Coal Extraction Using a Shield

N. A. Chinakal
Professor, Doctor of Technical Sciences

The system of mining for a mine is the same as the heart for the human organism: without a healthy heart there is no healthy person. Likewise, a mine, without a rational system of extraction, cannot fulfill its plan. Kuzbass, a young basin that developed under Soviet power, possessing enormous reserves of excellent clean coking coals occurring as thick seams with steep dip, could not operate normally without resolving the question of systems of extraction.

The most difficult question in working thick, steeply dipping seams is the question of support. Until recently this question was solved by using timber as the material for support, and therefore did not receive a satisfactory solution.

The decision of the Party and the government to establish a second metallurgical plant in Kuzbass raises before us the question of the further growth of coal output; moreover, in the Fourth Five-Year Plan it will be necessary at least to double coal production in Kuzbass. The task set will have to be solved both by mastering the designed capacities of the mines already built and by constructing new ones. In either case, the question of new effective systems and methods of opening acquires exceptional importance.

The resolution of the XVIII Party Congress speaks of the need to raise the labor productivity of workers, peasants, and the intelligentsia, and indicates the way in which the task set must be solved—“through the development of the Stakhanovite movement” and “the broad introduction of the newest achievements of science and inventions.”

The system of mining with shield support sets itself the task of resolving the question of systems for working thick, steeply dipping seams in Kuzbass, concretely putting into practice the directives of the XVIII Party Congress.

The idea of shield support in Kuzbass arose long ago and originates from the wooden mat used in the extraction of iron ores. A number of persons had long proposed various designs; in particular, I proceeded from the following premise: if it is possible to work under a wooden mat, the individual logs of which are not connected with one another, then it will be still easier and more reliable to work if, instead of an irregularly piled mass of timber, we have at our disposal a package of logs firmly connected into a single whole. It is clear that the connection could practically be achieved only with the aid of a metal base. The idea of a metal movable support was first advanced by I. A. Zhuravlev. In practical work similar proposals appeared repeatedly afterward as well. The idea of movable support was first expressed in print by me in 1934, when, on assignment from the mining commission of Glavugol, I was analyzing the work of the Emel'yanovskaya mine at the Leninsky mine field.¹

The Stakhanovite movement gave an impetus to the practical implementation of this idea. The first experiments were begun in 1936 at mine No. 3 of the Kiselevsk mine. The start was successful, but a fire that arose for an unknown reason (according to the commission’s conclusion, the fire could have occurred from arson or penetrated from a neighboring seam) greatly slowed the implementation of this proposal.

The experiments were successfully repeated at the beginning of 1938 under very difficult conditions at the Stalin Mine of the Stalingol trust. In the same year, 1938, and in 1939, the experiments were continued by the shield-support group of the Kuznetsk Scientific Research Coal Institute. These experiments confirmed the high efficiency of the proposed system.

A turning point in the history of this matter must be considered the presentation of a report on shield support at the conference for the exchange of world experience on systems of work at the USSR Academy of Sciences. The moral support of the conference, the firm position on this question of Comrade Onika, then head of Glavugol, and the order of Deputy People’s Commissar Comrade Kartashov of June 4, 1939, No. 237/a, played a decisive role in the history of shield support. At first, introduction proceeded very slowly; it was necessary to obtain metal, organize a base for manufacturing shields, select and prepare sites, and train personnel. Moreover, in practice the task of mining seams with shields had been solved only for seams up to 5–6 m thick.

Even at the beginning of 1940, mining seams 5 to 10 m thick with shields existed only in the plan of scientific research work. The first successful experiments with paired shields at mine 3–3-bis of the Stalingol trust at once opened the way for shield support, and by a resolution of the board of Narkomugol it was proposed that Kuzbasskombinat put 30 shields into operation before the end of the year.

If the first period, up to the conference at the Academy of Sciences and up to the report to the head of Glavugol, Comrade Onika, is characterized by exceptional difficulties and red tape in conducting experiments with shield support, then the second period is characterized by ever-increasing rates. When drawing up the plan of work on shield support for 1940, it was not known which shields, of the heavy or light type, would prove more effective in mining thick seams, and therefore, in order not to lose time, it was decided simultaneously to test in operation paired shields of light and heavy construction. Experience showed that it was possible to work with both light and heavy shields, but the cost of the shields, installation, and consumption of metal with heavy shields were considerably higher; and since experience showed that light paired shields successfully coped with the assigned task, the need to use shields of heavy construction fell away. In 1941, experiments were begun on the use of built-up shields in the Powerful seam (14.5 m) of the mine named for Comrade Stalin; here, with heavy shields, five pillars were taken, but the question was not finally resolved. Since then, a great deal of work has been done by Kuzbasskombinat and Narkomugol; during the days of the Patriotic War, the shield system became the leading one at a number of the largest mines of Kuzbass: the mine named for Comrade Stalin produces up to 55% of all its extraction output; the Kapitalnaya I mine of the Kaganovichugol trust, up to 70%; and the mine named for Comrade Voroshilov of the Prokopyevskugol trust, up to 80%.

