IMPROVING THE EFFICIENCY OF DRILLING AND BLASTING OPERATIONS IN OPEN-PIT MINES OF THE TOMSK DISTRICT

M. K. TSEKHIN

(Presented by the Scientific and Technical Conference on the Use of Mineral Resources of the Tomsk Economic District)

The extraordinary XXI Congress of the CPSU outlined a grand program of industrial and civil construction in our country. An important place in the fulfillment of this program is assigned to industrial and housing construction, as well as to road construction in Siberia. In recent years construction in Tomsk Oblast has noticeably expanded, especially in its regional center, the city of Tomsk. With each year the city’s need for local building materials increases; their extraction is carried out by blasting methods. The need is growing especially for rubble stone for laying foundations, construction crushed stone, and limestone for producing lime. Thus, whereas in 1940 the city consumed only 2,500–3,000 m³ of building stone, in 1959 its need reached 60,000 m³.

At present, the quarry in the Stepanovka district is still the only mining enterprise in Tomsk extracting rubble stone and crushed stone. The deposit is located near the city of Tomsk, 2 km east of the Tomsk-1 station.

The area of the quarry field is 27.5 ha. The site has the form of a steep watershed cape, washed on three sides by the waters of the Ushaika River and elongated in a northeastern direction. The site has steep slopes and flattens sharply toward the valley. On the north it borders the forest dacha of the Tomsk forestry enterprise. The deposit is being worked from the eastern and western slopes of the site. The main production structures, as well as residential premises, are located in the southern part of the deposit. The quarry is connected with the city by a motor road, which serves as the main route for transporting stone and crushed stone to construction sites.

Geologically, the Stepanovka rubble-stone deposit is represented by a sandy-clayey sequence of the Lower Carboniferous. The sandstones occur in clayey shales in the form of separate, strongly slaty beds in the transverse direction, with thicknesses ranging from several centimeters to 10 m and more. The entire sequence has a northeastern strike and a steep dip to the east at an angle of 53–89°.

The sandy-clayey sequence is cut by a dike of quartz monzonite-porphyry 27 m thick (including 6 m of weathered rocks suitable for processing into crushed stone). The length of the dike along the strike—

up to 189 m; the dip is almost vertical. The rock strength coefficient according to Prof. M. M. Protodyakonov’s scale ranges from 2 to 6. The thickness of the overburden is from 1.7 to 10.0 m.

The total reserves of the deposit as of January 1, 1957 amounted to 872,000 m³, of which about half are of industrial value. Most of the sandstone in the upper beds is of low quality, contains many interbeds of loam, and has insufficient mechanical strength (170–200 kg/cm²).

Mining operations in the quarry are carried out mainly in one shift. They are performed by a crew consisting of one driller, 3–4 blasters, a compressor operator, a shot-firer, an excavator operator, an assistant operator, and a mining foreman.

Stone breaking is carried out by blasting.

At the base of the face, which is a steep ledge (65–70°) up to 27 meters high, 1–2 rows of horizontal or slightly inclined blastholes 2.5–3.0 m long and 36–42 mm in diameter are drilled. The collapse of the ledge is achieved by the sequential detonation of charges, carried out by the fuse method. Each blasthole is loaded with 2.0–2.5 kg of ammonite No. 6.

As a result of primary loosening and subsequent scaling of the overhanging ledges and blocks at the base of the bench, a pile of crushed stone, rubble, and large blocks is formed, the linear dimensions of which range from several tens of centimeters to 2–5 meters. Characteristically, the large blocks obtained as a result of primary breaking here make up as much as 50–60% of the volume of the entire blasted rock mass, and secondary breakage with decked charges is required to bring them to standard size.

The consumption of explosives per cubic meter of rubble and crushed stone ranges from 200 to 300 grams.

During a shift, a driller using a handheld rock drill RP-17 or OM-506 drills 9–16 blastholes, 2.5–3.0 m long. With a shift standard of 54–82 m³ (depending on the rock strength), the actual yield of broken rock mass from a set of blastholes reaches 100–120 m³, and from 1 linear meter of blasthole, 2–4 m³.

The cost of stone, free-on-truck at the quarry loading site, in 1958 amounted to 10 rubles 99 kopecks per 1 m³.

Analyzing the drilling-and-blasting operations in the quarry, it is not difficult to see that the blasthole method adopted here for a bench almost 30 m high, composed of rocks prone to spontaneous collapse, is impractical and dangerous for the workers. There arises a need for repeated loosening of a large number of sandstone and montmorillonite porphyry blocks. Loading operations are difficult and dangerous. After the lower rows of blastholes are blasted, a steep overhang is formed in the bench, increasing the danger of spontaneous rock collapse during loading.

