only according to special designs. In these designs, the principal parameters must be determined with allowance for the laws and data on rock equilibrium, and concrete safety measures must be provided.

3. The extraction of burnt rock on waste heaps by a bench at their base is not guaranteed against major accidents and mishaps. Therefore this method of extraction must be prohibited and replaced by safer ones.

4. The procedure for work on the safe extraction of burnt rock from waste heaps in Kuzbass may also be recommended for other coal districts, for example, for the Karaganda district, where the rocks of mine dumps do not differ substantially from the rocks of Kuzbass.

REFERENCES

1. G. I. Knigina. Construction materials from burnt rocks of Kuzbass. Kemerovo, 1957.

2. Yu. N. Malyushnitskii. On the question of the stability of quarry slopes. Ugletekhizdat, 1952.

3. P. A. Leonov. Major deformations on burning rock dumps of Kuzbass mines. TsITIuglya, Moscow, 1959.

4. P. A. Leonov. Safe methods for eliminating burning waste heaps. Izvestiya vuzov, Gornyi zhurnal, No. 2, 1961.