CLASSIFICATION OF CRIB BREAKER SUPPORTS

E. A. BOBER

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

The Need for a Classification of Crib Breaker Supports

In the system of mine supports, an essential role is played by supports for stoping workings, which are subdivided into two large groups: face supports and special supports. The latter are of the greatest interest, since in the process of extracting a mineral, questions of roof control play a significant role. Special supports include timber packwall support, clusters of timber and metal props, cribs, packwalls of various designs, etc.

Cribs, as individual breaker supports¹, have been used in mining practice for a considerable period of time and continue to be used. The desire to reduce the labor required for moving cribs, to increase their resistance to rock pressure, to reduce metal consumption, and to lessen the danger of the work of crib setters has led to the creation of various designs of this type of breaker support, both in domestic and foreign practice.

In order to understand the structural features of cribs and to determine the possibility of using them as breaker supports, it is necessary to classify cribs. Despite the fact that there are many designs of crib supports, no attempts have been made to create a coherent classification. However, certain survey works on this question should be noted.

The largest is the work of V. G. Bochkarev [1]. In it, all crib designs are subdivided into three groups: portable, semi-mobile, and mobile. Such a division of designs does not reflect their essence, since the very definition does not take into account such an important factor as the degree of dismantlability of the crib.

In the article “The Use of Cribs in Stoping Faces” [13], the following subdivision of individual types of this support is given: timber, metal, and semi-dismantlable. Thus, the subdivision made

division does not encompass all constructions and is not exhaustive, since such a distinguishing feature as material cannot be taken as the basis for a classification of crib supports.

In Stassen’s survey work on metal supports [10], a description is given of dismantlable and semi-dismantlable cribs, while other types of this support are absent. There are also a number of other works devoted to the description of individual crib constructions [2; 4; 6; 7].

Thus, taking into account the existing works on crib supports, we consider it necessary to provide a classification of them that reflects their structural features and practical significance.

Features by which crib supports may be distinguished

The distinguishing features of individual types of cribs may be: the degree of complexity of the construction, resistance to rock pressure, yielding capacity, material, weight, the presence of devices for release from under pressure, the degree of dismantlability and the nature of movement, and the plan shape.

According to the complexity of construction, three classes of cribs should be distinguished: simple, frame-type, and complete. By simple cribs we mean the “primordial” construction of a crib, consisting of sections of timber, mine or railway rails, I-beams, etc. Frame-type cribs include cribs in the form of a frame or box, the height of which is approximately equal to \(2/3\) of the seam thickness. Complete cribs are cribs with cantilevers. The named constructions are shown in Figs. 1–2, 3–4, 5–6.

Depending on the material of which cribs are made, they may be wooden, metal, combined, and metal-wooden (wooden beams sheathed at the ends with sheet iron). In addition, in English practice, cribs made of bags filled with rock were used. The latter were nonportable.

According to yielding capacity, crib support may be subdivided into yielding and rigid. In this respect it is impossible to indicate any definite boundary. Wooden cribs, especially those made of round timber, are classified as yielding. In these cribs, settlement begins even under slight pressure. Cribs made of metal are rigid, or more precisely—conditionally rigid, since settlement

Fig. 1. Dismantlable portable crib MRK-1 of the Giprouglemash design

Fig. 2. Semi-dismountable movable chock made of metal beams

Fig. 3. Semi-dismountable movable chock of English design

Fig. 4. Non-dismountable movable chock PMK-2 of the Giprouglemash design.
1—chock post; 2—frame extensions; 3—stand; 4—locking beam; 5—front plate; 6—bosses

Figure 5. Demountable portable cribs with cantilevers in a longwall face

Figure 6. Buzikov’s non-demountable mobile cantilever chock.
1—base of the chock; 2–3—body of the chock; 4—fixed wedge; 5—vertical bolt; 6—movable wedges; 7—under-cantilever beams; 8—expanding mechanism—screw; 9—cantilevers; 10—extendable cantilever.

nevertheless occurs when the resistance arising in the structure reaches the yield point of the metal, on the one hand, and exceeds the indentation resistance of the roof rocks—of the soil, on the other. The crib must possess a certain yield immediately after its installation. For this purpose, in metal cribs wooden wedges are used for wedging.

