On the Design of Shield Supports

N. G. Kapustin

(Presented by Prof. P. M. Alabuzhev, Doctor of Technical Sciences)

In recent years, coal production in the mines of Kuzbass by the system with shield roofing has grown to 34% of the basin’s total output. Such a rapid spread of this system is explained by its major advantages and high techno-economic indicators, as is seen from the data of Table 1. However, the established practice of mining operations with shield roofing consists in the use of shields on steeply dipping seams.

The mining community has only limited experience in working thick coal seams of inclined dip by means of shield supports. Nevertheless, the operability of shields on rollers under conditions of seam dip angles of 37–45° has been established, as follows from operating experience in Kuzbass mines (mine No. 3—3 bis, on the Gorely seam, 8–10 m thick, with a dip angle of 34–40°; the “Maneikha” mine; the “Krasnotorskaya” mine; and the Dzerzhinsky mine).

Experimental work in Kuzbass made it possible to reveal the influence of the shape of the shield on its mobility. For example, a sickle-shaped shield on rollers, under the conditions of the Dzerzhinsky mine in 1945, advanced 33 m along the dip, showing good operability. Industrial tests of a tent-shaped sliding shield at the “Dalnie Gory” mine showed that its hemispherical profile contributed to its normal operation. The systematic guidance of Chinali’s paired shields also contains a valuable indication of the influence of the geometric shape of the shield on its operability, for by lowering the paired shields alternately we, as it were, transform a plane into a hemisphere.

The movement of the aforementioned sickle-shaped shield along the floor of the seam then proved to be more energetic than along the roof. The shield, as it were, rotated about its upper end, with the constant threat of waste rock breaking through under the shield from the floor side. These experiments, known in Kuzbass, are not accidental. On the contrary, they contain a direct indication from practice of the need to impart a convex shape to shield roofing. In this case it is important to understand that the points of support of the shield on the roof and floor of an inclined seam should preferably be brought forward relative to the outer convex surface of the roofing that takes the pressure of the caved rocks; the greater the curvature achieved in the shield, the better its expected operability. To these considerations one must add the constant mobility (“stirring”) of the caved rocks during the guidance of paired—

Table 1

Technical-and-economic indicators of shields on rollers and longwall mining systems under identical mining-and-geological conditions

…of shields, which guarantees the continuous movement of the caving behind the cover; this is especially important in the mining of inclined seams.

At present it appears possible to set forth the basic principles for the design of a protective shield support with a new kinematic scheme of guidance. The design development of such a shield would make it possible to put to industrial trial a shield mining system for extracting inclined thick seams, which would serve the purpose of radically improving methods for mining coal deposits.

New Kinematic Scheme for Advancing the Shield

When mining an inclined seam from top to bottom, above the working face we have caving, and below it—a coal pillar. If the seam is divided by a plane parallel to the bedding into an upper and a lower bench, with extraction of the upper bench leading by 2.5–3 m, then the working face will constitute a single working space for the entire thickness of the seam, expressed, as shown in Fig. 1, by the upper face 1, the berm 2, and the lower face 3.

We adopt the solution of enclosing the upper face with a shield that would take up the pressure of the caved rocks and would transmit this pressure, partly or completely, to the berm of the face through a rigid sector support 4. The working space of the lower face is likewise enclosed by shield 5, freely and hingedly suspended from the upper shield. Shield 5 can rotate about hinge \(B\), and, when lowered somewhat downward, can freely bear against the floor of the seam and rotate about point \(A\) of contact with the seam floor.

The motion of the shield occurs following the extraction of coal downward along the dip of the seam and consists of two successive stages, not simultaneous in time. The first stage is the rotation of the lower shield 5 about the suspension axis (hinge \(B\)). During this time the upper shield is immobile, which is achieved by its bearing against the face breast and against the berm under the action of the caved rocks.

Fig. 1. Elements of the shield and of the shield face.

The second stage is the rotation of the lower shield 5, bearing against the seam floor at point \(A\) (according to Fig. 1), and its drawing (pushing) of the upper shield downward along the dip of the seam. The upper shield is hingedly connected with the lower one and is drawn along by it, transmitting the external loads through the special sector supports 4 to the berm made of solid coal, along which the shield moves downward along the dip of the seam at the constant level of this berm.

