STUDY OF THE OPERATION OF NEAR-FACE AND SPECIAL SUPPORTS IN A LONGWALL

V. A. Shalavrov

(Presented by the scientific seminar of the departments of bedded-deposit mining, ore-deposit mining, mine construction, safety engineering, and mine ventilation)

The purpose of this article is, on the basis of observations carried out, to attempt to analyze the operation of near-face and special supports, taking into account the mining-and-geological conditions and the state of the “roof–support” system.

For the Leninsk district of the Kuzbass, gently dipping seams of medium thickness are characteristic. Coal extraction is carried out by longwalls from 80 to 200 m in length. For supporting longwalls, wooden near-face supports are used in combination with special supports; as the latter, clusters made of rail ends are the most widely used. Recently, clusters made of metal props of the SGK and SDT types have begun to be used.

However, the choice of support is often made without regard to the mining-and-geological conditions and is determined by the availability of support material, which leads to a mismatch between the supports used and the behavior of the rocks when the roof is exposed.

We conducted observations of the behavior of the roof rocks and the condition of the support in one of the longwalls, supported by clusters of SDT metal props in combination with wooden near-face support.

The solution of the problem is based on the hypothesis of G. N. Kuznetsov [1] on the caving and subsidence of rocks, confirmed by mine observations and laboratory investigations.

The analysis of support operation given in the article brings us closer to solving the problem of selecting special and near-face supports with consideration of their joint operation under definite mining-and-geological conditions.

Brief description of operations in the longwall

Longwall No. 58 was extracting a section of the Tolmachevsky seam of the S. M. Kirov mine. The length of the longwall was 130 m. The extracted seam thickness varies from 1.8 to 2.1 m. The dip angle is 6–8 degrees. The longwall face line is arranged along the dip of the seam.

The immediate roof consists of a bed of argillite of medium strength, 2–3 m thick. Above it lies a sandstone bed with a thickness of 10 m and more. The immediate roof is stable and permits exposures of up to 15 sq. m. The floor of the seam is argillite of medium strength.

No indentation of the props into the floor is observed. The hydrogeological conditions of the work are favorable; there is no inflow of water from the roof.

Coal extraction is carried out by a “Donbass” combine to a thickness of 1.4 m. The undercut depth is 1.45 m. A coal layer up to 0.5 m thick, left in the roof, either collapses by itself when exposed over 6–10 sq. m, or is broken down with a pick.

The working space is timbered. The frame of the near-face support consists of two props and a cap, placed perpendicular to the face line. In the middle part of the longwall, additional props made of split timber are driven under the cantilevers of the caps on the face side. The distance between frames is 0.9 m. The density of the near-face support is 1.56 props/m².

As special support, chocks made of timber props were used, 10–12 pieces in each. The timber consumption per 1000 t of output was 51.5 m³. Subsequently the timber chocks were replaced by chocks made of metal SDT props, 7–9 pieces in each. Timber consumption per 1000 t of output was reduced to 46.2 m³. The chocks are installed parallel to the face line and are moved after each cycle. The distance between chock centers is 2.7 m.

Roof control is carried out by the method of complete caving of the rock (Fig. 1).

Observation Method

For observations of the behavior of the roof in the middle part of the longwall as it advanced, two rows, I and II, of measuring stations were constructed, arranged along the strike of the seam (Fig. 1). Row I corresponded to the section of the longwall where the seam was extracted to its full thickness; row II, to the section where a coal layer 0.2–0.25 m thick remained in the roof. The distance between rows I and II was 10 m.

As the face advanced by one cycle, one measuring station was installed in each row at a distance of 0.8 m from the face. The measuring station consisted of wooden reference points driven into boreholes drilled in the floor and roof to a depth of 0.4 m. The point in the floor was located beneath the point in the roof. Measurements were made using a sliding measuring prop SUI-2 with an indicator head, making it possible to take readings of roof subsidence with an accuracy of up to 0.01 mm. Observations were made of the behavior of the roof rocks depending on the operations of the production cycle, and the character of roof subsidence was established as the observation points moved away from the face. To determine the influence of coal extraction by the combine on the behavior of the roof rocks, measurements were made of the rates of its subsidence at points located in rows I and II at distances of 2.3 and 0.8 m from the face.

