DETERMINATION OF THE SAFE HEIGHT OF MINE CONICAL ROCK DUMPS (TERRICONS) FOR KUZBASS CONDITIONS

P. A. LEONOV

The question of determining the safe height of mine rock dumps has until now been little studied and poorly developed. Correct calculation of the height of a rock dump is of great importance for operation and work safety. Dumps of small height require considerable areas, and their operation is much more expensive than that of dumps of great height. On the other hand, high conical dumps, as practice shows, are more dangerous than low dumps.

The optimal height of a rock dump should be considered to be that at which work safety, the lowest capital costs, and minimal operating expenses for dump formation are ensured.

Recently several major accidents have occurred at the rock dumps of Kuzbass mines; analysis of them gives grounds to assert that, in determining the height of a dump, the decisive factor must be the question of safety. From the standpoint of the amount of capital costs and operating expenses, conical dumps of great height will be the most economical. But on such dumps the equilibrium conditions of the deposited rocks are often disturbed and large deformations occur. These are the main causes of accidents and casualties.

Thus it may be said that determination of the safe height of a dump must be closely linked with establishing the conditions of equilibrium of the rocks in the dump.

To investigate the indicated conditions, a dump of conical form was taken (Fig. 1). It consists of three zones: the lower (I), composed of coarse-lump material; the middle (II), where rock of medium coarseness and fines predominate; and the upper (III), of pieces of rock of different sizes. The boundaries of the zones are shown by dotted lines. OBC is the transverse outline of a funnel-shaped depression during deformation of the dump slope.

The general equation of equilibrium of a mass of rock separating from the slope, proposed by Bernatskii [1], consists of three parts: 1) the product of the weight of the “abutment” prism by the tangent of the difference between the angle of friction between the bearing surfaces of this prism and the angle of inclination of this surface to the horizontal, minus; 2) the product of the weight of the prism of active pressure by the tangent of the difference between the angle \(45^\circ\) and half the angle of internal friction of the rock, plus; 3) twice the product of the values of the surface

the collapse force within the prism of “active pressure” and the cohesion force of the rock per unit area. Equilibrium is not disturbed if the indicated equation is equal to or greater than zero. The dangerous collapse surface passes at an angle of \(45^\circ\) minus half the angle of internal friction. The most severe equilibrium conditions occur when the boundary plane between the prisms passes through point “B,” where

Fig. 1. Section of a rock dump along the longitudinal axis.
OBC — deformation funnel; OBA — “abutment” prism;
ABC — prism of “active pressure”; BC — collapse surface;
\(\beta\) — angle of inclination of the abutment surface to the horizon

the collapse surface changes its inclination, encountering the supporting surface of the “abutment” prism. In the field, at point “B” a cylindrical collapse surface is obtained. The lateral faces of the prisms are at first almost vertical; then, as a result of rock collapse, they become inclined, and the depression occupied by the prisms of the displaced mass of rock assumes the form of a funnel.

The degree of stability of the dump slope is directly dependent on the magnitudes of the angles of internal friction, friction between the abutment surfaces of the “abutment prism,” the cohesion force, and the weight of the “abutment” prism, and inversely dependent on the value of the weight of the prism of “active pressure” and on the angle of inclination of the abutment surface to the horizon; in cohesive rocks it also depends on the height of the dump.

The indicated quantities may change under the influence of moisture, external loading, hydrostatic and hydrodynamic pressure, and burning of the rocks. Increased moisture and the burning of rocks in burning dumps reduce the magnitudes of the angles of internal friction, the friction between the abutment surfaces of the “abutment prism,” and the cohesion force.

The following deformations are observed on mine rock dumps: ejections, landslides, collapses, and a number of others. Their study has shown that: 1) the dump has coarse lump material at the bottom, medium-sized material and fines in the middle, and an unsorted mass of rock at the top; 2) intensive burning is observed in the middle zone of the dump; 3) at the bottom, lump material, under the action of high temperature and pressure, sinters and compacts; 4) the loosest material after burning is obtained in the middle zone; here the magnitude of the angle of internal friction and the cohesion force changes, collapses and combined deformations occur; 5) observations of the angles of natural slope on models

... established for burned rocks a reduction of these angles by \(2\)—\(2.5^\circ\); 6) large deformations on dumps in clear weather occur as a result of changes in the angle of internal friction and in the cohesion force as a result of firing and pressure; 7) deformations on dumps during downpours and heavy rains are ejections or combinations of ejection, landslide, and collapse; 8) in this case water increases the moisture content of the rocks, reduces their cohesion, and 9) steam formed when water falls on incandescent rocks has the same effect.

There are many different methods for calculating the height of a dump: Terzaghi’s, Fallekus’s, Maslov’s, Jaky’s, and others. Popov [2], comparing calculations by these methods, came to the conclusion that the deviations obtained by the indicated methods are insignificant (\(\pm 15\%\)). Popov [2] and Milonitskii [3], for calculating the height of dumps, recommend using Jaky’s method as the simplest.