For the purpose of substantiating the kinematics of the shield motion according to the scheme described, we take from theoretical mechanics\(^1\) the following statement on the degree of freedom of a kinematic chain composed of plane bodies, namely:

a) a free body has three degrees of freedom;

b) a supporting rod has one degree of constraint and two degrees of freedom;

c) a fixed hinge (Fig. 2) has two degrees of constraint and one degree of freedom;

Fig. 2. Fixed hinge with one degree of freedom (support on the floor).

d) a movable hinge (Fig. 3) has one degree of constraint and two degrees of freedom.

\(^1\) Prof. S. M. Rabinovich, Course of Structural Mechanics. ONTI, 1938.

In Fig. 4 a chain of two plane bodies is presented, for which the number of bodies is \(D=2\), the number of hinged connections of the bodies is “\(Ш\)” \(=1\), and the number of connecting rods is \(C_c=3\). If one disregards the supports \(A, B, C\), then the number of degrees of freedom of this system will be \(3D=6\); however, the support rods and the middle hinge restrict the freedom of these bodies by \((2Ш+C_c)\) degrees. Thus, for the chain of bodies in Fig. 4 the degree of freedom will be

\[ 3D-(2Ш+C_c)=6-(2+3)=1, \]

that is, the system has one degree of freedom, as was required to be proved.

In the figure: “Upper shield”; “design”; “scheme”.

Fig. 3. Hinged movable support with two degrees of freedom (support on a berm).

In the second stage of shield motion its kinematic scheme strictly corresponds to the scheme of Fig. 4, a; consequently, the shield of the proposed scheme has one degree of freedom—motion downward along the dip

In the figure: “motion”; “scheme”; “design”; \(A\), \(B\), \(C\).

Fig. 4. Kinematic pair with one degree of freedom.

In the figure: “roof”; “berm”; “soil”; “trajectory: \(A-A'\) ... first stage; \(B-B'\), \(C-C'\) ... second stage”; “Point \(C\) moves along a straight line; \(A'\) is the sliding point of contact of the sector with the berm”.

Fig. 4a. Kinematic scheme of the shield for one step.

of the seam; moreover, this motion is the only possible one. In the first stage, when the lower shield 5 freely rotates about the fixed middle hinge \(B\) according to Fig. 4, the upper shield proves to be

immobile, since under the action of the caved rocks it is pressed against the face and the berm.

On the basis of the considerations set forth, we draw the following conclusion:

a) in the first stage of motion, after blasting the supporting coal pillar on the lying side of the seam, the lower part of the shield can be lowered without disturbing the rest position of the upper part of the structure. Here, as it were, the principle of guiding paired shields, which occurred in the Kuzbass mines, is manifested;

b) after blasting the upper bench, the shield acquires the only definitely known degree of freedom, namely: rectilinear motion of the upper shield along the berm. This motion is carried out until the shield encounters an obstacle, i.e., the face in its new position after blasting;

c) in all cases the design scheme of the shields as a kinematic pair is statically determinate.

Labels in the figure: “Upper shield”; “Lower shield”.

Fig. 5. Design of shield elements.

The overall dimensions of the shield are determined by the thickness of the seam. It is desirable to reduce the height of the upper face in order to relieve the shield from the harmful frictional forces of its contact with the caved mass when rotating about point A. The dimensions of the shield will also depend, to a certain extent, on the strength dimensions of the coal berm that takes the pressure of the sector supports.

Under the influence of rock pressure, the berm will tend to cave from the side of the face. This caving of the berm of the lower bench will manifest itself in the form of Lüders cracks, which, as is known, are arranged at an angle of 30° to the destructive force. In addition, destruction of the lower bench may follow in the form of sliding at an angle (to the plane of the berm) \(\Theta = 45 + \frac{\varphi}{2}\), where \(\varphi\) is the angle of internal friction of the coal. Proceeding from the orientation of the Lüders cracks and the Coulomb sliding plane, there arises an extreme position of the shield, which (position) should be determined by an angle \(\gamma\) of less than 30° to the line of the supporting force on the berm, i.e., to the line of the true seam thickness. In order to preserve the berm, the rotation of the shield during the first plus second stages of motion cannot exceed the angle \(2\gamma = 60^\circ\).

