ON THE QUESTION OF THE CLASSIFICATION OF ROCKS OF THE PROKOPYEVSK DEPOSIT BY DRILLABILITY

M. K. Tsekhina

(Presented by the scientific seminar of the departments of the development of bedded deposits, the development of ore deposits, mine construction, safety engineering, and mine ventilation)

The rates of driving horizontal workings through rock in the mines of the Prokopyevsk mine administration determine, to a considerable degree, the progress of extraction work.

As a result of the author’s analysis of the heading cycle, from the standpoint of the expenditure of time and labor on individual operations, it was established that drilling operations are the most labor-consuming and lengthy. Depending on the strength of the rocks and the number of column-type electric drills operating simultaneously at the face, the process of drilling blastholes under Prokopyevsk conditions occupies up to 45–65% of the cycle time.

The low rate and high cost of drilling blastholes, as established as a result of the studies carried out, are explained by the discrepancy between the operating conditions of electric drills and the mining-geological conditions, and by the incorrect standardization of drilling operations, carried out on the basis of the departmental classification of rocks [1], shown in Table 1.

Table 1

The classification of rocks adopted in Kuzbass is subjective in nature. For assigning rocks to one group or another under this classification, there are no objective and quantitative indicators. In classification, only the external features of these rocks are taken into account.

This state of affairs with the classification of rocks leads to a whole series of errors and abuses in the standardization of mining operations and especially of blasthole drilling. In mines, rocks are often assigned to higher categories, and drilling work is paid for at higher rates. Thus, of 94 workings in rock studied by the author during 1952–1954, in 70 cases the work was standardized for rocks of the second category, in 21 cases—for the first category, and only in three cases—for the third category. Thus, in the overwhelming majority of workings, when drilling work was being standardized, the rocks were assigned to the strongest, although in a number of cases the drilling passports for these rocks provided for drilling blastholes with hand electric drills as for rocks of categories IV and V.

To construct a classification of the rocks of the Prokopyevsk deposit by drillability, we carried out a number of investigations. The most characteristic rocks were subjected to microscopic analysis and studied with respect to their strength and hardness. For microscopic analysis, 64 thin sections were used. The temporary compressive strength and hardness of the rocks were determined on samples of these rocks made in the form of cubes (49 pieces) and plates (24 pieces). Analysis of the investigations, as well as study of the geological structure of the deposit, made it possible to establish the following.

1. The principal rocks of the coal-bearing deposits of the Prokopyevsk deposit are rocks of sandy and clayey composition. In the mines of the mine administration, crosscuts, field drifts, and other mine workings are driven mainly through these rocks.

The predominant rocks in the section of the coal-bearing series are sandstones. They occupy 57.7% of the thickness of the series. A considerable part of it (32.9%) is represented by aleurolites and argillites and, finally, 9.4% by coal seams.

1. The content of silicon dioxide in most sandstones and sandy-clayey rocks exceeds 10% and reaches 60–70%.

2. The rocks of the deposit have a highly varied structure (Figs. 1, 2, 3, 4, 5, 6), changing from silty (Figs. 1, 2, 3) to coarse- and very coarse-grained (Figs. 5, 6).

3. Owing to the large quantity of quartz grains, the rocks of the deposit possess increased abrasiveness, which strongly affects the wear of cutters and the results of drilling.

4. The Shore hardness of the rocks studied ranges from 26 units for carbonaceous argillite (Fig. 1) to 81 units for very coarse-grained quartz sandstone occurring in the roof of the Kharakterny seam (Fig. 6). The greater part of the sandy and sandy-clayey rocks of the deposit has an average Shore hardness within the range of 35–65 units.

5. The temporary compressive strength of the rocks studied is observed within the range of 246–1762 kg/cm². For the greater part of these rocks it varies from 450 to 1250 kg/cm².

In the course of the laboratory investigations carried out on the physicomechanical properties of the rocks of the Prokopyevsk deposit, as well as the investigations of the drillability of these rocks conducted under the conditions of the mines of the mine administration, it was found that the strength, hardness, and abrasiveness of the rocks ...

