Study of Friction Angles of Crushed Rocks Used for Backfilling at Mines of the Prokopyevsk Deposit in Kuzbass
Abstract Generated abstract
This study examines the friction angles of crushed argillite and sandstone backfill materials from the Novo-Usyatsky quarry used in mines of the Prokopyevsk deposit in Kuzbass. Using an experimental tilting-bottom apparatus, the author measured friction angles for different particle-size classes and mixtures on iron, smooth and unfinished wood, concrete, and crushed-rock surfaces, with additional tests on the effect of moisture. The results show that sliding-surface material has the strongest influence, followed by particle size and moisture: friction angles decrease with increasing particle size, rise and then fall with increasing water content, and are greatest on rougher or rock-fill surfaces. The paper provides coefficients of friction and recommended inclination angles for gravity transport and backfilling devices, comparing calculated values with mine practice.
Full Text
STUDY OF THE FRICTION ANGLES OF CRUSHED ROCKS USED FOR BACKFILLING IN THE MINES OF THE PROKOPYEVSK DEPOSIT OF KUZBASS
Yu. A. Ryzhkov
(Presented by the scientific seminar of the Department of Mining of Bedded Deposits)
The angle of friction of bulk materials is an important parameter in the calculation and design of devices for transport under the action of their own weight. The value of the angle of friction must be known, for example, when designing various hoppers, chute devices, bunkers, ramps, pipe chutes, screw and stepped descents, as well as when selecting mining systems with backfilling of the worked-out space.
The rubbing surfaces encountered in backfilling work are most often, on the one hand, various crushed country rocks, sand, and slags; and, on the other hand, wood, iron, concrete, reinforced concrete, and the ground of mine workings. In some gravity-flow devices—for example, stepped descents—the sliding surface consists of broken rock of various sizes.
The angle of friction of bulk materials in air depends on the material of the sliding surface, the petrographic composition, size, moisture content, shape, character of the surface, strength, specific weight, and temperature of the particles.
Most researchers who have studied this index of bulk bodies come to the conclusion that, for rocks similar in petrographic composition, the angle of friction depends mainly on the material of the sliding surface, the size, and the moisture content of the particles [1, 2, 3]. Studies carried out with the aim of establishing the possibility of using gravity backfilling on an inclined dip in the Ruhr mines showed that the angle of friction depends mainly on the material of the sliding surface [4].
The other factors listed above also influence the angle of friction; however, only very general and, in most cases, qualitative data about them are given in the literature. For example, spherical, rounded, cubic, and rhombic particles roll at a smaller angle of inclination of the surface than flat, elongated pieces [5, 6]. Particles with a smooth surface have a lower coefficient of friction [6].
This paper presents the results of a study of the friction angles of crushed country rocks from the Novo-Usyatsky quarry, used—
... in the mines of the Prokopyevsk deposit of Kuzbass, as a function of size and moisture content, on wooden, iron, concrete, and rock-fill bottoms.
Properties of the rocks
The rocks of the Novo-Usyatsk backfill quarry are represented mainly by argillites (70%) and sandstones. Hardness on the Mohs scale is 4–8; compressive strength is 400–1100 kg/cm². The shape of the pieces is more often elongated-platy, less often platy, cubic, and columnar. The pieces are sharp-edged, with a conchoidal fracture. The bulk density of the rocks in a loosened state and the natural moisture content of rocks of different sizes are presented in Table 1.
Table 1
| Rock size, mm | Bulk density, t/m³ | Moisture content, % |
|---|---|---|
| 0—3 | 1.43 | 3.04 |
| 3—6 | 1.17 | 2.53 |
| 6—13 | 1.21 | 2.11 |
| 13—25 | 1.26 | 1.84 |
| 25—50 | 1.26 | 1.65 |
| 50—100 | 1.28 | 1.08 |
Sampling and preparation of specimens
Sampling of specimens for studying the friction angles was carried out at the crushing plant of the Novo-Usyatsk quarry and at Plant No. 2 of the Usyatsk Quarry Administration. From a large number of partial samples, two averaged mixtures were compiled, representing: the first, the charge for gravity backfilling; the second, the charge for hydraulic (pneumatic) backfilling. The average size of mixture No. 1 was 63.5 mm; of mixture No. 2, 21.9 mm. To obtain homogeneous size classes, the material was screened on hand sieves with opening diameters of 3, 6, 13, 25, 50, and 100 mm.