Fig. 1. Longwall support layout

At points located at a distance of 2.3 m from the face, observations of the behavior of the roof began before the combine reached the observation point, at a distance of 15–18 m from it, and ended after it had passed by a distance of 15–20 m. Indicator readings were taken every 2–5 min. This made it possible to determine the influence of coal extraction on the intensity of roof subsidence.

At the point located at a distance of 0.8 m from the face, only the average rate of roof subsidence during the passage of the combine was determined. The general character of roof subsidence as the face advanced was established by daily observations of the displacement of the roof rocks at observation points in rows I and II.

Observations were carried out from 2.X to 10.X 1955.

Influence of Coal Extraction by the “Donbass” Combine on Roof Subsidence

In the process of coal extraction by the combine in the longwall, as the observations showed, very intense roof subsidence takes place both ahead of the combine and behind it. At a distance of 2.3 m from the face ahead of the combine, an increase in the intensity of roof subsidence is observed at a distance of 12–16 m from it, increasing as this distance decreases from 0.8–3 to 9–15 mm/hour. At the same time, the pressure on the near-face and special supports increases.

After the combine has passed the observation point, the magnitude of the exposed roof space behind it increases, and the intensity of roof subsidence continues to increase for some time. The greatest rate of roof subsidence is observed at a distance of 2–4 m behind the combine and amounts to 9–21 mm/hour. With further movement of the combine, roof subsidence behind it begins to decrease and reaches a value close to the minimum after 15–20 m (Fig. 2),

Fig. 2. Influence of coal extraction by the combine on the rate of roof subsidence.
I—measurements of 4.10.55; II—measurements of 5.10.55; III—measurements of 6.10.55; IV—measurements of 7.10.55.

depending on the speed of advance of the combine, the condition of the support, and the completeness of collapse of the rocks in the worked-out space. Incomplete removal of the support in the caved space increases the period of intense roof subsidence.

The coal pack remaining in the roof because of insufficient height of the cutter-loader bar separates more quickly and with a smaller exposed area when roof subsidence is more intense.

The support is set after the cutter-loader’s loader, but its lag by 3 m is not accompanied by falls of pieces of rock.

During the time required for the cutter-loader to pass the observation point, located at a distance of 0.8 m from the face, and for the support to be installed at this place, roof subsidence amounted to from 18 to 34 mm, which corresponds to an average rate of subsidence over this period of from 4.7 to 14 mm/h.

Influence of Transferring Packs on Roof Subsidence

Especially intensive roof subsidence in the working face is observed during the transfer of packs and continues until the rock of the immediate roof collapses beyond the limits of the supported space. At this time the roof rests only on the near-face support: the special support does not take up the roof pressure for some time, since the props in the packs are set without preliminary wedging. During this period the props of the near-face support carry a large load. The caps of the near-face support at a distance of 2.3 m from the face are compressed by up to 50–60% of their thickness or break.

The rate of rock subsidence at a distance of 3.6 m from the face reached 20–30 mm/h. In row I the rate of roof subsidence increases almost linearly with distance from the face (Fig. 3, line I), while in row II the increase occurs in a diminishing progression (Fig. 3, line II).

Fig. 3. Influence of transferring packs on the rate of roof subsidence.
I—section of the longwall without a coal pack; II—section of the longwall with a coal pack.

On the basis of such a distribution of the rate of roof subsidence, it may be assumed that it proceeds by rotation of the broken-off cantilever, with a hinge along the face line. Subsidence resulting from the influence of transferring packs at a distance of 2.3 m from the face amounts to 120–150 mm. The magnitude of roof subsidence of 70–75 mm at this distance from the face, corresponding to the period from coal extraction to the transfer of the packs, did not exceed the permissible roof deformations. Only crushing of the lagging in the near-face support of the preceding cycle was observed.

Immediately after the transfer of the packs, the magnitude of roof subsidence exceeds the limit of elastic deformation of the rocks, and cracks appear in the roof at the face, with a slight displacement of the rocks into the working space.

The amount of roof subsidence at which the cantilever of the immediate-roof rocks at the face breaks is 110–120 mm at a distance of 2.3 m from the face.

The nature of roof subsidence as the face is retreated is shown in Fig. 4.

