ON THE INTENSITY OF EXTRACTION OF ROOM RESERVES IN THE MINING OF STEEPLY DIPPING THICK ORE DEPOSITS BY ROOM-AND-PILLAR MINING SYSTEMS

G. E. Bakanov

(Submitted by the Department of Ore Deposit Mining)

The intensity of extraction of room reserves in the mining of thick ore deposits by room-and-pillar mining systems, as determined by the technique and organization of stoping operations in the rooms, must correspond to the rates of preparation and development of the rooms. Questions relating to determination of room productivity in breaking and drawing ore have been studied quite well in the USSR, and their solution in particular cases now presents no special difficulties. As for methods of establishing the possible productivity of rooms according to the rate of carrying out preparatory and development workings, they have not yet been covered in the mining literature, despite their importance and great significance for production.

To ensure rhythmic operation of a mine and to achieve the maximum possible intensity of reserve extraction under room-and-pillar mining systems, it is necessary that the rates of preparation and the intensity of room mining constantly remain in definite quantitative relationships.

If \(N\) is the number of rooms simultaneously being mined in a wing of a level, \(T_1\) is the time spent on stoping the ore from them, \(T_2\) is the time required for preparation and development of the same number of new rooms, and \(k_0\) is the coefficient of the reserve of prepared reserves (advance of preparation), then, for timely preparation and development of rooms, the following relationship must exist between \(T_1\) and \(T_2\):

\[ T_1 = k_0 T_2 . \tag{1} \]

On the other hand, the time required for stoping ore from the rooms is

\[ T_1 = \frac{NQ_k}{NP_k} = \frac{Q_k}{P_k}, \tag{2} \]

where \(Q_k\) is the average industrial reserves of ore in rooms extracted by stoping operations, \(t\);

\(P_k\) is the average productivity of a room per unit time, \(t\)/month or year.

The time spent on preparing and cutting rooms depends on the mining system, the equipment, and the organization of the preparatory and cutting operations. In general form this dependence may be represented as follows:

\[ T_2=t'_n+t''_n+t'_{\!н}+t''_{\!н}. \tag{3} \]

Here \(t'_n\) is the time required to carry out preparatory workings, dependent on the number of rooms \(N\) being prepared simultaneously;

\(t''_n\) is the time for preparing rooms that does not depend on \(N\);

\(t'_{\!н}\) is the time required for cutting operations that do not coincide either with preparatory or with stoping operations;

\(t''_{\!н}\) is the time required for cutting operations that can wholly or partially coincide with preparatory, other cutting, or stoping operations.

For most room-and-pillar mining systems the time

\[ t'_n=\frac{N(s+c)}{v_{\!ш}}, \]

where \(s\) is the room width, \(c\) is the thickness of the inter-room pillars, and \(v_{\!ш}\) is the rate of driving a field or ore drift. The quantities \(t''_n\), \(t'_{\!н}\), and \(t''_{\!н}\), other conditions being equal, depend on the structural features of the mining systems and on the procedure for preparing and cutting the rooms. To establish this dependence, let us consider two types of room-and-pillar mining systems: with an open mined-out space and with ore storage.

When rooms are mined with an open mined-out space, ore extraction may be carried out without dividing the levels into sublevels, with ore broken by blasting charges in long boreholes, or with the levels divided into two or more sublevels. In the latter case, depending on the adopted sublevel height, either long boreholes or long blastholes (“rods”) are used for breaking the ore.

Figures 1 and 2 show schemes for preparing and cutting rooms during level extraction of ore using deep vertical boreholes. For the scheme shown in Fig. 1, preparation of the room and its cutting,

Fig. 1. Scheme for preparing a room with level extraction of ore by deep vertical boreholes (with a grizzly level)

carried out during the time interval \(t''_{n}\), consists in driving the haulage crosscut \(O_1\), the drift along the hanging wall of the deposit \(\Ш_{р}\), the raise along the axis of the pillar at the footwall \(B_1\), and the slot raise along the axis of the chamber at the hanging wall \(B_2\). If the chambers are prepared according to the scheme shown in Fig. 2, then instead of a crosscut along the axis of the pillar, a crosscut is driven along the axis of the chamber, and no raise is driven in the pillar at the footwall of the ore body. In all other respects the procedure for preparing the chambers remains the same here as in the scheme depicted in Fig. 1.

Fig. 2. Scheme for preparing a chamber with sublevel extraction of ore by deep vertical boreholes (with a scraping horizon)

When chambers are mined by an advancing front, the crosscuts \(O_1\), the raises \(B_1\), \(B_2\), and the drift \(\Ш_{р}\) are driven as the field drift \(\Ш_{п}\) is advanced. The raise \(B_1\) is begun before the raise \(B_2\), or else it is not driven at all. Therefore, the driving of this raise falls within the time interval during which the crosscut \(O_1\), part of the drift \(\Ш_{р}\), and the raise \(B_2\) are driven.

