Results of Observations of the Displacement of Side Rocks Ahead of the Shield Face

G. E. Posokhov

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

The study of manifestations of rock pressure and of the process of displacement of the enclosing rocks provides initial materials for designing supports and for refining the dimensions of elements of mining systems. Therefore, in most cases, changes in the loads acting on the support are measured simultaneously with instrumental observations of the displacement of the rock mass.

With respect to the shield system of mining, such comprehensive investigations have not been carried out. Measurements of rock pressure on the shield, performed by the Mining-Geological Institute of the West Siberian Branch of the Academy of Sciences of the USSR [2], give some idea only of the loads on the upper platform of the shield. The pressure on the shield directly from the side rocks was not measured. Meanwhile, as practice has shown, this pressure cannot be disregarded. It is known that, when the longwall face advances slowly, the shield support is deformed, and after prolonged stoppages it, as a rule, becomes unsuitable for further operation.

Observations have shown that in individual cases the canopy of emergency shields bends toward the mined-out space. In all probability, the deflection of the canopy toward the mined-out space is caused by pressure on the shield from the side rocks undergoing displacement. It is interesting to note that, when the shield pillar is being extracted, the support of coal chutes at a distance of 6–7 m below the shield is strongly deformed; moreover, as the shield face approaches, the deformation of the support continuously increases. The broken support falls into the chutes and is often the cause of their clogging.

From what has been said it is clear that displacements of the side rocks in the shield face and ahead of it may reach considerable magnitudes. This is also confirmed by instrumental observations carried out by the author of the article in 1955 at the Voroshilov mine of the Kuzbassugol combine.

The purpose of the instrumental observations was to determine the magnitudes of the relative displacements of the side rocks along the normal to the bedding plane ahead of the shield face, depending on the change in the distance between the measurement points and the working shield.

Geological Characteristics of the Seam

The observation station was established in the extraction area of seam IV Vnutrennii, opened at horizon +60 m by crosscut No. 6. The seam in this area had a regular occurrence. The seam thickness varied from 9.4 to 10.6 m, and the dip angle of the seam from 64 to 65°. The seam was divided into two benches by an interbed of graphitic argillite 0.05 m thick. The thickness of the lower bench was 7.2 m, and of the upper one, 2.2 m. The seam was dangerous with respect to gas and dust. According to the strength of the coal, in accordance with the classification adopted in the standards handbook of the Kuzbassugol combine, the seam belonged to the second category. The immediate roof of the seam was an interbed of soft argillite 0.1 m thick. Above it lay a five-meter layer of strong medium-grained sandstone. The floor of the seam was represented by a layer of argillite of medium stability, 4.5 m thick, which separated seam IV Vnutrennii from seam III Vnutrennii.

Mining Operations in the Extraction Area

The extraction area of seam IV Vnutrennii, where the observation station was established, was worked using a shield mining system. The opening of the seam at the haulage and ventilation horizons was carried out by crosscuts driven from group drifts.

The dimensions of the extraction area were adopted as 270 m along the strike and 89 m along the dip. The boundaries of the mining operations were adopted as 270 m along the strike and 89 m along the dip. The area was bounded along the strike by unworked fields, and along the rise by worked-out extraction fields.

The upper horizons of seam IV Vnutrennii were worked using chamber mining on horizon +220 m and, along the dip, 89 m. The depth of horizon +140 m was worked by a shield mining system with caving. In the worked-out space of the upper horizons, coal pillars of considerable size were left (Fig. 1).

Adjacent seams at horizon +60 m had been opened, but were not yet being worked.

At the time the observation station was established, stoping operations were being carried out in the second shield pillar (No. 39), counting from the boundary of the extraction area (Fig. 1). Roof control was effected by undermining the roof through the transfer of the caved rocks of the upper horizons.

During the extraction of the first shield pillar, after the shield had descended 10–12 m down along the dip of the seam, on the ground surface

Fig. 1. Vertical projection of seam IV Vnutrennii

a cave-in formed. Above the second shield pillar the cave-in formed only during the extraction of the third shield pillar (No. 41).

Observation Method

The observation method was based on the many years of experience of the Siberian Branch of VNIMI in measuring the displacements of side rocks during the working of thick steeply dipping seams of the Kuzbass.

The observation station was set up in the bypass ventilation crosscut “a,” driven from the haulage drift in the plane of the coal chutes to the lower common breakthrough “b” (Fig. 1).

To measure the relative displacements of the side rocks ahead of the shield face downward along the dip of the seam, in a plane perpendicular to the plane of bedding, four pairs of reference marks were installed (Fig. 2).