Fig. 1. Carbonaceous argillite. M. I. Kalinin Mine. Main crosscut. Seam 24. Nicols X

Fig. 2. Fine-grained structure of sandy argillite. M. I. Kalinin Mine. Crosscut No. 11. Seam 24. Nicols X

Fig. 3. Argillaceous sandstone. “Chernaya Gora” Mine.
Crosscut No. 18. Magn. 24. Nic. X

Fig. 4. Structure of quartz sandstone from the “Chernaya Gora” mine. Field crosscut. Carbonate cement.
Magn. 24. Nic. X

Fig. 5. Medium-grained quartz sandstone.
“Yuzhnaya” Mine. Crosscut No. 2. Calcite cement.
Magnification 24. Nicols X.

Fig. 6. Coarse-grained structure of quartz sandstone of the roof of the Characteristic seam.
M. I. Kalinin Mine. Siliceous-carbonate cement.
Magnification 24. Nicols X.

rocks exert a combined influence on the principal indicators of rotary drilling and determine the drillability of the rock. Neither temporary compressive strength, nor hardness, nor the external features of the rock, without taking its abrasiveness into account, can serve as indicators of the drillability of sandy and sandy-clayey rocks. For abrasive rocks of the sandstone type this proposition is also confirmed in the works of L. A. Schreiner [2] and E. F. Epstein [3].

The classification adopted at the mines of the mine group is built on a basis such that the physical-mechanical properties of rocks most important for assessing drillability—strength, hardness, and abrasiveness—are essentially not taken into account. For assessing the drillability of sandy rocks, the classification of Professor M. M. Protodyakonov also proves of little use. An attempt to classify the rocks of the Prokopyevsk deposit according to the criterion of feed of the cutter per one revolution, made by the Kuznetsk Scientific-Research Coal Institute, did not lead to a positive solution, since with drills having mechanical spindle feed this criterion of rock drillability is of little suitability. In addition, the indicators of specific feeds were obtained as a result of drilling blastholes with core drills having different numbers of revolutions and different axial spindle-feed forces; the standard character of the drilling tool used was not observed.

Other classifications of rocks by drillability, such as the “Classification of soils (rocks) according to unified norms of output and rates for 1944” [4], and the “Unified classification of rocks by drillability,” likewise cannot be used to assess the drillability of the sandy-clayey rocks of Kuzbass, since in compiling them the data taken as a basis were not for rotary drilling, but for percussion drilling.

To establish the drillability of the rocks of the Prokopyevsk deposit, it proved possible partially to use the method for determining the categoricity of rocks proposed by Prof. A. F. Sukhanov [5].

The method of classifying rocks by drillability adopted by us was based on the principle of determining the specific energy expenditures for drilling one unit of length of a blasthole. The energy consumption of drilling one unit of blasthole length fairly fully covers the set of factors influencing the relative drillability of a rock, and can be calculated from the equation:

\[ \omega = \frac{w \cdot t}{l} \quad \text{or} \quad \omega = \frac{w}{v}\left(\frac{\text{watt min}}{\text{cm}}\right), \]

where \(\omega\)—energy expenditure for drilling one cm of blasthole, \(\dfrac{\text{watt min}}{\text{cm}}\);

\(w\)—average load on the motor of the electric drill, watts;
\(t\)—drilling time, min;
\(l\)—length of the blasthole, cm;
\(v\)—average drilling speed, cm/min.

Here it is taken into account that the stronger the rock with respect to drillability, the lower the drilling speed and the higher the consumption of electrical energy observed when drilling a blasthole of established length with an electric drill of definite mechanical characteristics. The influence of cutter wear when drilling such abrasive sandy rocks as the rocks of the Prokopyevsk deposit is, in this case, reflected in changes in the drilling speed and in the load on the motor of the electric drill.

The following are adopted as constant parameters of the drilling regime: number of revolutions \(n = 300\) per minute, axial force \(P = 500\) kg, and specific spindle feed \(h = 0.9\) mm/rev in the dry method of drilling horizontal blast-holes up to 40 cm deep, perpendicular to the face plane.