Jaky’s formula is a fraction in which the numerator is the product of the coefficient 4, the magnitude of the cohesion force of the rock, and a multiplier depending on the slope angle, internal friction, and cohesion force; while the denominator is the value of the unit weight of the rock. The indicated multiplier is determined from specially compiled graphs [2].

Below are given the results of determining the height of a dump by the indicated formula for the following specific conditions: the dump is composed of rock materials with \(5\)—\(10\%\) clay particles, the slope angle is \(40^\circ\), the angle of internal friction is \(30^\circ\), the unit weight is \(1.6\ \text{t}/\text{m}^3\), the cohesion force is \(0.5\), \(0.8\), and \(1.1\ \text{t}/\text{m}^2\), and the multiplier is 20. The value of the height according to the calculations is \(25\), \(40\), and \(55\ \text{m}\), with an average of \(40\ \text{m}\). As is evident from the results of the calculations, a change in the cohesion force strongly affects the height of the dump.

The complex structure of the body of mine dumps, and the presence in them of rocks with the most diverse properties, make it impossible to use, for calculating the optimum height, the formulas adopted for designing dumps in open-pit workings and earthworks in railway construction. The indicated formulas should be considered suitable only for approximate calculations. The limiting height of flat dumps of rock materials with an external load in quarries and open cuts is recommended by Mel’nikov [4], Sheshko [5], Zurkov [6], and others to be on the order of \(25\)—\(30\ \text{m}\). The Rules for Technical Operation [7] determine this value as \(30\)—\(40\ \text{m}\). According to the same sources, the height of dumps formed with the use of transport-dump bridges should not exceed \(40\)—\(60\ \text{m}\).

Thus, it may be considered that approximate calculations and the practice of open-pit work determine the maximum height of conical dumps of waste rock as \(40\ \text{m}\) (allowing for the external load in the form of a hoisting skip installation).

It is necessary to check this height by the safety factor. For this purpose, the results of a study of 129 rock dumps at Kuzbass mines were used. Their data are as follows: 1) there were no landslides or landslide-collapses on dumps \(30\ \text{m}\) high; 2) on waste heaps where the height was \(31\)—\(40\ \text{m}\), there were observed three ejections and landslides without casualties, and two collapses in clear weather (deformation in the form of a bench one meter high); 3) on dumps \(41\)—\(80\ \text{m}\) high, ten large deformations were recorded in various weather conditions, with a volume of displaced rock from \(500\) to \(8000\ \text{m}^3\) and with casualties, and two in clear weather; 4) the runout of pieces of rock during the indicated deformations at dumps \(40\ \text{m}\) high was \(20\ \text{m}\), while at dumps \(70\)—\(80\ \text{m}\) high it reached \(60\ \text{m}\); 5) the distance of the dispersion of dust and fines was respectively \(60\) and \(200\ \text{m}\).

Analysis of the factual data presented shows that on dumps up to 40 m high there are no major deformations and no accidents. Dumps of great height, especially 70–80 m, have favorable conditions for the occurrence of major accidents; they are often the causes of serious and fatal accidents.

Dumps up to 30 m high do not undergo significant deformations either from external loading or as a result of combustion of the rock and other causes. They are also acceptable from the standpoint of safety. A substantial shortcoming of these dumps should be considered their small volume and the large area of ground surface under the base (foot). Dumps up to 40 m high have somewhat greater deformations than waste heaps 30 m high. In terms of safety, dumps of 30–40 m should be considered identical.

A major advantage of 40 m dumps is the more economical use of the ground-surface area and the sharp increase in their volume.

The investigations carried out give grounds, proceeding from the requirements of safe operation of conical rock dumps, to determine the maximum (safe) height of these dumps as 40 m.

Conclusions

1. Deformations of the landslide type, landslide–collapse type, and combined types on burning rock dumps 30–40 m high occur considerably less often, and in magnitude these deformations are smaller than on high dumps of 60–80 m.

2. The danger of deformations on dumps 30–40 m high is very slight; there have been no accidents there. On large dumps (60–80 m high), some deformations (landslides, a combination of outburst, landslide, and collapse) are distinguished by their large scale, are very dangerous, and are the cause of accidents.

3. Calculations, data from open-pit practice, and investigations of equilibrium and work-safety conditions give grounds for determining the maximum height of mine rock dumps as 40 m.

LITERATURE

1. Bernatskii L. N. Conditions of Stability of Earth Masses, 1925.

2. Popov S. I. On the question of calculating the height of a dump bench. Scientific Works of the Magnitogorsk Mining and Metallurgical Institute. Issue II, 1945.

3. Miliushitskii Yu. N. On the question of the stability of quarry slopes. Ugletekhizdat, 1952.

4. Melnikov N. V. Handbook for Engineers and Technicians on Open-Pit Mining Operations. Ugletekhizdat, 1956.

5. Sheshko E. F. Open-Pit Mining of Mineral Deposits, 1954.

6. Zurkov P. E. Development of Ore Deposits by the Open-Pit Method. Metallurgizdat, 1953.

7. Rules of Technical Operation in the Development of Coal Deposits by the Open-Pit Method. Ugletekhizdat, 1956.