Types of shields and the system of mining

The guiding idea in designing shields is the requirement to provide the worker with a sufficiently free area, allowing him to carry out the work conveniently and effectively, and at the same time to provide suffi-

...the precise flexibility and mobility of the entire system, capable of adapting to all changes in the conditions of occurrence of the seam and of operation.

The basic element of the shield is a section. Its size is chosen so that each section independently rests upon the coal pillars and, at the same time, would make it possible to make passages beneath it and to extract coal freely. The standard size of a section along the strike is taken as 5 m; across the strike, the size and number of sections are determined by the thickness of the seam. The sections are connected to one another by means of cables 22–25 mm in diameter. In Fig. 1 a shield of lightweight construction is shown. The fundamental difference between shields of heavy and lightweight construction consists in the fact that, in shields of lightweight construction, the calculation is made for the roll timber; the metal part is not included in the calculation and serves only for tying and preserving the dimensions of the shield. In shields of heavy construction, the calculation is made for the metal part; the roll timber is not included in the calculation and serves auxiliary purposes.

Fig. 1. Single shield of lightweight construction.

Visible labels in the figure: “along A–B”; “shield of lightweight construction”; dimensions 4250, 5000, 1210, 500, 625.

If the thickness of the seam is covered by one section, we call such shields single shields. Single shields may be used for seam thicknesses up to 5.5–6 m (although recently at the mine named after Comrade Voroshilov single shields have been used on seams up to 8–8.5 m thick); moreover, the size of the section across the strike is usually made 0.25–0.75 m smaller, since the working face dimensions must always be larger than the shield dimensions. And coal is always extracted over a larger area than the area of the shield. With greater seam thickness, the load on the section increases, individual parts become heavy and bulky, and therefore the problem has to be solved with the aid of two sections. If the seam thickness is covered by two sections, the shield is called a doubled shield.

In essence, a double shield consists of two shields arranged side by side and connected to one another by flexible connections into a single unit.

Fig. 2 shows a double shield. In a double shield the leading section is the lower one, i.e., the one located on the footwall side; therefore it is usually made somewhat larger than the upper one, located on the hanging-wall side of the seam. Sections may be made of equal

[Figure 2. Double shield. Visible dimensions include 0524, 1200, and 6000.]

sizes; making the lower section smaller than the upper one is not recommended. The most flexible is the single shield, since it bends freely and almost without restriction in one plane. Double and triple shields have to bend in two planes, and therefore they are less flexible. For operation, freer connections are required

between the sections and more careful conduct of the work, not allowing large sags along the strike or bends across the strike.

The greater the dimensions of shields along the strike, the higher the qualification required of the workers to control them, and the higher the work culture must be. At the present stage of mastering the shield system, it is recommended that shields be made 25 m in size along the strike. Even with such shield dimensions along the strike, the working front is far from being fully utilized, and so far shields of this length give the same output as shields measuring 15–20 m along the strike.

Recently, the practice of using shields has begun to take the path of simultaneous operation of two shields—either on two adjacent seams located close to one another, the extraction of which can be carried out simultaneously, or on one and the same seam. In the latter case the shields are arranged along the strike and work with them is carried on simultaneously. We call the operation of such shields paired. In the latter case, what is meant is obtaining double output from one seam or simultaneous extraction of closely spaced seams.

There is now a tendency to reduce the amount of metal for shields. At present there are already sections in operation which in essence are the skeleton of the former section without transverse channels; this makes the construction of a light shield 50% lighter and, undoubtedly, as work with shields is mastered and personnel are selected and trained, the consumption of metal for manufacturing shields will decrease. Shields of lightened construction are called “wooden shields.” This, of course, is incorrect: there are no completely wooden shields, but only shields in which the consumption of metal has been brought down to a minimum. But practice has shown that one must not become carried away with excessive lightening of the structure, since this entails an increase in accidents. The basic dimensions of sections of light shields adopted in production are taken with rounding to 0.25 m, and there are 5 sizes: 5 × 4.5 m; 5 × 4.25 m; 5 × 3.75 m; 5 × 3.25 m; and 5 × 2.25 m. From these sizes, by means of various combinations, one can obtain a single and a double shield for all cases of seam development, beginning from 2.50 m up to 9.5–10 m, taking into account that the shield is always made 0.25–0.75 m less than the thickness of the seam. The weights of the standard sections are respectively: 2.30 t; 2.05 t; 1.9 t; 1.64 t; and 1.20 t. In practice, considerable deviations are encountered owing to the use of different numbers of channels.

The cost of manufacturing a shield of light construction, including the cost of the metal, at the Kuznetsk plant in Stalinsk is 725 rubles per 1 t, and with transport taken into account it should be reckoned as 750 rubles. The manufacture of shields in mine workshops is cheaper—600–650 rubles per 1 t. Recently the cost of 1 ton of finished structure has risen to 1000 rubles.