Method for determining friction angles
To determine the friction angles of crushed backfill materials, the method of the Institute of Mining, Academy of Sciences of the USSR, which had conducted analogous studies with iron ores [2, 3], was adopted. The experimental setups for determining the friction angles of ores and backfill materials differed from one another structurally. The latter consisted of a frame in which a bunker with a discharge opening closed by a slide gate was rigidly fixed. To the lower part of the bunker a wooden bottom with an inclinometer was attached on hinges, making it possible to determine the angles of inclination of the bottom. Lowering the bottom and raising the rock into the bunker were carried out by means of a hand winch connected by a metal cable and a pulley to the bottom or bucket. The design of the setup made it possible to study the regularities of the movement of crushed rocks over wooden, iron, concrete, and rock-fill bottoms. To the wooden bottom, made of untreated boards with dimensions of 950×500 mm, an iron sheet was nailed or a platform with concrete was inserted. The composition of the concrete—
gravel with particle sizes of 0.5–25 mm, sand, and cement. The hopper bottom was a wooden box with dimensions corresponding to the dimensions of the bottom and a height of 60 mm. The box was divided by partitions into 3 compartments, into which rock of different sizes was poured.
On average, 150 kg of rock was loaded into the hopper. The weight of a portion for the fine fractions was taken to be smaller than for the coarse fractions. In each experiment, one portion was discharged from the model.
The start of the experiments was preceded by loading the hopper with rock, setting the bottom in a horizontal position strictly according to the level and adjusting the inclinometer. Then the bottom, with the aid of a winch, was slowly lowered until the mass of rock located on the bottom and in the hopper began to slide along the inclined plane. At this same time the angle of inclination of the bottom (the friction angle) was recorded with the aid of the inclinometer. The accuracy of measuring the angle of inclination of the bottom was 30 sec. The experiments for each homogeneous class and for the mixtures were repeated from 4 to 8 times. The friction angles were taken as averages. About 350 experiments were carried out with rocks of natural moisture content.
The dependence of the friction angle on moisture content was obtained for mixture No. 1 and for the 12–25 mm class on the iron bottom.
Results of the experiments
The friction angles of rocks of natural moisture content from the Novo-Usyatskii stowing quarry, as a function of size and sliding surface, are presented in the graph (Fig. 1), and the corresponding coefficients of friction in Table 2.
Table 2
| Rock size, mm | Iron | Smooth wood | Unfinished wood | Concrete |
|---|---|---|---|---|
| 0–3 | 0.56 | 0.61 | 0.82 | 0.87 |
| 3–6 | 0.51 | 0.57 | 0.75 | 0.77 |
| 6–13 | 0.50 | 0.56 | 0.73 | 0.74 |
| 13–25 | 0.49 | 0.54 | 0.68 | 0.70 |
| 25–50 | 0.48 | 0.54 | 0.64 | 0.68 |
| 50–100 | 0.45 | 0.53 | 0.63 | 0.68 |
| Mixture No. 1 | 0.47 | 0.55 | 0.63 | 0.69 |
| Mixture No. 2 | 0.49 | 0.57 | 0.64 | 0.72 |
Experimental studies of the friction angles of stowing materials confirmed the regularities established by the Institute of Mining, Academy of Sciences of the USSR, for ores. Analyzing the experimental data, it should be noted that the friction angle (and, correspondingly, the coefficient of friction) is most strongly affected by the character of the sliding surface. If the average value of the rock friction angle for homogeneous classes on iron is taken as 100%, then the friction angles for other surfaces increase as follows: for smooth wood by 12%, unfinished wood by 34%, and concrete by 38%.
It should be noted that in the studies of the Institute of Mining, Academy of Sciences of the USSR, the friction angles of iron ores on a wooden bottom proved to be greater than on a concrete one; in our case, however, the opposite is true. This is explained by the different quality of the sliding surface and is confirmed by studies of the friction angle on wood with smooth and unfinished surfaces (Fig. 1).
With an increase in the size of the homogeneous classes, and also with an increase in
the proportion of coarse fractions in the mixtures, the angle of friction decreases. In the investigated size interval of homogeneous classes, the difference between the values of the minimum angles of the extreme classes (the coarsest, 50–100 mm, and the finest, 0–3 mm) is: for an iron bottom—5°, for wood with a smooth surface—3.5°, for wood with an untreated surface—7.5°, for concrete—7°.
Fig. 1. Dependence of the friction angle of rocks of natural moisture content on particle size and sliding surface:
1—on concrete; 2—on untreated wood; 3—on smooth wood; 4—on iron.
The friction angle of mixture No. 1 is 2–3° smaller than that of mixture No. 2 for all sliding surfaces.
As can be seen from Fig. 1, the friction angle decreases most sharply with an increase in size up to the limit corresponding to the 13–25 mm class. For example, in the case of sliding on a concrete surface, the friction angle of the 0–3 mm fractions was 41°, whereas already for the 13–25 mm fractions it decreased to 35°, varying only within insignificant limits for coarser fractions. An analogous regularity is also observed for the other sliding surfaces.