Fig. 4. Roof subsidence with retreat from the face

Influence of Roof Subsidence on the Operation of the Support

When directly above the coal seam there lies a layer of easily caving rock whose thickness is insufficient for the caving, upon its collapse, to provide support for the overlying bed of hard-to-cave rocks, two possible cases of support operation are distinguished.

1. The cantilever of the immediate roof breaks over the face under the influence of its own weight. It is considered that in this case the main roof does not affect the behavior of the immediate roof, since the overhang of the main-roof cantilever has not reached the magnitude at which its collapse occurs. The working-space support takes only the weight of the broken-off cantilever of immediate-roof rocks and operates under conditions of a specified load.

At the moment when the special support is being moved, the load on one prop of the face support will be:

\[ P=\frac{h\gamma r}{n}, \tag{1} \]

where: \(P\) is the load on one prop, t;
\(h\) is the thickness of the immediate roof, m;
\(\gamma\) is the unit weight of the rocks, t/m³;
\(r\) is the coefficient of nonuniformity of pressure distribution on the prop;
\(n\) is the installation density of the face support, props/m².

When the support operates under conditions of a specified load, the magnitude of the pressure on a prop is directly proportional to the thickness of the immediate roof and inversely proportional to the installation density of the support.

2. The case will be fundamentally different if a collapse of the main roof occurs above the face. As it subsides, the main roof will exert very great pressure on the support, which will begin to deform. During subsidence, the collapsed cantilever of the main roof does not lose its connection with the rock mass; rather, it rotates about an axis whose projection is point \(O\) (Fig. 5). The rock mass is one of the supports that sustain the collapsed cantilever. The support cannot serve as the other support, since this would require very great resistance from it; the collapsed rocks of the immediate roof will serve as such a support. The support will deform until the collapsed block of the main roof encounters the second support. These will be the operating conditions of the support under a specified deformation. After the roof subsides by the magnitude of the specified deformation, the support will again operate under the conditions of a specified load.

Fig. 5. Mechanism of subsidence and collapse of rocks

The magnitude of subsidence \(\eta_0\) of the main roof at point \(B\) (Fig. 5) will depend on the thickness of the seam \(m\), the thickness of the rocks of the immediate roof \(h\), and the degree of their loosening \(k\), or

\[ \eta_0 = m - h(k - 1). \tag{2} \]

The subsidence of the immediate roof will be governed by the subsidence of the main roof. We assume that, while subsiding, they rotate through an equal angle. Then the magnitude of subsidence of the immediate roof will depend on the same factors on which the subsidence of the main roof depends, and also on the distance to the face, and is determined from the following relation:

\[ \frac{\eta}{\eta_0} = \frac{l}{L}, \quad \text{whence, by formula (2)} \]

\[ \eta = l \frac{m - h(k - 1)}{L}, \tag{3} \]

that is, the magnitude of subsidence of the main roof, for constant \(m\), \(h\), and \(k\), will be directly proportional to the distance to the face \(l\) and inversely proportional to the step of collapse of the main roof \(L\).

In the case considered by us, the thickness of the immediate roof is \(h = 3\) m. Above it lies a thick bed of sandstone. The extracted thick—

ness of the seam \(m = 1.8\) m. The caving interval of the main roof is \(L \simeq 9\) m. At a distance of 3.9 m from the face, corresponding to the greatest value of the cantilever of the immediate roof, the amount of its subsidence under the conditions of prescribed deformation will be

\[ \eta = 3.9 \frac{1.8 - 3(1.3 - 1)}{9} = 0.39\ \text{m, or } 390\ \text{mm}. \]

The greatest amount of subsidence observed in longwall No. 58 at this distance from the face was 401 mm.

The insignificance of the difference between the roof-subsidence values observed and those calculated by formula (3) makes it possible to suppose that, in the longwall, the support operates under conditions of prescribed deformation. This is also indicated by the presence, on individual days, of large fluctuations in the rates and magnitudes of roof subsidence and by the nature of the distribution of subsidence rates when the packs are moved. Therefore we shall analyze the operation of the support on the basis of the principles applicable to conditions of prescribed load and prescribed deformation.

Let us consider how changes in the quantities entering formula (3) affect the operation of the support.