Development System

The standard development system is shown in Fig. 3. Work is carried out with caving; moreover, as a rule, in all cases of work with shields a retreating system is adopted. The preparatory work consists in driving the main and ventilation drifts. Since the coal is stored in furnaces and loading is carried out from several chutes simultaneously, for the work it is sufficient to have a single-track haulage drift with a passing siding. Practice has shown that having horizontal workings, especially of considerable cross-section, in a field extracted by means of shield support is harmful; therefore, in preparing the section it is not recommended to drive even the first parallel drift. Ventilation is effected through drilled boreholes 350 mm in diameter from the main drift to the ventilation drift and with the aid of electric fans. The drilled ...

the boreholes are then enlarged to dimensions of \(1 \times 1\ \text{m}\) (coal-discharge furnaces) and \(1.2 \times 2\ \text{m}\) (traveling furnaces). Practice has shown that such a solution to the problem of mechanizing the driving of furnaces fully justifies itself, and explosives become unnecessary where their use is least effective and more dangerous; moreover, the most dangerous and least productive labor of the face worker becomes easier and safer. Driving furnaces by drilling ensures their straightness and a more correct position, while deviations from the specified direction of up to \(2\)—\(2.5\ \text{m}\) are easily corrected when they are subsequently enlarged with a pneumatic hammer.

When the driving of the main and ventilation entries has been completed and two pillars have been cut, the initial cut is made for assembling the shield. The cut is a horizontal layer \(2.5\ \text{m}\) high, extracted through the entire thickness of the seam from the floor to the roof, and along the strike—over the length of the shield. This is, in essence, the beginning of stoping operations, since making the cut differs from extraction by horizontal layers with caving only in a somewhat different method of support. The cut is not made at once over the entire area covered by the shield, but only for one or two sections; later the work is carried out with the intention that assembly of the shield and installation of the ramp should proceed without delay. It is very important, before starting work with the assembled shield, to assemble, set back along the strike by \(2\ \text{m}\), one section for a single shield or two sections for a paired shield. If these conditions are observed, the calculated pillar is left and safe work on assembling the next shield is ensured.

Coal extraction in the cut, its support, and assembly of the shield are the most labor-intensive operations, and nevertheless the productivity of workers at the face in the first stage of work (including assembly) is \(4\ \text{m}\) per shift, i.e., no lower than with other systems. The cut is supported by incomplete door sets with the roof tightened by lagging; the coal along the furnaces

by its own weight, is delivered to the main drift; the delivery of parts of the sections and of timber is carried out through boreholes from the surface or along the ventilation drift. When the first shield has been put into operation, the next one is assembled. With a lift height of 50 m, extraction of a pillar takes 20–25 days; assembly of a single shield takes 7–8 days, and of a double shield 10–12 days. When the shield reaches the main drift, the shield is dismantled.

Dismantling a shield section, provided it has been correctly assembled and operated, is carried out over the course of 2–3 working shifts, with 2 men assigned per shift; normally 80–90% of the metal is recovered. Part of this metal is suitable for direct use, part goes to the shop for straightening, and part goes for scrap. In practice it may be considered that a shield is normally used on average 3 times. In cases where the shield is badly deformed and, during disassembly, its bolts and ties have to be cut, assembly requires a greater expenditure of labor, and the percentage of metal recovered decreases. Coal extraction is usually carried out with explosives, the best results being obtained when the shield is set down simultaneously along the entire front onto the broken coal—in such cases all the boreholes are fired at once from the power network by means of a Guk button. Recently an electric firing machine for 100 boreholes has appeared. The positive results obtained in working with shields on the upper levels naturally gave rise to the desire to use shield support on the lower levels as well. Trials were begun in seams 4–5 m thick, i.e., in those whose mining had previously also been carried out by longwalls with caving, but only with large losses, since the seam was not extracted to its full thickness, but with a band of coal left behind. Later the trials were transferred to thicker seams, the upper level of which had been worked with comparatively small losses. At present, on the lower levels, a large number of pillars have already been worked by shields, and the question is being raised of further expanding the work. On assignment from KNIUI, the Tomsk Industrial Institute named after S. M. Kirov developed a plan of operations on the lower level with shields, as well as a series of preventive measures. These measures are basically as follows.

1) If the upper level has been worked with large losses, then before beginning the mining of the lower level it is necessary to carry out preliminary exploration of the worked-out reserves.

2) Mining should be carried out on the retreat in blocks, with the block consisting of 4 pillars; every 110 m, another 4 m is added to the usual 2-m barrier pillar between two pillars, thus forming a fire barrier pillar that ensures the installation of chock stoppings on the clay on the ventilation level and in the crosscuts; in the main drift, a brick stopping is installed.

3) By means of boreholes drilled from above, clay packing of the main drift is immediately carried out to the height of the seam thickness, and undercutting is made for the last pillar adjoining the fire barrier pillar.

4) The cave-ins are filled; in the worked-out block one control borehole is left, through which observation of the condition of the worked-out block is carried out.

Calculations show that the systematic implementation of preventive measures will increase the cost per 1 ton by 40–60 kopecks and will make it possible to increase the distance between district crosscuts to 300–400 m and more.