The presence of moisture in the rocks has a considerable effect on the magnitude of the friction angle (Fig. 2). An increase in the amount of water initially leads to an increase in the friction angle, and then to its decrease. This effect is manifested to a greater degree in fine fractions and mixtures and to a lesser degree in coarse homogeneous classes. Subsequent addition of water to coarse-lump materials has almost no effect on the friction angle, whereas fine fractions acquire flow properties.
The data from the study of the friction angles of rocks of various sizes on the fill bottom are given in Fig. 3, and the corresponding values of the coefficients of friction are given in Table 3.
The friction angles (and, correspondingly, the coefficients of friction) increase with a decrease in the size of the discharged material and with an increase in the size of the rocks of the fill bottom.
The experimentally obtained friction angles may be used in establishing the minimum inclination angles of various
devices for transport under the action of the rocks’ own weight. The minimum angles of inclination are recommended to be taken as 5–10° greater than the angle of friction of the rocks [2, 3, 5].
Fig. 2. Dependence of the rock friction angle on moisture content on an iron bottom: 1—mixture No. 1; 2—class 13–25 mm
Table 3
| Rock size, mm | Bottom material—crushed rocks of various sizes, mm | Bottom material—crushed rocks of various sizes, mm | Bottom material—crushed rocks of various sizes, mm | Bottom material—crushed rocks of various sizes, mm | Bottom material—crushed rocks of various sizes, mm |
|---|---|---|---|---|---|
| Rock size, mm | 0–6 | 6–13 | 13–25 | 25–50 | 50–100 |
| 50–100 | 0.8 | 0.81 | 0.83 | 0.9 | 0.95 |
| 25–50 | 0.81 | 0.83 | 0.87 | 0.92 | 1.0 |
| 13–25 | 0.82 | 0.84 | 0.90 | 0.98 | 1.09 |
| 6–13 | 0.87 | 0.90 | 0.97 | 1.05 | 1.19 |
| 0–6 | 0.92 | 0.99 | 1.04 | 1.07 | 1.24 |
Table 4
| Mine | Structure or working | Sliding surface | Angle of inclination, deg |
|---|---|---|---|
| 1 | 2 | 3 | 4 |
| Koksovaya-1 | Bunker | Wood covered with iron sheets | 50 |
| Koksovaya-1 | Raise along the Lutuginsky seam | Concrete, reinforced concrete | 50–58 |
| Koksovaya-1 | Raise along the Provodnik seam | Metal pipes 50 mm in diameter | 55 |
| Koksovaya-1 | Manway of raise along the Lutuginsky seam | Iron sheets | 48 |
| 1 | 2 | 3 | 4 |
|---|---|---|---|
| Bunker | Concrete covered with iron sheets | 50 | |
| 3-3-bis | Skat-bunker | Reinforced concrete | 50 |
| 3-3-bis | Hatch of stowing shaft No. 5 | Iron sheets | 45 |
| 3-3-bis | Inclined layers with gravity stowing along the Moshchnyi seam | Wooden chutes | 40--46 |
| Chernaya Gora | Long pillars with gravity stowing along the Dvoinoi seam | Wooden chutes | 39--45 |
| Chernaya Gora | Diagonal entries with gravity stowing along the Dvoinoi seam | Wooden flooring | 45 |
Table 4 summarizes data on the minimum angles of inclination used in Kuzbass practice for various sliding surfaces in the transport of crushed rock from the Novo-Usyatsky quarry, while Table 5 gives the minimum angles of inclination established by calculation.
Vertical axis: Angle of friction of the rock, degrees.
Horizontal axis: Rock size, mm.
Fig. 3. Dependence of the angle of friction of rock on the filling bottom on rocks of different sizes.
The calculated values of the inclination angles for wooden and concrete sliding surfaces, as is evident from Tables 4 and 5, coincide with practical data. However, the inclination angles for iron surfaces are overestimated relative to the calculated ones.
Table 5
| Sliding surface | Friction angle of fine fractions, deg. | Calculated angle, deg. |
|---|---|---|
| Iron | 29 | 34—39 |
| Wood | 39.5 | 44.5—49.5 |
| Concrete | 41 | 46—51 |
REFERENCES
- Sinakovskii A. O., Mine Transport, Ugletekhizdat, 1953.
- Baron L. I., Simonyan E. A., Hatch Loading in Underground Ore Mining, Metallurgizdat, 1959.
- Golobokaya G. V., Inclination angles of ore-receiving and discharge workings in ore blocks. In: Problems of Mining, Ugletekhizdat, 1958.
- Experience in the use of gravity backfilling on inclined seams in the Ruhr Basin, “Bergfreiheit,” No. 6, 1954.
- Alferov K. V., Bins, Gates, Feeders. Mashgiz, 1946.
- Derkach V. G., Kopychev P. A., Special Methods of Beneficiation of Minerals, Metallurgizdat, 1956.