Taking the above into account, one can write the equation for determining the duration of preparation and partial development work for chambers in the following form:

\[ t'_{n}+t''_{n}+t'''_{n} = \frac{N(s+c)}{v_{\ш}} + \frac{m_2+l_0}{v_0} + \frac{c+s}{2v_{\ш}} + \frac{h}{v_{в}\sin \alpha}, \tag{4} \]

where \(m_2\) is the horizontal thickness of the ore body, m;
\(l_0\) is the distance of the field drift from the ore deposit (the length of the part of crosscut \(O_1\) driven through rock), m;
\(h\) is the height of the level, m;
\(v_0\) and \(v_{в}\) are the driving rates of crosscut \(O_1\) and raise \(B_2\);
\(\alpha\) is the dip angle of the deposit.

As applied to the schemes shown in Figs. 1 and 2, the development work covered by the period \(t'''_{n}\) consists in driving crosscuts \(O_2\) (at the caving or scraping horizon), in forming

chutes, the construction of ore passes, the driving of crosscuts on the drilling level, and the formation of the cut slot. The time required for development of rooms depends on the equipment, the organization and sequence of development operations, and also on the method of cutting off the room. The formation of chutes can be combined in time with stoping operations. Ore passes may be driven together with the drifts \(O_2\). In turn, the drifts \(O_2\) may be driven in parallel with the drifts \(O_1\) and raises \(B_2\), or immediately after these workings have been driven. When the cut slot is formed by blasting charges in deep boreholes, the operation of cutting off the room may be assigned to stoping operations (provided that a definite sequence is observed in blasting the charges in the first two rows of boreholes). In this case, when determining the duration of development operations \(t''_n\), this operation may be disregarded. If the slot is formed by some other method (by benches, with ore storage, etc.), then the appropriate time must be allowed for carrying it out during the development of the room.

Thus, with respect to the development operations carried out during the period \(t''_n\) according to the schemes shown in Figs. 1 and 2, the following cases are possible:

a) All these development operations are performed in parallel with the preparatory and stoping operations in the room; then the time \(t''_n = 0\).

b) The drifts \(O_2\) are driven together after the raise \(B_2\) has been driven; the other development workings are driven in parallel with the driving of these drifts and with the performance of stoping operations; hence

\[ t''_n = \frac{m_2}{v_{op}}, \]

where \(v_{op}\) is the rate of driving the drifts \(O_2\).

c) The cut slot is formed together with the driving of the drifts \(O_2\), before the beginning of stoping operations in the room; in this case the time required for development of the rooms is

\[ t''_n = \frac{m_2}{v_{op}} \quad \text{or} \quad t''_n = \frac{h-a}{v_{\shch}\sin\alpha}, \]

depending on which of these two quantities proves to be larger (\(v_{\shch}\) is the rate of formation of the cut slot).

d) The drifts \(O_2\) are driven together with the drifts \(O_1\) and the raise \(B_2\); the cut slot is made before the beginning of stoping operations; the duration of development is

\[ t''_n = \frac{h-a}{v_{\shch}\sin\alpha}. \]

e) The driving of the drifts \(O_2\) and the formation of the cut slot are carried out consecutively after the raise has been driven and before the beginning of stoping operations in the room; under this development sequence the value is

\[ t''_n = \frac{m_2}{v_{op}} + \frac{h-a}{v_{\shch}\sin\alpha}. \]

Thus, when rooms are prepared and developed according to the schemes shown in Figs. 1 and 2, the duration of development operations \((t''_n)\) may vary from 0 to

\[ \frac{m_2}{v_{op}} + \frac{h-a}{v_{\shch}\sin\alpha}. \]

The faster these operations are carried out, the more intensively the rooms will be mined. The order of room development considered in item “e” is the least advantageous and may be used when the scale of production is limited and the enterprise has limited technical and organizational capabilities.

On the basis of equations (1, 2, 3, and 4), for the mining systems under consideration (Figs. 1 and 2), one may write:

\[ k_0\left[ \frac{N(s+c)}{v_{\mathrm{sh}}} +\frac{m_z+l_0}{v_0} +\frac{c+s}{2v_{\mathrm{sh}}} +\frac{h}{v_v\sin\alpha} +t''_n \right] = \frac{Q_k}{P_k}. \]

Since

\[ \frac{N(s+c)}{v_{\mathrm{sh}}} +\frac{c+s}{2v_{\mathrm{sh}}} = \frac{(c+s)(2N+1)}{2v_{\mathrm{sh}}} \]

and

\[ Q_k=k\mu\gamma m_z s(h-a), \]

then

\[ k_0\left[ \frac{(c+s)(2N+1)}{2v_{\mathrm{sh}}} +\frac{m_z+l_0}{v_0} +\frac{h}{v_v\sin\alpha} +t''_n \right] = \frac{k\mu\gamma m_z s(h-a)}{P_k}, \tag{5} \]

where \(k\) is the coefficient of ore recovery in extracting room reserves;
\(\mu\) is the coefficient of stoping extraction of ore in mining rooms;
\(\gamma\) is the bulk weight of the ore, \(t/m^3\).