At the same time, as an experimental measure, a plan has been developed for mining the lower levels with shields and with the use of complete stowing. In a number of cases we cannot avoid the use of stowing, and therefore this question must also be resolved.

Shield Control and Accidents

In January 1940 the question was still being discussed as to whether shields should or should not be used. Double shields for seams more than 6 m thick were only in the program of the research work of the KNIUI. After obtaining the first positive results, it became necessary to take stock of the work done, to select and prepare areas for shields, to train personnel, to obtain metal, to organize the manufacture of shields, and to ensure the management of all the work. Within such an extremely compressed period, naturally, there could not fail to be “childhood diseases”; not everything proceeded as smoothly as we would have liked. Therefore there were persons inclined to see in the first failures and accidents not childhood diseases, but an organic defect of the system that somehow had to be eliminated. The information collected on accidents in the trusts Stalinu gol and Prokopievskugol for 11 months of 1940 gives the following picture.

In all, 18 accidents were registered in the Stalinu gol trust; of these, 5 occurred in crosscuts from improper support and lagging, and 13 during work with shields. The number of pillars worked out was 70. Consequently, the number of accident pillars amounts to 18.6%. In the Prokopievskugol trust, during the same period, 14 accidents were registered, of which 5 were in crosscuts and 9 with shields. Seventeen pillars were taken. Consequently, the number of accident pillars amounts to 52.7%. Such a high accident rate in the Prokopievskugol trust is explained first of all by the fact that this trust began mastering shield timbering later, did not manage to study the natural conditions of seam occurrence, or to prepare personnel. It must be said that the Prokopievskugol trust also has more disturbed seam occurrence. On average for Prokopyevsk, out of four shields one was in an accident.

According to the nature of the accidents, they are divided into the following categories:

According to the causes that brought about the accidents, they can be distributed into the following categories:

The largest number of accidents occurred as a result of section breaks. The breaks occurred because of improper assembly, expressed in uneven tightening of the sections, the use of old ropes and ropes of unsuitable dimensions, but, chiefly, because of incorrect conduct of the work, when entire sections, together with the collapsed rock lying on them, were held only by the ties, without support in the form of supporting packs. With proper conduct of the work it is possible to work even with sections that have no connections between them, whereas, if one does not know how to work, any connection can be broken. The second large category of accidents occurred even before the shield began working, during the driving of the cut for assembling the shield. This phenomenon resulted from improper support, because the cut was driven prematurely in pursuit of production, the shields were not ready, and the cut lagged behind. There were cases of collapses also from intensified blasting; as a result, recently they have switched to driving the cut with the aid of pneumatic hammers. Preparatory work carried out with the expectation of using other systems, and improper division of the areas, in reality played a greater role than was reflected in the figures presented; a considerable number of cases of section breaks and of rock entering under the shields is explained by improper preparation and division of the areas. Increased pressure of the collapsed rocks was observed at mine 3—3-bis (the Lutuginskii seam), during the simultaneous working of the Gorelyi and Lutuginskii seams, separated from one another by a rock parting of 1.5—2 m. The Gorelyi seam was being worked with a certain advance, and with the collapse of the overlying rocks the rock parting was crushed and the shield along the Lutuginskii seam was pinched. To eliminate this undesirable phenomenon, it was decided to extract by shields in both seams with an advance along the Gorelyi seam of 2 pillars, calculated so that extraction along the Lutuginskii seam would be carried out after the movement of the rocks along the Gorelyi seam had subsided. Accidents from causes dependent on the shields relate to shields of heavy construction. Finally, there were two cases of accidents from untimely and incorrect seating of the upper side at the Voroshilov mine; the first time the shield was started without seating the side, and when the shield had moved more than 10 m, a collapse occurred that shattered the shield. The second case occurred in the same place: to seat the side, boreholes were drilled from the surface, and when blasting was carried out, it turned out that the boreholes were under the shield, and not above it, as they should have been. The blasting damaged the shield and threw it onto the lying side of the seam.

People’s Commissar of the Coal Industry T. Vakhrushev, during a visit to Kuzbass, drew attention to accidents and to rock being let under the shields. On the basis of his instructions, measures were developed and carried out by the Kuzbass Combine that yielded the following results: in the Prokopevskugol trust, accidents in November and December 1940 decreased sharply, while in the Stalinugol trust during the same period there was not a single case of an accident or of rock being let under the shields. During the period of the Patriotic War, many new unskilled workers poured into Kuzbass, and this adversely affected the accident rate when working with shields.

Technical-and-Economic Indicators

Every mining system is characterized by the length of the longwall face, the productivity of the longwall face, the productivity per 1 linear meter of face, the advance of the longwall face, the output of workers per shift, and the degree of saturation of the face with workers, or the density of workers in the face, the losses of the useful mineral during operation, and the consumption of materials.

Table 1 presents, according to materials of the technical department of the Stalinugol Trust, the data for the trust over 9 months for the various systems.

