SELECTION OF THE LOCATION FOR A FIELD DRIFT IN MINING A GROUP OF CLOSELY SPACED STEEPLY DIPPING SEAMS

V. V. PROSKURIN

(Presented by Professor D. A. Strelnikov)

The basic principles for locating group field drifts when mining closely spaced steeply dipping seams are briefly set forth in the book by Academician L. D. Shevyakov [1, p. 628]. Questions of the technical and economic substantiation of the location of field drifts relative to the footwall of the underlying seam have not yet received sufficient coverage in the mining-engineering literature. By the present time a certain amount of production experience has been accumulated which, in the author’s opinion, is sufficient for establishing the basic provisions for solving the problem posed. The scientific development of a method for determining the location of a field drift is especially necessary in connection with the transition of mining operations to deeper levels and the broader development of field preparation of panels. The aim of the present work is to set forth a method and procedure for determining the location of a field drift when mining closely spaced steeply dipping seams¹. It is expected that this will fill a gap in the mining literature and facilitate the work of engineers and technicians in solving problems of this kind.

Requirements Imposed in Selecting the Location of a Field Drift

The requirements imposed in selecting the location of a field drift intended for opening and mining closely spaced steeply dipping seams reduce to the following:

a) if the field drift is subsequently intended to be used as a ventilation drift, it must be located beyond the limits of the rock-displacement zone (Fig. 1);

b) in order to reduce the costs associated with maintaining the field drift during operation, it should, as far as possible, be located in sufficiently stable rocks;

c) the field drift must be located from the footwall of the lowest seam at a distance convenient for arranging entries to the seams, provided that minimum protective

pillars under an intermediate crosscut, which are in practice abandoned forever;

d) the costs associated with driving and maintaining the field drift and the intermediate crosscuts, as well as the costs associated with transporting loads along the intermediate crosscuts, must be minimal over the entire service life of these workings.

Some of the requirements listed above were already partly noted earlier in the works of D. D. Karpukhin [2] and K. I. Tatomir [3].

Fig. 1. Diagram for determining the location of the field drift on the basis of observing the condition that it not be undermined

Factors influencing the choice of the location for laying out the field drift

The choice of the location for laying out a field drift when working a group of closely spaced steeply dipping seams is influenced by the following factors.

Height of the planned lower level. If the displacement angle of the rocks of the footwall, \(\beta_1\), is less than the dip angle of the seam, \(\alpha\), then when working a panel toward the boundaries of the mine field, in order that the field drift not be undermined, it must be moved away from the floor of the lower seam by an amount the greater, the greater the height of the level (Fig. 1). The height of the planned lower level will not affect the choice of the location for laying out the field drift if \(\alpha \leq \beta_1\) and if the panel of the mine field is to be worked by retreating.

Dip angle of the lowest seam in the group. If \(\alpha > \beta_1\), then the steeper the seam dips, the greater must be the distance of the field drift from the floor of the lower seam; the field drift must not be located in the zone of rock displacement.

Structure of the rocks of the footwall, their strength and stability. If rocks that differ sharply from one another in stability occur in the footwall, driving the field drift along the contacts of such rocks is not recommended. In order to reduce the costs of maintaining field drifts, they should be driven in the more stable rocks; however, in doing so it is necessary to take into account their service life and length. The longer the service life of the field drift and the greater its length, the more grounds there are for laying it out in stable rocks. The disturbed condition of the rocks of the footwall, their strength and bedding, influence the magnitude of the displacement angle \(\beta\) and, consequently, the choice of the distance from the axis of the field drift to the floor of the lower seam.

Number of seams in the group, their thickness, and the distance between seams. If a group of closely spaced steeply dipping seams consists of 2–3 seams and the seams are of small thickness, then the arrangement of entries on the seams can be resolved more simply, without significant capital costs and coal losses in protective pillars. A large number of seams in the group, with considerable distances between them, often necessitates the use of two-sided extraction fields, which complicate the scheme of entries onto the seams, and the arrangement of intermediate crosscuts perpendicular to the strike of the seams. A perpendicular arrangement of intermediate crosscuts—

gobs toward the seam moves the field drift away from the floor of the underlying seam, as compared with any other arrangement of them.

Type of transport. The type of electric locomotive and rolling stock determines the radii of curves at the entries to the seams, the cross-sectional area of the field drift and of the intermediate crosscuts.

Length of the field drift and its service life. The service life of a field drift is often determined by its length. The longer the field drift, the longer its service life. The service life affects the choice of support type, the cost of driving the mine working, and the cost of maintaining it. The rate of advance of the stoping front affects the service life of the field drift and the intermediate crosscuts, and the cost of maintaining them.

Length of the intermediate crosscuts and the distance between them. The farther the field drift is located from the floor of the underlying seam, the longer the intermediate crosscuts are; the more expensive the cost of driving and maintaining them will be, and the higher the cost of hauling loads will be. The smaller the distance between intermediate crosscuts, the greater their total length will be. These expenses must accordingly be taken into account in the technical-and-economic comparison of alternatives for selecting the site of the field drift.