When \(h(k - 1) < m\), the caved rocks of the immediate roof will not undercut the main roof. In this case the support will be observed to operate both under conditions of prescribed load and under conditions of prescribed deformation. The smaller \(h\) is, the more the main roof will subside and the more strongly its subsidence will affect the condition of the support. Caving of a cantilever of immediate-roof rocks of small thickness does not cause difficulties in controlling it.

When \(h(k - 1) = m\), subsidence of the main roof can occur only as a result of further compaction of the caved rocks.

For \(h(k - 1) > m\), subsidence of the main roof will be very insignificant, since, when the rocks of the immediate roof cave, complete undercutting of the overlying strata will occur. Thus, an increase in the thickness of the immediate-roof rocks reduces the influence of main-roof subsidence.

However, as the thickness of the immediate roof increases, the load on the support will increase when it operates under conditions of prescribed load, since, when the cantilever of the immediate roof caves at the face, the increase in pressure on the props will be directly proportional to the thickness of the immediate roof (formula 1).

Let us consider the combinations of face-end and special supports used under the conditions of the longwall under discussion.

Wooden packs and wooden face-end support

In this case the characteristics of the face-end and special supports are similar: the very high resistance and rigidity of wooden props in frames and in packs. Yielding is possible only through crushing of the caps or by indentation of the props into the floor.

Under conditions of prescribed load, wooden packs fully satisfy the requirements for supporting the caved cantilever of immediate-roof rocks above a row of special support possessing great rigidity and resistance. Under conditions of prescribed deformation, the presence of large roof subsidences pri-

leads to compression of the clusters, which makes their dismantling difficult, and to disturbance of the face support: crushing and breakage of the caps, pulling of the props out from under them, and breakage of the props. In the support of the preceding cycle, on some days breakage of caps reached 30%, and fractures of props 15% of their total number. Local roof disturbances often occur above the clusters. For the convenience of dismantling the clusters and for detecting dangerous magnitudes of roof displacement, it is necessary to leave behind the row of special support a so-called control path, the maintenance of which increases the unsupported cantilever of the rock by 1 m or more. This greatly complicates the possibility of reusing the timber of the face support.

Thus, the use of wooden face and special supports under large subsidence of the rocks, i.e., under conditions of prescribed deformation, should be considered irrational, as not meeting the requirements imposed on supports.

Clusters of metal props and wooden face support

At present, in the stoping workings of the mines of the Leninskii district, metal props are used only as special support—clusters—in combination with wooden face support. Types of props with increasing resistance under yielding are used (STK and SDT). The characteristic of the props in a cluster is determined by a rapid increase in resistance with insignificant settlement. Clusters of SDT props are yielding, and during their operation neither deformation of the props nor roof disturbances above the clusters are observed. The resistance of the clusters is also quite sufficient for achieving collapse of the rocks of the immediate roof behind the row of special support. The use, as special support, of clusters of metal props is quite effective in its resistance for conditions of prescribed load, and in yielding it satisfies the conditions of prescribed deformation.

Wooden face support does not possess the necessary yielding. Being more rigid than the special support, as it is moved away from the face it carries an ever-increasing load and is damaged. Increasing the diameter of the props in this case increases the consumption of timber per unit of output, but does not bring about any significant improvement in the condition of the roof.

Thus, the combination of clusters of metal props and wooden face support should be considered irrational, because of the rigidity of the wooden props of the face support.

On the basis of the observations made of roof-rock movement, and applying the division of support action into operation under conditions of prescribed load and prescribed deformation, we shall determine rational characteristics of supports for the conditions of the longwall under consideration.

Determination of the rational characteristic of the special support

The special support serves as a cutting abutment when the roof caves beyond the limits of the supported space and does not allow the caving to spread into the working space supported by the face support [2].

In studying the operation of support, G. N. Kuznetsov notes that, in order to achieve breakage of the rocks of the main roof above an organic support, in most cases a very high bearing capacity of the special support is required, together with very small yielding of the “support—

ground.” Meeting these requirements in many cases is very difficult and would require structurally very heavy special supports. The author further notes that in some cases caving of the main roof over the face does not create special difficulties in roof control if, in the immediate roof, there is a stratum of readily caving rocks whose thickness is less than 6–8 times the seam thickness, while above it lies rock that caves only after a considerable area has been exposed. Caving of the main roof over the face makes it possible to use lighter special supports. This is possible if the support is yielding, since the load on it increases as the yielding capacity decreases.