An important indicator characterizing crib support is its resistance to rock pressure. Cribs possessing considerable yield have low resistance. These include, first of all, wooden cribs made of round timber. In contrast to them, metal cribs possess high resistance.

By weight, crib support may be subdivided into light, heavy, and lightweight. Of course, such a division is purely conventional. Wooden cribs are classed as light, metal cribs as heavy. An example of a lightweight design is the proposal of P. N. Onishchenko [6].

When a crib is moved, it must be relieved of pressure. For this purpose several designs of wedge locks were proposed. In simple cribs such devices are sometimes absent. Therefore, one should distinguish crib support with special devices for facilitating release from pressure and without them.

The method of moving cribs is a question of enormous importance. In this respect cribs may be divided into portable and movable. The former are characterized by the fact that the crib is carried in parts (elements). For the second type, the application chiefly of mechanical means for moving them is characteristic. In this case the crib has such a design that approximately \(2/3\) of the height of the crib is non-dismantling. Only the upper part is dismantled. In some literary sources [1] cribs of this type are called semi-movable. We consider such a definition unfortunate, since it does not reveal the essence of the design. It would be more correct to call such cribs semi-dismantling, i.e., partially dismantled movable cribs. Those crib structures which are not dismantled during movement should be called non-dismantling movable cribs.

Crib support may also be distinguished by shape (in plan). In this respect they may be square, rectangular, triangular, and trapezoidal.

Finally, it is necessary to note the following. Crib support has different purposes. According to the functions performed, it may be divided into two types: yielding and supporting (supporting the sides of development workings, domes, etc.). However, the principal function of cribs is to serve as yielding support in stoping faces. In this role it is of practical interest.

Thus, according to a whole series of the distinctive features cited, crib support may be subdivided into various types.

Basic scheme for constructing a classification of crib yielding support

The classification of cribs according to some of the distinctive features indicated above (weight, yield, resistance) cannot be correct, since these may vary within wide limits for cribs of one and the same class. And such distinctive-

features as material, the presence of devices to facilitate release from under pressure, and shape in plan cannot be taken as the basis of classification, since in the present case they are not essential.

For a classification to be correct, it must satisfy the following requirements: first, there must be such a totality of classes as embraces all the objects being classified; second, there must be a more or less clear distinction between classes. The task is to find those distinguishing features that would satisfy the requirements of a correct classification.

First of all, crib support is an artificial (engineering) structure. The essence of this concept lies in the constructive formation of the object. We therefore believe that the main feature by which crib support may be classified will be the complexity of the structure. From this point of view, crib support may be subdivided into three classes: simple, frame-type, and composite. The degree of structural complexity is the starting point in our classification.

As regards the subdivision of classes into groups, it is necessary to dwell on the following. The distinguishing feature by which classes may be subdivided into groups will be the degree of dismantlability of cribs. An important factor is also the method of moving the crib. The method of moving and the degree of dismantlability of the crib are closely connected with each other and, taken together, constitute a distinguishing feature within a class. Taking the foregoing into account, we distinguish dismantlable portable, semi-dismantlable, and non-dismantlable mobile cribs.

In considering groups, one may distinguish individual structures or types of cribs depending on the material, the nature of the arrangement of the elements, etc.

Thus, the totality of classes, groups, and types provides a classification of crib yielding support (table).

Brief remarks on the use of metal cribs of various designs as yielding support

The need for rigid support in roof control by the method of complete caving led to the use of metal cribs laid up from sections of mine or railway rails, I-beams, and the like. Cribs made of such material are metal-intensive, and as the seam thickness increases the labor productivity of crib setters decreases. In order to reduce the labor input of work involved in moving cribs, a semi-dismantlable mobile structure was created by welding or bolting a dismantlable portable crib to a certain height (up to \(2/3\) of the seam thickness). The upper part of the crib was dismantled during movement. Such a structure has substantial shortcomings (great weight, difficulty in removing rock that has fallen inside), owing to which it did not become widespread.