Economic factors. The following are to be taken into account: the cost of driving and maintaining one running meter of the field drift and of the intermediate crosscut, as well as the cost of transport per 1 ton-km.

The procedure for establishing the site of a field drift may be recommended in accordance with the requirements imposed:
1) establish the site of the field drift on the basis of the condition that it not be undermined; 2) establish the site of the field drift on the basis of the conditions for convenient arrangement of entries to the seams; 3) finally establish the site of the drift, taking account of technical factors and minimum costs per 1 t of commercial reserves.

Selection of the Site of a Field Drift on the Basis of the Condition That It Not Be Undermined

On the basis of the condition that the field drift not be undermined by the overlying seam, the choice of its site should be determined only in the case where \(\beta_1 \leq \alpha\),
where \(\beta_1\) is the angle of displacement on the footwall side of the seam,
\(\alpha\) is the dip angle of the seam.

In this case, the following must be taken into account:
\(h\) — vertical height of the level, m;
\(b\) — width of the protective berm, m;
\(a\) — width of the field drift at the bottom, m;
\(X_n\) — minimum horizontal distance of the axis of the field drift from the floor of the underlying working seam in the group, based on the condition that it not be undermined, m.

The conditions under which the field drift will not be undermined in the direct sequence of extraction of a wing of the mine field will be satisfied (Fig. 1) if

\[ X_n \geq h(\operatorname{ctg}\beta_1 - \operatorname{ctg}\alpha) + b + \frac{a}{2}. \tag{1} \]

Selection of the Site of a Field Drift on the Basis of the Conditions for Convenient Arrangement of Entries to the Seams

On the basis of the condition of maintaining minimum coal losses in protective pillars under the intermediate crosscut, the latter is expediently

it is more expedient to locate them perpendicular to the line of strike of the seams [4, p. 13].

Let us denote:

\(X_T\) — the minimum horizontal distance of the axis of the field drift from the footwall of the lower working seam, determined from the conditions of convenient arrangement of entries, m;
\(R\) — radius of curvature, m;
\(U\) — length of the straight section between switches, m;
\(c\) — width of a mine car, m;
\(m\) — distance between mine cars on a double-track road, m;
0.2 — clearance between the support and the mine cars, provided for by the PTE, m;
\(p\) — width of the pedestrian passage, m;
\(r_1\) — distance from the axis of the field drift to its side wall at the bottom, m;
\(\psi\) — angle of inclination of the field-drift support, degrees;
\(h_1\) — height from the floor of the working to the upper edge of the mine car, m.

Fig. 2. Diagram for determining the location for laying out a field drift from the conditions of convenient arrangement of entries into the seams

From the diagram (Fig. 2) it follows:

\[ X_T = 2R + U + \left(\frac{a}{2} - r_1\right) - r_2, \tag{2} \]

but

\[ r_1 = h_1 \operatorname{ctg}\psi + p + \frac{c}{2} \]

\[ a = b + 2c + m + 0.2 + 2h_1 \operatorname{ctg}\psi, \]

therefore, finally,

\[ X_T = 2R + U + \frac{1}{2}(c + m + 0.2 - p) - r_2. \tag{3} \]

Selection of the Location for Laying Out a Field Drift from the Conditions of Minimum Costs per 1 t of Industrial Reserves

Field drifts, from the standpoint of minimum costs for their maintenance, should be driven through stable rocks; in this case it is ne—

it is necessary to take into account the service life and the length of the working. But this is possible only if the following conditions are observed:

\[ X_y \geq X_n \]

and

\[ X_y \geq X_t, \tag{4} \]

where \(X_y\) is the distance from the axis of the field drift to the floor of the working seam when the drift is driven in more stable rocks, m.

When establishing the location for driving a field drift, the following four cases are possible (Fig. 3).

First case, when
\(X_y = X_n = X_t\).
There is no need for a techno-economic comparison of variants.

Second case, when
\(X_y > X_n > X_t\). It is necessary to carry out a techno-economic comparison of two variants: when the distance to the axis of the field drift is equal to \(X_y\) and \(X_n\). Consideration of the variant \(X_t\) is excluded by the conditions of undermining.

Third case, when
\(X_y > X_t > X_n\). It is necessary to carry out a techno-economic comparison of three variants: \(X_y\), \(X_t\), and \(X_n\); however, when comparing the variant \(X_n\), one should take into account the lengthening of the intermediate crosscuts owing to the entries onto the seam, as well as coal losses in protective pillars.

Fig. 3. Variants of the location for driving a field drift for techno-economic comparison

Fourth case, when \(X_y > X_n > X_t\), but

\[ X_n < X'_n < X_y, \]

where \(X'_n\) is the distance from the axis of the field drift to the floor of the working seam under the conditions of not undermining the drift and when it is located outside the contact of the rocks, m.