Table 1

To characterize the shifts that have taken place recently, the following data may be cited. For the month of November 1940, the trust-wide average productivity of 1 linear meter of the stoping front was, for horizontal seams with caving, 116 tons; for longwalls with caving, 81.1 tons; for inclined seams with stowing, 56.2 tons. At the same time, under the shield system the productivity of 1 linear meter of the stoping front increased to 272.5 tons, i.e., by 74%, and at the Comrade Stalin mine to 536.9 tons; the average monthly output per shield increased to 6300 tons, i.e., by 128%, and the output per worker per attendance rose to 16.8 tons, i.e., to the level attained in experimental work.

The standard designs developed by the Department of Mining of Bedded Deposits of the Tomsk Industrial Institute on assignment from the Kuznetsk Scientific Research Coal Institute provide for obtaining a daily output from a single shield of 6750 tons and from a paired shield of 8900 tons, with worker productivity for the section per attendance of 10–11 tons. In December, the Stakhanovite brigades produced in a month per shield: Bloshkin’s, 9500 tons; Romanovsky’s, 10000 tons; Latyshev’s, 11500 tons—and this is far from the limit. On record days some shields produced up to 650 tons per day.

Shield sections showed very good results. Section No. 7 of mine 3—3-bis on 5/XII produced 783 tons, on 12/XII—814 tons, on 21/XII—888 tons, against a plan of 327 tons. At the Comrade Stalin mine, on 21/XII, the following produced: section No. 1—867 tons against a plan of 290 tons; section No. 14—880 tons against a plan of 288 tons; section No. 15—1195 tons against a plan of 452 tons. On December 5, section No. 14 produced 901 tons against a plan of 288 tons, i.e., 312% of the plan. On these days the productivity of a worker at the stoping face under shields rose to 75 tons per attendance. All this indicates that we still have far from exhausted reserves opened up by the shield system, and that these reserves will be realized as the new system is mastered. During the days of the Patriotic War, the best shield brigades of Kornakov, Baranov, Lazarev, and others systematically produced 12000–14000 tons per month, while Borisov’s section, where 2 shields operate, produced up to 25000 tons monthly.

The achievement of the effect and the average figures given above in the table were significantly influenced by cases of the use of shields in abnormal

conditions; thus, for example, at the Comrade Stalin mine, seam IV Inner was being worked in the 1st district by diagonal slices with stowing; the lower sublevel lagged behind the upper one by 50 m, and, owing to large losses and the slow advance of the extraction work, the coal began to heat up and the smell of benzene was noticeable in the ventilation drift. The question arose: either to abandon the remaining coal reserves and isolate the district, or to try by some means to take the coal in the lower sublevel. It was decided to do the latter: shields were installed and work was begun with the stowing being let through. The reserves were taken, the district was isolated, but the height of the pillars extracted was only 20 m. At mine 3–3-bis, on the Gorely seam, work was begun with inclined slices; the first slice was taken, and then the remaining two were taken with short shields, letting them down along the floor of the first slice. From the standpoint of extending experience in the use of shields under various conditions, such cases are of interest, but these same cases considerably reduce the average figures, and for the Stalinugol trust over 9 months the average height of a pillar taken with shields came to 35 m. It is clear that the lower the height of the panel, the greater the overhead cost, per 1 ton of coal mined, of installing and dismantling the shields. Consequently, increasing the height of the panel and of the pillars extracted with shields is one of the ways to raise labor productivity when working with shields.

From the standpoint of metal losses, the question of dismantling shields is of very great importance. At first it was very difficult to move this matter from a standstill, despite the fact that in experimental work it was possible to recover up to 90–95% of the metal. The reasons hindering recovery of the metal were the lack of interest on the part of district chiefs in recovering the shields and certain design shortcomings of individual parts. Thus, for example, in the first designs of double shields the sledges had to be cut apart during dismantling, which caused a large expenditure of labor and time, complicating and delaying dismantling. According to data from the technical department of the Stalinugol trust, in 1940, at the Comrade Stalin mine alone, 212 sections were installed in all; of these, as a result of accidents and of leaving sections undismantled in worked-out spaces, 89.4 were left for various reasons, i.e., 42%; at mine 3–3-bis, out of 149 sections, 37.4 were left, i.e., 25%; for the trust as a whole, out of 361 sections, 126.8 were left, or 35.2%. Thus, the percentage of sections lost or left undismantled still remains very high, although recently a noticeable improvement has been observed in this matter and, as a rule, 70–90% of the metal is recovered. Part of the recovered metal goes directly into the mine, part is issued to the shops for straightening, and part is used for various needs or sent as scrap metal. For 1 m² of a shield of light construction, on average 100 kg of metal is consumed. With a pillar height of 50 m, 1 square meter of shield in one stowing ensures extraction of 65 t and, consequently, if the entire shield is lost, the write-off will amount to 1–15 kg per 1 t. In standard designs a threefold turnover of the metal is assumed in the calculations, and then depreciation charges amount to 38.4 kg per 1 t of coal mined, while metal consumption per 1 t is 500 g.