The strength dimensions of the shield and the berm will require a preliminary calculation of the external loads on the shield. The external forces tending to destroy the shield or the coal berm, and in a favorable case to move the shield along the line of dip of the seam, should be determined by constructing force diagrams. An example of such diagrams is given in Fig. 6.

Let us name the external forces acting on the shield:

1. Rock pressure from the caved rocks on the upper shield — \(Q_v\).
2. The friction force caused by this pressure — \(T_{Qv}\).
3. Rock pressure on the lower shield — \(Q_n\).

1. The friction force caused by this pressure—\(T_{QH}\).

2. The rock pressure of the seam roof on the canopy of the upper shield—\(P\).

3. The friction force caused by this pressure—\(T_p\).

4. The reaction of the shield abutment against the upper coal face—\(E\).

5. The reaction of the berm on the supporting sector of the upper shield—\(N\).

6. The friction force caused by this reaction—\(T_N\).

7. The reaction of the abutment of the middle hinge of the shield (point \(B\) in Fig. 4) against the edge of the berm—\(W\).

8. The friction force caused by this reaction—\(T_w\).

9. The reaction of the seam floor—\(U\).

Fig. 6. External forces acting on the shield in the second stage of its motion.

In addition, of interest is the internal reaction between the upper and lower shields, arising in the middle hinge, \(R\); and the weight of the shield, \(G\).

In the first stage of motion, the stability of the upper shield is ensured by the equilibrium of the forces \(Q_B\), \(P\), \(E\), \(W\), \(N\), \(T_{QB}\), \(T_W\), \(T_N\), \(R\), \(G\).

In the second stage of motion, the driving force is the resultant of the forces \(Q_B\), \(Q_H\), while the retarding force is the resultant of the forces: the weight of the shield, \(G\), and the friction forces \(T_p\), \(T_{QB}\), \(T_N\).

From the condition for possible motion of the shield in the second stage (rotation about point \(A\) in Fig. 1), it follows that, for inclined seams (with an angle greater than \(25^\circ\)), reliable fixation of the upper shield occurs under the action of rock pressure, which makes it possible to unload the lower shield from longitudinal (along the beams) forces and to turn it about hinge \(B\) through an angle \(2\gamma\).

It should be noted that, in gently dipping seams (with an angle of less than \(25^\circ\)), the rock-pressure forces are insufficient for fixing the upper shield. For this purpose it is possible to use a system of tension tackle blocks that constantly pull the upper shield downward along the dip. By this measure the range of application of “walking” shields is expanded.

By constructing force diagrams for the second stage of shield motion (about point \(A\) in Fig. 1), one can verify that motion of the shield under the action of collapsed rock occurs at all angles of inclined dip of the seams, i.e., greater than \(25^\circ\). Moreover, the greater the dip angle of the seam, the more reliably the process of self-motion of the shield is carried out.

Coal extraction under the shield is carried out in four successive operations (Fig. 7):

1. Coal is removed from area 1 under the lower shield, but the side pillar 2 is left.

Fig. 7. Sequence of coal breaking under the shield in one cycle.

1. Pillar 2 is blasted, with simultaneous turning of the lower shield through an angle \(2\gamma\).

2. Coal is removed in area 3 under the upper shield; move-

the shield has not yet occurred, in view of the bearing of the middle hinge against the brow of the coal ledge and of the sector against the berm.

1. The coal is blasted over area 4; the shield, now having point \(A\) as its support on the floor of the seam, turns through an angle \(2\gamma\) and, while retaining its dimensions in thickness owing to the sector supports, moves at the level of the berm until it bears against the face.

The process of extracting the coal in 4 stages constitutes one cycle of setting the shield. Since shields on an inclined dip do not experience dynamic impacts to the same degree as is observed when driving shields on a steep dip, the setting step in our case may be allowed up to \(1.5\ \text{m}\), which must be established experimentally under specific conditions. It should be borne in mind that the stronger the coal and the roof of the seam, the greater the setting step.