The variants \(X'_n\) and \(X_y\) are subject to techno-economic comparison. It is advisable to drive the field drift at a distance \(X_y\), if

\[ R_y \geq R_n, \tag{5} \]

where \(R_y\) is the comparable costs associated with selecting the location for driving the field drift at a distance \(X_y\);

\(R_n\) is the comparable costs associated with selecting the location for driving the field drift at a distance \(X_n\).

The total comparable costs when driving the field drift at distances \(X_y\) and \(X_n\), referred to 1 linear m of a group of seams by

along strike, are determined (Fig. 4)

\[ R_y = K_{pshy} + \frac{(S + S_{vp}) r_y}{L} + r_{py} + \frac{\Delta l K_{kv}}{S_{kv}} + \]
\[ + 2 \Delta l \left( \frac{r_{kv}}{L} + \frac{r_{pkv}}{S_{kv}} \right) + \frac{Q \Delta l q}{S_{kv}} . \tag{6} \]

\[ R_n = K_{psh} + \frac{(S + S_{vp}) r_n}{L} + r_{pn}, \tag{7} \]

Fig. 4. Diagram for determining the location for driving a field drift with allowance for the stability of the rocks

where \(K_{pshy}\) is the cost of driving one linear m of field drift in stable rocks, rubles;

\(K_{psh}\) is the cost of driving 1 linear m of field drift in less stable rocks, rubles;

\(S\) is the total length of the field drift, m;

\(S_{vp}\) is the length, along strike, of the outermost extraction field located at the boundary of the mine field, m;

\(L\) is the rate of advance of the stoping faces along the seams of the entire suite (group), m/year;

\(r_y\) is the cost of maintaining 1 linear m of field drift in stable rocks per year, rubles;

\(r_n\) is the cost of maintaining 1 linear m of field drift in less stable rocks per year, rubles;

\(r_{py}\) is the cost of supporting 1 linear m of field drift driven in stable rocks, rubles;

\(r_{pn}\) is the cost of supporting 1 linear m of field drift driven in less stable rocks, rubles;

\(\Delta l\) is the difference between the values \(X_y\) and \(X_n\), m;

\(K_{kv}\) is the cost of driving 1 linear m of intermediate crosscut, rubles;

\(S_{kv}\) is the distance between intermediate crosscuts, m;

\(r_{kv}\) is the cost of maintaining 1 linear m of intermediate crosscut per year, rubles;

\(r_{pkv}\) is the cost of supporting 1 linear m of intermediate crosscut, rubles;

\(Q\) is the freight transported over the section \(\Delta l\) of the intermediate crosscut, t;

\(q\) is the cost of moving freight per tm, rubles.

When the intermediate crosscut is located at an angle to the strike of the seam (Fig. 5), it is advisable to lay the field drift at a distance \(X_n\), if

\[ R_n \leq R_t, \tag{8} \]

whereas

Fig. 5. Diagram for determining the location for laying a field drift, taking into account losses in protective pillars

\[ R_n = K_{psh} + \frac{(S + S_{vp}) r_n}{L} + r_{pn} + \Delta l_{kv}\left[ \frac{K_{kv}}{S_{kv}} + 2\left(\frac{r_{kv}}{L} + \frac{r_{rkv}}{S_{kv}}\right) + \frac{Qq}{S_{kv}} \right] + \frac{\Delta P q_y}{S_{kv}} . \tag{9} \]

\[ R_t = K_{psht} + \frac{(S + S_{vp}) r_T}{L} + r_{pT}, \tag{10} \]

where \(\Delta l_{kv}\) is the difference between the length of the intermediate crosscut when it is located at an obtuse angle or at a right angle to the strike of the seam, m;

\(\Delta P\) is the difference in the amount of coal losses in protective pillars under the intermediate crosscut when it is located at an obtuse angle or at a right angle to the strike of the seam, t;

\(q_y\) is the economic loss from the loss of 1 t of coal in protective pillars, rubles;

\(K_{psht}\) is the cost of driving one running meter of the field drift in rocks located from the seam at a distance \(X_T\), rubles;

\(r_T\) is the cost of maintaining 1 running meter of the field drift in rocks located from the seam at a distance \(X_T\), rubles;

\(r_{pT}\) is the cost of supporting 1 running meter of the field drift in rocks located from the seam at a distance \(X_T\), rubles.

References

1. Shevyakov L. D. Development of Mineral Deposits. Ugletekhizdat, 1953.

2. Kaplukhin L. D. Group Drifts. Moscow Mining Institute, 1943.

3. Tatomir K. I. Opening and Development of Coal Deposits with the Main Workings Located in Stable Rocks. Academy of Sciences of the Ukrainian SSR, 1950.

4. Rules for Protecting Structures from the Harmful Influence of Mining Operations in the Donets and Moscow-Area Basins. GONTI, 1939.

1. For normal conditions of successive mining of panels (author). ↩