In order to eliminate the shortcomings inherent in simple semi-dismantlable mobile cribs, semi-dismantlable mobile cribs of the frame type were designed. The latter solved two problems: first, the quantity of metal required for their manufacture was reduced; second, labor productivity in moving them increased, especially as the seam thickness increased. Cribs

Table

Proposed classification of crib landing supports

this group have become widespread in foreign practice, and have also been tested in a number of basins of the Soviet Union. As for the attempt to create a non-dismantlable crib structure of the frame type, it has in practice not been crowned with success.

In considering complete crib support, it is necessary to dwell on the following point. Cribs of this type performed the role not only of pack support, but also, in part, of near-face support. The first trials showed that complete support, especially non-dismantlable movable support in crib form, owing to its bulkiness, cannot become widespread.

Finally, it should be noted that the attempt to create semi-dismantlable, non-dismantlable, and complete crib supports led to the design of organ walls, pack props, and complete mechanized movable supports of the supporting type.

At the present time, various types of pack support are used in mining practice. Among them, metal cribs occupy an important place. Thus, as of January 1, 1956, according to data of the “Kuzbassugol” combine, metal cribs were used in 61 longwalls; in the Donbass, as of January 1, 1955, 206 longwalls were equipped with them [3]. As regards the type of cribs, dismantlable portable ones are used. This is explained by the availability of material at hand for their manufacture (mainly mine rails). Other types of cribs are not used. This must be recognized as abnormal. To ease the labor of crib-setters and to reduce timber consumption, in appropriate conditions frame-type cribs should be used; these are very simple to manufacture.

Along with the creation of pack supports more convenient in operation, it is necessary also to improve crib support. This follows from the available experience in roof-control work: for specific mining-and-geological conditions, workable individual portable (movable) supports, as well as mechanized supports, must be created. In particular, in longwalls working seams with a large number of rock partings, on seams with disturbed bedding, at least in the next few years portable cribs will become widespread; and for longwalls with weak and unstable roof or floor, workable designs of semi-dismantlable movable frame-type cribs must be created. This is noted in the domestic mining-engineering literature [3; 5].

Thus, the proposed classification of crib pack support:

1) makes it possible to select a workable crib design for particular mining-and-geological conditions;
2) reflects the tendency toward the creation of more advanced pack supports of various designs;
3) makes it possible to generalize the experience of designing crib support.

LITERATURE

1. Bochkarev V. G. Roof control in longwalls by means of metal cribs. Ugletekhizdat, 1948.
2. Erofeev V. F. Roof control using wooden movable cribs. Collection “News of Coal-Mining Technology.” Ugletekhizdat, 1947.
3. Ilshtein A. M., Lykov I. F., Bushuev N. P. Experience in improving methods of roof control in the longwalls of Donbass mines. Ugletekhizdat, 1956.
4. Kobishchanov M. A., Gleskul M. N. Metal support of mine workings. Ugletekhizdat, 1952.
5. Korzhikov M. I. Pack support. Ugletekhizdat, 1955.

1. Onishchenko P. N. On lightweight metal chocks. Ugol’, No. 9, 1949.

2. Pshenko M. M. A new non-dismantling mobile chock, in the collection News of Coal-Mining Technology, Ugletekhizdat, 1950.

3. Faerman E. M. Basic principles of the classification of mine supports. Izvestiya AN SSSR, OTN, No. 11, 1948.

4. Exhibition of roof-control equipment, Col Eng, 1950, vol. 27, No. 332.

5. Stassen P. Le soutenement metallique en taille, Ann. des Min. de Belg., 1951, Numero special.

6. Steel E. Experience with steel chocks, Iron and Coal Trades Rev., 1945, vol. 150, No. 1401.

7. Steel E., Hebblethwaite J. Steel chocks in the longwall face roof-support system, Trans. of the Inst. of Min. Eng., 1947, vol. 106, part 7.

8. The use of chocks on coal-faces, Col. guard., 1944, vol. 169, No. 4380.

1. Cribs are sometimes called “cutting support.” The term “breaker support” should be regarded as more appropriate, since crib support is used not only in roof control by the caving method (author). ↩