Structural Description of the Walking Shield

The covering structure consists of two shields, hinged to one another: an upper one—on the side of the hanging wall, and a lower one—on the side of the lying wall. Their description will be given in five directions: the upper and lower shields, the hinge joint, assembly of the shield, and the essence of the methods of driving it.

According to the existing classification, the proposed structure resembles paired non-sectional shields. The upper, as well as the lower, shield is represented by a lagging fastened with flexible steel bands made of channel beams. The logs of the lagging are fastened together in pairs by clamps in such a way that one bolt of the clamp passes through an opening in the channel beam. The ends of the logs are bound with boiler iron. On the side of the hanging wall, aprons are fastened to prevent rock from breaking through under the shield. Similar aprons cover the hinge joint. Two rows of wire mesh, sewn to the logs with staples, are laid over the logs. On the side of the coal face, each metal spatial truss (Fig. 5), performing the role of a sliding sector support, is suspended from the upper shield on five clamps.

The lower shield is similar to the upper one, but has no sector support.

The logs of the lagging of the upper and lower shields form a crest along the hinge line, since they are of different lengths. Along this crest of the upper shield a double-T beam is installed, serving as the axis of the hinge. At the same time, the elongated logs of the lower shield have corresponding openings into which the hinge-forming double-T beam enters.

Thanks to the adopted design of the hinge and to the oval ends of the logs bound with steel, the shields rotate reliably and firmly relative to one another and bear one against the other.

The length of the shield is taken, based on experience with non-sectional shields, as \(35\ \text{m}\).

Assembly of the structure is carried out in a horizontal slice, excavated with caving (possibly with stowing).

The following sequential list of the principal operations for assembling the structure is recommended.

1. Three slice drifts are driven on the lower level, including a drift along the middle of the seam.

2. The coal between the drifts is extracted by a short longwall along the strike with the support plan of an ordinary horizontal slice.

3. On the floor of this slice, as it is formed, the shield is assembled.

1. The roof of the installation crosscut above the completed shield is caved or packed on the basis of experience gained from work in the horizontal layer.

2. As a result of work on the minus level in the installation layer, a situation will be reached in which a paired flexible shield rests on the floor of the layer over an area of approximately 300 sq. m. By this time, coal-discharge chutes should have been driven.

3. A ditch is driven beneath the upper shield for transporting and assembling the demountable metal elements of the sector support. It should be borne in mind that its main fork-shaped belt was fastened to the shield beforehand, during the initial work on the minus level.

4. After completing the suspension of approximately 20 sector supports on the upper shield, we lower the lower shield by 60° relative to the horizon. In essence, after this the shield is considered to be in working condition, provided, of course, that the control part (say, 6 m) of the new shield has already been laid in the adjacent shield pillar.

Conditions of application and advantages

The field of application of the proposed shields is very thick seams of 7–12 meters with a dip angle from 25 to 55°. The coals must not be weak, and the roof rocks must cave by themselves.

It is realistic to create a subvariant of the proposal for strong roof rocks, with stowing of the mined-out space. At seam dip angles of 25–30°, in all cases the use of stowing is necessary, since the caved rocks at such angles will not exert the necessary pressure on the shield; at these same seam dip angles, scraper delivery of coal along the shield is required.

At mines with hydraulic mechanization, hydraulic breaking and transportation of coal will be rational.

If the sliding-friction force of a sector support against the coal face, under the conditions of a seam with a dip angle of 25–30°, proves excessive, then sliding friction should be replaced by rolling friction. For this purpose, cast-iron balls 8–10 cm in diameter are “poured” onto a steel grooved track under each support. As the support advances, the balls emerge from under the load and are carried forward onto the same steel track.

The main advantages of the proposed mining system consist in the fact that it makes it possible to work very thick seams of inclined dip with a shield covering, instead of the prevailing principle of slice extraction.

The use of shield mining instead of slicing systems, as shown by the experience of work at mines 3–3 bis, “Manezha,” and “Krasnogorskaya,” will make it possible to save no less than 10 rubles per 1 t of coal; the productivity of face workers will increase from 69 to 110 t per month, and output from a single face will increase from 2500 to 6000 t per month.