For any mining system, the volume of development and preparatory work is of substantial importance. Under the shield system, the volume of development and preparatory work remains almost unchanged for seams of different thickness. With double shields, in order to accelerate installation, a second auxiliary drift with a cross section of 4 m² is often merely added in the ventilation drift.

The greater the panel height, and the greater the thickness of the seam, the smaller the percentage yield of coal from development and preparatory work. With a panel height of 50 m and a seam thickness of 5 m, the yield of coal from development and preparatory work amounts to 11%, while with a seam thickness of

9 m—7.5%. With a lift height of 100 m, respectively, 7.2% and 4.5%. Until now, shields have been used at lift heights reaching 85 m. On the basis of the available experience it may be said that shields can be used with a lift height of 100 m. When the lift height is doubled, the costs of preparing a new horizon, which in large mines amount to several million rubles, are correspondingly reduced. The costs of driving district crosscuts are reduced by half, and the costs of installing and dismantling shields are also reduced.

As the lift height increases, the coal discharged through the chutes is increasingly crushed, and the chutes themselves are observed to break; this phenomenon is especially noticeable in weak coals and at places where the chutes intersect with crosscuts. To prevent the chutes from breaking, it is necessary to support the latter. First of all, the floor of the chutes breaks; moreover, it may happen that, after the lower bench of coal has broken, destruction of the floor begins, and then the shield will be unable to cover the chute clearance and rock will begin to fall into the chute. It is therefore necessary to monitor the condition of the chutes and not allow them to be destroyed, and in weak coals it is necessary to support the chutes with continuous timbering.

Coal losses are a very important indicator. The standard design provides for system-dependent losses of 9.5–10%. Total losses are 17–18%. In experimental work it was possible to reduce system-dependent losses to 10–12%, and total losses to 18–20%.

Actual losses in the Kuzbass over three quarters of 1940 in shield districts were: system-dependent losses 13.3%, total losses 28.3%. Losses not dependent on the system are distributed as follows: 1) pillars at workings 2.6%, pillars at district boundaries 2.8%, pillars under the upper horizon 9.4%. For the system of work on inclined seams with stowing, over the same period the losses were: system-dependent 21.2%, system-independent—8.6%, total—29.8%. The actual increase in losses under the shield system as compared with the design figures depends on the fact that in a number of cases shields 10–15 m long were used instead of 25 m. A two-meter pillar left at the boundary of two panels, with a shield length of 25 m, gives losses of 7.4%, while with a shield length of 10 m—16.2%. In a number of cases losses across the thickness of the seam were observed, resulting from the use of shields that did not correspond to the seam thickness. These losses averaged 3%. Losses not dependent on the system, amounting to 15%, are divided into losses under the upper horizon, 9.4%; to reduce these losses it is necessary to drive the split as high as possible; moreover, in order to prevent falls, it is recommended to drive the split without the use of explosives, using jackhammers. The next category of losses—at district boundaries, 2.8%—occurs because a wedge is left at the boundaries of the protective pillars under the crosscuts; when working with shields, these losses can be entirely avoided, since, as the shield is lowered, a retaining wall of rock is constantly formed at the cut-off boundary pillar and, as observations through travel crosscuts over worked-out panels have shown, crushing of the pillar—at least visible to the eye—does not occur. The last category of losses consists of losses in pillars above the main drift—2.6%. At the beginning of the work there was fear of bringing the shield out onto the main drift, and it was stopped 3–4 m short of it. With proper organization of the work these losses can also be reduced by half. With the elimination of the indicated deficiencies and the proper conduct of work, losses can be brought to the figures indicated in the design; this has already been achieved in experimental work.

The consumption of materials is of great importance. The design specifies timber consumption for preparatory work, support of split workings, and construction of the chute in the amount of 18–19 m³ per 1000 tons of coal extracted, at a cost of

57–60 kopecks per ton. A further reduction in timber consumption can be achieved by increasing the height of the story; if the height of the story is doubled, the expenditure under this item will likewise be reduced by half. Under other systems, the consumption of timber materials amounts to from 1.5 rubles to 2.5 rubles per ton of coal extracted, or 45–60 m³ per 1000 tons of coal extracted.

The consumption of explosives in practice amounts to 70–90 kg per ton and is high as a result of the improper use of explosives, as well as of the driving of coal chutes for breaking down. Despite the fact that the coal is squeezed out by the shield, no appreciable saving in explosives has yet been obtained.

In a socialist economy, one of the most important questions is the question of work safety. During the entire period of work with shields there has not been a single serious accident involving workers under the shields. Accidents occurred with people who had only an incidental connection with the shields. Thus, for example, at the mine named after Comrade Kaganovich of the Prokopyevskugol trust, a shield, having encountered a disturbance, was stopped; a surveyor was sent to sketch the character of the disturbance, and he descended below the grating, fell into the chute, and received severe bruises. At the mine named after Comrade Stalin of the Stalinugol trust, during shield operation there was a breakthrough of water from the overlying horizon. Under any other system a mass accident would have been inevitable; here all the workers survived, and only a mining-school pupil who happened to arrive, who, becoming confused, rushed toward the travelling chute, was swept away by the water. At the same mine there was an accident with a mining foreman, who sent a horse-driver to clear a chute under the shield, while he himself sat on top of the clogged chute. The horse-driver unloaded part of the rock, and only after this did upward movement begin: the mining foreman went into the chute together with the rock. At the mine named after Comrade Voroshilov there was a case of liquid clay breaking through from the worked-out space. All the accidents listed were, in the main, not dependent on the system.