Thus, by using yielding special supports, it becomes possible to control the roof when the main roof caves over the face.

No sharp alternation of conditions of prescribed load and prescribed deformation is observed as the production face advances, although observations of roof behavior clearly show that subsidence of the main roof on different days has a very different effect on subsidence of the immediate roof. Therefore, the yielding capacity of the special support should be taken according to the greatest daily value of roof subsidence at a distance of 2.3 m from the face. For the conditions of longwall No. 58 this amounts to 180–200 mm. According to their working characteristics, these may be supports of the organ-pillar types OK, OKU, MOK.

Determination of the rational characteristic of the near-face support

The near-face support is intended to protect the working space from the fall of individual pieces and blocks of rock of the immediate roof; with the method of complete caving of the rocks, it is used only together with a special support.

The advance increment of the props of the near-face support as the production face advances is considerably greater and is approximately 2.4 m. The greatest distance of the props from the face is approximately 3.7 m.

Roof subsidence under conditions of prescribed deformation is determined by formula (3), and for our conditions $\eta = 370$ mm. The yielding capacity of the props of the near-face support should be selected with reference to this magnitude of roof subsidence.

The yielding capacity of the support is directly dependent on the magnitude of roof subsidence in the supported space, while remaining smaller than it because of crushing of the caps and indentation of the props into the floor. The relationship between them remains to be clarified; Prof. P. M. Tsimbarevich [5] was the first to draw attention to this.

The resistance of the props must be such that the near-face support can support the immediate roof. We determine the magnitude of the resistance of the props by formula (1). Under our conditions, when $\gamma = 2.2$, $h = 3$ m, $n = 1.56$ t/m$^2$, $r = 2$ [1], $P = 8.6$ t.

The insignificant resistance and the large required yielding capacity make it possible to determine that, under the given conditions, it is advisable to use lightweight metal props with very large yielding capacity and low resistance as the near-face support. These may be props having working characteristics of the TS or ST types of constant resistance props.

Conclusions

As a result of the observations made of the behavior of the roof rocks and the analysis of support operation, the following conclusions can be drawn:

1. Coal extraction by a cutter-loader causes intensive subsidence of the roof over a section of the longwall measuring 25–35 m: ahead of the cutter-loader the influence of extraction is manifested at a distance of 12–16 m; behind it, at a distance of 15–20 m. The most intensive subsidence of the roof occurs during the shifting of the special support.

2. Observations of the behavior of the rocks in the longwall make it possible to conclude that the support operates sometimes under conditions of a prescribed load, and sometimes under conditions of prescribed deformation, as indicated by the presence of large roof subsidences on certain days.

3. An increase in the thickness of the immediate roof reduces the effect of subsidence of the main roof on the working space; it reduces the magnitude of subsidence and requires an increase in the resistance of the support.

4. Increasing the caving step of the main roof has a favorable effect on the condition of the immediate roof, reducing the magnitude of its subsidence per unit distance from the face.

5. The use of timber face support and timber special support satisfies the requirements for the state of the roof under conditions of a considerable thickness of the immediate roof, and is irrational under conditions of large subsidences, i.e., under conditions of prescribed deformation.

6. Sets made of SDL metal props, in terms of their operating characteristic, may be used both under conditions of prescribed load and under conditions of prescribed deformation; however, their use in combination with timber face support is irrational because of the low yielding capacity of timber props.

7. In the case where the support operates under conditions of prescribed deformation, which occurs in longwall No. 58 of the Tolmachevsky seam, yielding supports should be used.

8. The yielding capacity of the support should be selected with allowance for its operation under conditions of prescribed deformation, and its resistance under conditions of prescribed load.

References

1. Kuznetsov G. N. On the mechanism of interaction between side rocks and support in stoping workings of gently dipping seams. Ugletekhizdat, 1954.

2. Korzhikov M. I. Yielding support. Ugletekhizdat, 1955.

3. Tsimbarevich P. M. Mechanics of rocks. Ugletekhizdat, 1948.

4. Avershin S. G. Calculation of the displacement of rocks. Metallurgizdat, 1950.

5. Tsimbarevich P. M. On the theory of rock pressure in a coal longwall. Scientific Transactions of the Moscow Mining Institute, Collection No. 11, 1953.