Fig. 2. Layout of the observation station and relative displacements of the side rocks

Each pair of reference marks constituted an observation point. The reference marks consisted of screw-together metal rods 19 mm in diameter (Fig. 3). The design of the reference marks for measuring displacement of the side rocks during the working of thick steeply dipping seams was developed and introduced into practice by the Siberian Branch of VNIMI. The reference marks were installed in boreholes drilled with sectional drills normal to the plane of bedding. The boreholes were drilled to such a depth that the reference marks could be securely anchored in the rocks of the floor and roof of the seam.

To determine the magnitude of the relative displacements of the side rocks at the observation stations, systematic measurements of the distances between the benchmarks were made using a telescopic rod of the SU-2 type, providing a measurement accuracy of up to 0.2 mm.

Fig. 3. Composite benchmark of the design of the Siberian Branch of VNIMI:
1—component parts of the benchmark; 2—connecting coupling; 3—wooden plug for wedging the benchmark in the borehole.

It should be noted that longitudinal bending of the benchmarks is possible during measurements. However, the effect of longitudinal bending on the accuracy of the measurements is so small that it may be disregarded.

The first measurements were made on April 22, 1955. At that time the shield of the adjacent shield pillar (No. 39) was located above the level of the upper pair of benchmarks at a distance of 11–13 m.

The distances between the benchmarks of each observation station, established from the first measurements, were entered in the observation log as initial values, with which the results of all subsequent measurements were compared.

Simultaneously with the measurements of the distances between the benchmarks, the distances from the observation stations to the working shield were measured each time. In addition, the observation log systematically recorded the entire course of mining operations during each shift, as well as visual observations of surface displacement within the extraction area.

During the first month, measurements and observations were carried out every other day, and subsequently daily.

Results of observations

The observations continued for three months, from April 22 to July 21, 1955. Some of the observation results are summarized in the table.

Figures 2 and 4 show graphs of the changes in the magnitudes and rates of relative displacement of the side rocks along the normal to the bedding plane ahead of the stoping face, depending on the distance between the measurement points and the working shield.

From the graph in Fig. 2 it is evident that a noticeable increase in the rate of displacement of the side rocks was observed 20–25 m ahead of the shield face. The greatest rate of rock displacement was observed at a distance of 5.8 m below the shield. At this moment, at this measurement point (benchmarks Nos. 1, 2), the total magnitude of displacement of the side rocks reached 60.8 mm.

Table

Measurements ahead of the shield at distances of less than 5.8 m and directly at the shield face were not carried out, since the coal pillar above the bypass ventilation stove collapsed.

From June 18 to 21, the bypass ventilation stove was packed with coal, and measurements were made only at the lower observation point (bench marks Nos. 7; 8).

On June 23, during the first shift, the shield was stopped, because water began to enter the working face from the flooding of the first shield pillar (No. 34). By the time of the stoppage, the shield had descended below the upper pair of bench marks and was 6.5 m above bench marks Nos. 3; 4. The stoppage of the shield face made it possible to continue observations at the three remaining points throughout the entire period of extraction of the next shield pillar (from June 25 to July 21, 1955). After the shield stopped, the rate of displacement of the side rocks began to decrease and reached almost its initial value. Only at the moment when the neighboring shield approached the main drift was a slight increase in the displacement rate noted.

Fig. 4. Graph of the change in the rate of relative displacement of the side rocks as a function of changes in the distances from the measurement points to the shield: 1—advance of the working face in shield pillar No. 39; 2—advance of the working face in shield pillar No. 41; 3—advance of the working face in shield pillar No. 47; 4—rates of displacement of bench marks Nos. 1–2; 5—rates of displacement of bench marks Nos. 3–4; 6—rates of displacement of bench marks Nos. 5–6; 7—rates of displacement of bench marks Nos. 7–8.

Over the entire observation period, the greatest magnitude of relative displacements of the side rocks normal to the plane of bedding was shown by bench marks Nos. 3; 4. This value reached 118.3 mm.

The above results of instrumental observations do not permit general conclusions to be drawn. However, the results show that, in designing shield supports and supports for coal chutes, one cannot fail to take into account the magnitude and character of changes in the displacements of the side rocks ahead of the working face.

LITERATURE

1. Guide to Questions of Rock Movement for the Conditions of the Prokopyevsk–Kiselevsk District of Kuzbass. Ugletekhizdat, 1951.

2. Chinakal O. N. Results of measuring rock pressure on the shield. Ugol’, No. 10, 1954.