Finally, it is necessary to say a few words about the quality of the coal. At first there were complaints about contamination of the coal by rock. Indeed, during accidents the penetration of rock under the shield is unavoidable, and if one does not carefully monitor the elimination of the consequences of accidents, contamination of the coal is inevitable. With proper work with the shield, rock does not get into the coal and no grounds are created for contamination of the coal. The struggle for quality is a struggle against accidents; from the moment accidents ceased, complaints about contamination of the coal by rock also ceased. In any case, such large mines as the mine named after Comrade Stalin and 3–3-bis of the Stalinugol trust meet the standards, and there are no complaints about contamination of the coal; this, of course, should in no way reassure us or weaken our attention to questions of coal quality.

Economic Effect of the Use of Shield Supports

In the industrial testing of shields of various designs and under various conditions, the economic results presented in Table 2 were obtained.

The indicated results were obtained under conditions of absence of experience, technical norms, and rates. The tests aimed to reveal the advantages and disadvantages of shields of heavy and light construction, and also to prove the possibility of working with double and paired shields. As the system is mastered, the results obtained can, of course, be significantly improved. Industrial testing showed the unquestionable economy of shields of light construction, in the form in which they were proposed from the very beginning, in comparison with shields of heavy construction. These same tests showed the advisability of using shields in seams close to...

Table 2

laid and separated from one another only by a small interbed of rock, about 1.5--2 m.

To determine the savings from the use of shield support, the results obtained during industrial testing must be compared with something. On this question, as also in the application of mechanization, there is no sufficiently developed methodology for determining the economic effect. Each system of work has its own field of application, and often the question is decided not by which system gives the greatest economic effect, but by which system can be applied under the given specific conditions. Where it is expedient to use shields, it is inexpedient to use inclined layers with gravity stowing. Only within narrow limits, i.e., where the boundaries of the field of application of one system and the other meet, can a comparison be made, i.e., at a dip angle of 60°. Owing to the bedding conditions of the surrounding rocks and to safety conditions, horizontal layers with caving cannot always be used on the upper levels, and still less on the lower levels; therefore comparison with this system will likewise be of an accidental and unsubstantiated character. Horizontal layers with stowing in descending order can apparently be used in all cases where shields are used on thick seams, but here, on the upper levels, horizontal layers with caving may also find application in individual cases. A comparison of results obtained under different systems, in different administrative-production and natural conditions, at different times, and in the presence of different accompanying factors that often distort the results in one direction or another, will be entirely unconvincing. Some attempt to make a compar-

…by sections using predominantly one system or another, forgetting that, when operations have not yet stabilized, the degree of fluctuation of the results for one and the same section often exceeds the degree of accuracy of our determinations.

For lack of accounting of the results of work by systems, because it has so far been impossible to establish such accounting, and because there is no developed and tested methodology for identifying the economic effect, one has to resort to methods that are somewhat conditional in character.

The first method of identifying the economic effect from the use of a new system may consist in the following. A given section or seam was worked by a certain system; when this system was applied, certain results were obtained; and after another system was applied in the given section or seam, different results were obtained. The first results may be compared with the second if not too long a time elapsed between the use of the first and second systems, during which the production situation in the basin might change very substantially.

At mine 3—3-bis of the Stalinugol Trust, the shield system is used on the Gorelyi and Lutuginskii seams, where formerly inclined slices with stowing and horizontal slices with stowing were used. The cost of 1 t by section in the first case reached 11 rubles, and in the second up to 13–15 rubles. After the introduction of shield supports, the cost of 1 t of coal by section, taking into account the amortization of the shields, fell to 5–6 rubles, thus becoming 6–9 rubles cheaper. When working with horizontal slices with stowing, it was necessary to install equipment costing 160,000 rubles. The cost of two shields was 18,000 rubles. In the first case, several times more metal also had to be mobilized than in the second.

The actual data for shield supports in section No. 14 of the mine named after Comrade Stalin for the month of November 1940 were as follows.

At the same mine, for the same month, in section No. 4, using inclined slices with stowing and caving, the actual cost of 1 t by section was as follows.

In section No. 3, where inclined slices and longwalls with stowing are used, for the month of November the cost of 1 t by section was as follows:

Thus, the saving in direct costs obtained when using the shield system ranges from 4 rub. 33 kop. to 7 rub. 62 kop. per 1 t of coal mined, as compared with other systems of work.

The indirect benefits from the use of the shield system are still harder to determine: with every ruble paid to the worker, the enterprise bears a whole series of additional expenses, which may amount to approximately the same

magnitudes. Fulfillment and overfulfillment of the plan also lowers the cost of coal, quite apart from the fact that it ensures the proper operation of transport and of all industry. The preservation of the health and lives of workers in the USSR is such an advantage that it cannot be measured in rubles.