Owing to the presence in the roof of unremoved overburden deposits, the caved rock becomes contaminated with clay and turf; the quarry’s operation is complicated, and the quality of the stone material obtained is reduced.

Good blasting indicators are characterized by maximum safety and cost-effectiveness of the work. In this connection, the main requirements imposed on drilling-and-blasting operations in the quarry should be:

a) proper primary crushing of the blasted mass at a bench height optimal for the given conditions;

b) proper separation of the rock along the floor and preservation of the profile of the entire bench;

c) observance of the optimum width and height of the muck pile of the blasted mass;

d) the maximum reduction in the volume of auxiliary drilling and blasting, including blasting with surface charges;

e) ensuring productive operation of the excavator;

f) reduction of expenditures of time, labor, energy, and materials (including explosives) per unit of output.

The operating experience of many open pits of small and medium productivity convincingly shows that these requirements are feasible. In order to increase the efficiency of drilling-and-blasting operations in the open pits of the Tomsk economic district, it would first of all be necessary to replace the shothole method with the method of blasting column charges in small-diameter boreholes. This is especially important in connection with the planned development of dimension-stone extraction in the area of the large Larinskoye deposit of strong sandstones, 20 km from the city of Tomsk, and of limestone in the area of the Kamenskoye deposit.

Vertical and inclined boreholes 100 mm in diameter can be successfully drilled with the new high-performance BA-100 drilling unit. This machine uses the M-1700 percussion-rotary submersible pneumatic hammer. The unit consists of a machine mounted on an anchor column (for underground drilling conditions) or on a mobile carriage (under open-pit drilling conditions), a drill-rod string, and a pneumatic hammer with a drill bit. During drilling, the drill-rod string with the hammer fixed to it is set in rotation and fed to the bottom of the borehole. Compressed air and water (an air-water mixture) at a working pressure of 5–6 atm is supplied through the inner cavity of the rod. The machine is sufficiently convenient in operation and has a relatively low weight (220 kg).

Industrial tests of the unit at the Temir-Tau mine (Gornaya Shoriya) showed that even in hard rocks ($f = 10\text{--}18$) it provides a shift productivity of 12–23 linear meters of borehole per machine [1]. With an average yield of ore and rock of 10.2 m³ per linear meter of borehole, the shift productivity of the unit is 120–240 m³ and is very high in comparison with the indices obtained at the Stepanovskii open pit, although here the rocks are of insignificant strength ($f = 2\text{--}5$).

When using the BA-100 drilling unit, it is possible to adopt any bench height within limits of up to 30 meters and to drill boreholes parallel to the bench face.

The arrangement of boreholes parallel to the bench face and the dispersed arrangement of the charge along the entire length of the borehole, as shown by the experience of conducting blasting operations at the Blyavinskii open pit [2], makes it possible to improve rock fragmentation considerably, reduce secondary breakage and the specific consumption of explosives to a minimum, maintain a constant angle of the bench slope, eliminate the formation of overhangs, and reduce the labor intensity of cleaning the faces. It should be noted that for open pits of high productivity it is currently planned to use powerful self-propelled drilling machines BM-150 KS with the M-150 submersible hammer. According to the technical specifications, the productivity of this machine for rocks of strength 10–12 on the scale of Prof. M. M. Protodyakonov reaches 60–70 m per shift [3].

An important reserve in drilling-and-blasting operations when blasting charges in deep boreholes, as has been established by numerous data from domestic and foreign practice, is also short-delay blasting of charges [4, 5, 6, 7]. At present, standard electric detonators for short-delay blasting are being supplied to the mining enterprises of the country—

of short-delay action with delay intervals of 25, 50, 75, 100, 150, and 200 msec. Experience shows that a delay interval of 25 msec for the successive blasting of charges in boreholes spaced 5–6 m apart is optimal.

Short-delay blasting makes it possible, to a considerable extent, to control the action of the explosion; it ensures a reduction in the seismic effect, a decrease in the yield of oversize material, and the obtaining of a more compact muck pile of the blasted rock mass. It substantially reduces the formation of backbreak; the face is better prepared and contoured for subsequent drilling of boreholes. The possibility is provided for effective blasting both with single-row and multi-row arrangements of boreholes. As the operating experience of the quarries of the Soyuzasbest trust, the Amvrosievka, Karakub, Kara-Tau, and many other quarries, including foreign ones, shows, short-delay blasting makes it possible to conduct mining operations with greater safety, to increase labor productivity significantly, and to reduce the cost of extracting the useful mineral.

The scientific and technical conference on questions of the theory and practice of short-delay blasting, held on November 26–28, 1958, in Moscow, convened by the Interdepartmental Commission on Blasting of the Academy of Sciences of the USSR and by the All-Union Scientific and Technical Mining Society, recommended the wide introduction of this blasting method into the practice of both underground and open-pit mining operations.