The number of such blast-holes at the face is five or six. The first 20 cm of each blast-hole are drilled with an auxiliary new bit without measuring the indicators. The drilling of the subsequent 20 cm of the blast-hole is carried out with a new standard RP-type bit, with measurement of the time and the load on the electric drill motor.

The standard spiral drill rod must have a length of 60 cm and permissible wear of the ribs around the circumference of not more than 10%. The drilling time is measured with a stopwatch, and the average consumed power with a VIT-type wattmeter or a self-recording wattmeter. Both types of wattmeters, for work in dust-and-gas mines, must be of gas-safe design. Adjustment of the EBK-2M drill to the specified parameters of rotational speed and specific spindle feed is performed by installing standard factory pairs of gears in the reducer; as for adjustment of the spring of the friction mechanism to an axial force of 500 kg, it is carried out by means of a stand with an oil pressure gauge, the diagram of which is shown in Fig. 7.

Fig. 7. Diagram of the setup for regulating the axial force of spindle feed. 1—electric drill; 2—tightening clamps; 3—spindle; 4—support bearing; 5—crosspiece with rod; 6—cylinder; 7—pressure gauge

Such a drilling regime is not optimal for all rocks of the Prokopyevsk deposit; however, it makes it possible to classify these rocks fairly clearly according to their relative drillability and to determine for each of them a rational drilling regime, taking account of which drilling operations are subsequently planned and standardized.

The work carried out on testing the rocks of the Prokopyevsk deposit made it possible to classify them by drillability into five most characteristic categories. The proposed classification of rocks is given in Table 2.

Rocks assigned to the first category are especially difficult to drill. Their specific energy consumption is 180 and more \(\dfrac{\text{W·min}}{\text{cm}}\).

Such rocks include very strong quartz sandstones with siliceous and siliceous-calcareous cement, very strong quartz sandstones with calcareous cement, and other especially difficult-to-drill sandy rocks. Under the conditions of the Prokopyevsk deposit, such rocks occur in the roof of the Kharakternyi seam; they were also encountered during the driving of crosscut No. 2 at the “Yuzhnaya” mine.

Predominant under the conditions of this deposit are rocks of the second, third, and fourth categories, with characteristic for them

Table 2

Classification of rocks of the Prokopyevsk deposit by drillability

with a specific energy consumption of drilling from 179 to 40 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$. These rocks were traced in the driving of many horizontal workings in the mines of the mine field. Thus, for sandy argillite (crosscut No. 11, M. I. Kalinin mine) the specific energy consumption proved to be equal to 43 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$; for a fine-grained sandstone of the interbed between the Kharakterny and 1-Internal seams—74 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$; for a fine-grained sandstone of the interbed between the Gorely and Kharakterny seams—68 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$; for a fine-grained sandstone in the field drift of the “Chernaya Gora” mine—116 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$; for clayey sandstone (crosscut No. 18 of the same mine)—45 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$; for sandy-clayey shales (field drift, “Yuzhnaya” mine)—62 $\dfrac{\text{W}\cdot\text{min}}{\text{cm}}$, and others.

The proposed classification of the rocks of the Prokopyevsk deposit by drillability rather fully covers the natural features of these rocks, gives an objective characterization of their relative resistance to destruction in the process of rotary drilling,

makes it possible, for each rock, to determine the optimal drilling regime and to plan and standardize drilling operations most correctly.

REFERENCES

1. Standard-Setting Handbook for Mining Operations in the Mines of the Kuzbassugol Combine. Ugletekhizdat, 1948.

2. Schreiner, L. A. Physical Foundations of the Mechanics of Rocks. Gostoptekhizdat, 1950.

3. Epstein, E. F. Theory of Drilling–Cutting of Rocks with Hard Alloys. GONTI, 1939.

4. Handbook of the Mining Engineer. Ugletekhizdat, 1950.

5. Sukhanov, A. F. Drillability and Blastability of Rocks. Gostoptekhizdat, 1940.