Finally, the use of shield support made it possible to improve the general working conditions in mines where the delay in mining thick seams did not permit the working of conserved reserves of lower-lying, less thick seams.

The data cited above undoubtedly indicate the great economic effect of applying the shield system under production conditions. Such an effect is logically and inevitably predetermined by the high labor productivity obtained with the shield system. This once again confirms the correctness of the position of the Party and the government that the main link to be grasped in order to raise the country’s economic well-being is the increase of labor productivity.

Conclusions and Proposals

The practically tangible effectiveness of the shield system under the conditions of Kuzbass entailed, alongside experimentation, its simultaneous mass industrial application. In January 1940 a total of 6,150 t was mined by shields, while in December 106,593 t was mined, and the shield system took first place in specific weight among the systems working thick steeply dipping seams in the Prokopievsk district (104,793 t was mined by shields, 55,043 t by inclined seams with stowing, and 74,328 t by horizontal seams with caving). The use of shield support provided Kuzbass in 1941 with an additional output of about 1,500,000 t of coal. Since then, output by the shield system has steadily increased.

The rapid growth of extraction with the aid of the shield system led to a number of negative factors, which primarily entailed an artificial decline in the technical and economic indicators obtained as a result of such rapid spread of the shield system.

In essence, all the principal ailments inherent in the first trials and revealed in the course of the initial experiments are at present widely disseminated and in many cases even legalized in the mass application of this system. What has been said applies both to the shield, which in mass application has remained in its original design, still far from perfected, and to the technological process of seam extraction and to the very principle of the shield system, which has not been tested in its full scope (the size of the pillar along the strike has remained 20–25 m).

Nevertheless, in the comparatively short time spent on practical trial work, not only was the possibility of applying and the correctness of the principle of the shield system for mining thick steeply dipping seams proved, but it was also definitively established that the shield system, as an entirely new field of mining technology, is the most effective, safe, and at the same time cultured system, carrying with it the richest prospects for the entire national economy of our country.

The Second All-Kuzbass Conference on systems for the mining of coal seams, from among the numerous systems used in Kuzbass, selected the most rational ones; among them the conference placed the system of shield mining of thick steeply dipping seams in one of the first places, permitting its application not only on the upper horizons, but also on

lower levels with the simultaneous implementation of a number of preventive measures.

The advantages of the shield system were especially evident during the days of the Patriotic War, when Kuzbass, with Donbass temporarily out of operation, had to bear on its shoulders the entire burden of supplying industry with fuel.

The accumulated experience, the observations carried out, the trained personnel, the available theoretical prerequisites and generalizations place before us as a task for the immediate future the question of further expanding the field of application of the shield system not only in Kuzbass, but also in other basins.

This question is extremely important, since even in Kuzbass, owing to such a rapid spread of the shield system, the latter has, again artificially, acquired a somewhat restricted field of application: undisturbed and regular seams with a dip of 65°—90°, which is by no means the limit for this system. Further, it is extremely necessary to use the experience of the shield system on steeply dipping seams for inclined, gently dipping, and horizontally bedded seams, as well as for extra-thick seams. This requires serious work to generalize all the materials at our disposal, accumulated over the period of our many years of work in this field.

The practical use of the results obtained from the application of the shield system has led to the fact that questions of a scientific-research nature, which ought to have been laid at the foundation of further improvement of this system, are lagging behind in their development; therefore, simultaneously with the work of generalizing the experience of applying the shield system, it is necessary to establish systematic scientific-research work that would ensure the further improvement of this system and the expansion of its field of application in Kuzbass and in other coal basins.

A very important question, and for a new field of technology such as the shield system a decisive one, is the question of personnel training. This question acquires especially great importance when raising the issue of transferring experience in working with shields to other basins. At present there is not, and few seriously consider, the need to develop a methodology for management and proper operation when applying the shield system; therefore the organization of training courses for managerial personnel to work with the shield system and the publication of corresponding guidance material is also one of the urgent tasks of the present day.

Finally, this extremely important matter must be given the proper organizational direction, since what exists now not only fails to ensure the improvement of this system for the conditions of Kuzbass, but also has no prospects for further expanding the field of application of this new, effective method of working coal seams both in the coal industry as a whole and in other branches of mining.

It seems to us highly advisable and timely to establish, under the People’s Commissariat of the Coal Industry, a unifying and directing center that would ensure the implementation of the questions put forward by life itself, and thereby this entire great undertaking would receive the necessary organizational direction.

The “Chinakal Shield Bureau,” organized in Tomsk and directly subordinate to the technical department of the People’s Commissariat of Coal, has to a considerable extent advanced the solution of the question posed, but has not yet fully resolved it, since, simultaneously with work of an engineering character, it is necessary to organize scientific-research work and broad experimentation in laboratories and in production.

1. ONTI, NKTP—1935. Collection of articles on the working of coal seams in the Kuznetsk Basin, p. 4. ↩