According to the conclusion of the Production and Experimental Administration of Soyuzvzryvprom, made on the basis of numerous observations carried out, an especially good result is obtained in short-delay blasting of rocks in a bench “through a borehole.”

An analysis of practice and of the technical-and-economic indicators of drilling-and-blasting operations at quarries of low productivity, similar to the quarries of the Tomsk district, shows that the best fragmentation of rock as a result of the primary blast, high productivity with sufficiently good economy, and safety of operations can be achieved under the following conditions.

The height of the bench with vertical boreholes is 8–10 m; with boreholes parallel to the bench slope, 10–15 m. It is known that even in large quarries the height of benches is taken within the range of 10–15 m. True, there are examples of good operation of quarries with a considerably greater bench height, but they are isolated and are characteristic of specific mining-geological conditions. Thus, for example, at the quarries of the Amvrosievka cement plant (Stalin Oblast, Ukrainian SSR), the benches, composed of marl, have a height of 20 m and more. In view of the weak stability of the slopes, the use of vertical boreholes here proved impossible, since in the lower part of the bench it is necessary to place very powerful concentrated charges that destroy the mass beyond the borehole line. The transition to blasting column charges in inclined boreholes ($d = 130$–$135$ mm), parallel to the bench, reduced the value of the toe resistance by 8–10 m. In this connection, a real possibility appeared for effectively working the deposit with benches up to 25 m high, using self-propelled rotary drilling rigs KA-2M-300 mounted on crawler track carriages.

With a vertical arrangement of boreholes in section No. 2 of “Volchanskugol,” a scheme has proved itself well that combines fragmentation of the main part of the mass by a crater charge at the toe of the bench with column charges that fragment the upper part of the mass and back-slope the newly formed side surface of the bench.

In strong and monolithic rocks, when the bench face is almost vertical, vertical boreholes with overdrilling or with a kettle-shaped enlargement at the base are effectively used. However, as quarry operating experience shows, the best results for high and insufficiently stable benches are obtained by arranging the boreholes parallel to the bench slope.

The charge weight for vertical boreholes can be calculated from the specific consumption of explosives, \(q\), and the volume of rock being blasted:

\[ Q = qWa\frac{H}{\sin \alpha},\ \text{kg}, \]

where \(q\) is the consumption of explosives per \(1\ \text{m}^3\) of blasted rock in the solid mass, kg;
\(W\) is the calculated line of resistance at the toe, m;
\(a\) is the distance between boreholes in a row, m;
\(\alpha\) is the angle of inclination of the bench slope;
\(H\) is the bench height, m.

The charge weight according to the length of the charging capacity of the borehole is

\[ Q = \gamma l_{\text{ch}}, \]

where

\[ \gamma = \frac{\pi d^2}{4}\Delta \]

is the charge weight in one meter of borehole, kg;

\(d\) is the borehole diameter, m;
\(\Delta\) is the charging density, kg/m\(^3\);
\(l_{\text{ch}}\) is the length of the charged part of the borehole, m (for benches with a height within 10–20 m, \(l_{\text{ch}} = 0.7H\)).

The parameters of the borehole grid can be calculated from the equation

\[ W = \sqrt{\frac{\gamma k}{qm}}; \]

here \(k\) is the coefficient of filling the borehole with charge;

\[ m = \frac{a}{W} \]

is the charge convergence index. It is rational to take its value as: 0.8 for dense monolithic rocks and for rocks with horizontal bedding; 1.0–1.2 for rocks with vertical bedding parallel to the blast-face line; 0.6–0.8 for rocks with vertical bedding perpendicular to the blast-face line.

With an inclined arrangement of boreholes in the bench, the method for calculating the charges remains analogous.

In the interests of more uniform fragmentation of the rock throughout the bench, it is advisable to distribute the charge. The positive operating experience of the Elenovskoye Ore Administration indicates the advisability of placing 60–80% of the charge in the lower half of the borehole and 40–20% in the upper half. The length of the column of separating stemming is usually taken as approximately 15% of the bench height, and the height of the column of main stemming at the collar of the borehole as equal to or somewhat greater than the line of least resistance.

References

1. S. I. Dekhtyarev and N. A. Chinakal, Mining Journal, No. 1, 1956.
2. F. M. Bychkov, Mining Journal, No. 4, 1956.
3. P. M. Emelyanov, S. N. Kuznetsov, Mining Journal, No. 5, 1959.
4. A. S. Tugarev, Coal, No. 1, 1959.
5. S. P. Levchik, Mining Journal, No. 11, 1956.
6. A. V. Kovazhenkov, A. M. Pugo, M. E. Doks, L. K. Vertleib, Mining Journal, No. 4, 1957.
7. D. M. Kushnarev, Mining Journal, No. 6, 1957.