From equation (5), the output of a room, according to the conditions of preparing reserves for stoping extraction, is

\[ P_k= \frac{k\mu\gamma m_zs(h-a)} {k_0\left[ \frac{(c+s)(2N+1)}{2v_{\mathrm{sh}}} +\frac{m_z+l_0}{v_0} +\frac{h}{v_v\sin\alpha} +t''_n \right]}. \tag{6} \]

Example 1.

An ore deposit is mined by a sublevel-room system with ore breaking by deep vertical boreholes. Determine the possible output of a room according to the conditions of development of preparatory and cutting works, if \(k=0.98\); \(\mu=0.95\); \(\gamma=3.5\,t/m^3\); \(m_z=40\,m\); \(s=20\,m\); \(c=10\,m\); \(h=70\,m\); \(a=15\,m\); \(k_0=1.25\); \(l_0=30\,m\); \(v_{\mathrm{sh}}=60\,m/\text{month}\); \(v_0=v_v=v_{\mathrm{sh}}=50\,m/\text{month}\); \(\sin\alpha=0.9\); \(t''_n\) (according to item “g”)

\[ =\frac{h-a}{v_{\mathrm{sh}}\sin\alpha}=1.22\ \text{months}; \quad N=3\ \text{rooms in the wing}. \]

By equation (6), the monthly output of the room is

\[ P_k= \frac{0.98\cdot0.95\cdot3.50\cdot40\cdot20\cdot55} {1.25\left(\frac{210}{120}+\frac{70}{50}+\frac{70}{45}+1.22\right)} = 19.3\ \text{thousand }t. \]

If all cutting operations of the period \(t''_n\) are combined with preparatory, other cutting, and stoping operations, then such cutting of the room will require no additional time (\(t''_n\) will be equal to 0), and the output of the room can be brought up to 24.3 thousand \(t/\text{month}\) (rounded). With a reduction of the rates \(v_{\mathrm{sh}}\) to \(50\,m\) and \(v_0, v_v, v_{\mathrm{sh}}\) to \(40\,m/\text{month}\), the possible output of the room will decrease to 15.6 thousand \(t\) per month (with cutting as indicated in item “g”). If it were necessary simultaneously to mine and prepare 5 rooms in the wing, then, with the numerical data adopted in Example 1, the possible monthly output of the rooms would be only

16.5 thousand tons (instead of 19.3 thousand tons). In order to intensify mining, it is necessary to increase the rate of driving the workings, especially the field drift, or to change the sequence of cutting the rooms. In the same way one can find numerical values of the quantity \(P_k\) for any other conditions of sublevel-room mining with ore breaking by deep vertical boreholes.

Let us consider the determination of room productivity under the same conditions for mining systems with sublevel ore extraction. Figure 3 shows the scheme of preparation and cutting of a room in such mining.

Fig. 3. Scheme of room preparation with sublevel ore extraction (with a grizzly level)

In this case, it is advisable to drive the sublevel crosscuts \(O_3\) immediately after driving the raise \(B_2\). It is also desirable to make the cut-off of the room after the completion of driving the sublevel crosscuts \((O_3)\). Therefore, here, when determining the value \(T_2\), only the component element \(t''_n\) will differ. To the value \(t'_n\) found for sublevel-room mining systems (items “a”—“d”), it is necessary here to add the interval of time needed for driving the sublevel crosscuts. To drive these crosscuts requires a time equal to

\[ \frac{n m_2}{v_{on}} \]

or, when driving the crosscuts from two sides, equal to

\[ \frac{n m_2}{2 v_{on}}, \]

where \(n\) is the number of groups of sublevel crosscuts being driven simultaneously, and \(v_{on}\) is the rate of driving them. Consequently, as applied to the mining systems under consideration, the duration \(t''_n\) will range from

\[ \frac{n m_2}{2 v_{on}} \]

to

\[ m_2\left(\frac{n}{2 v_{on}}+\frac{1}{v_{op}}\right)+\frac{h-a}{v_{\shch}\sin\alpha} \]

or from

\[ \frac{n m_2}{v_{on}} \]

to

\[ m_2\left(\frac{n}{v_{on}}+\frac{1}{v_{op}}\right)+\frac{h-a}{v_{\shch}\sin\alpha}. \]

In each specific case, the value of \(t''_n\) is found in accordance with items “a”—“d” and the explanations given here. In the remain-

the determination of the value \(P_k\) is carried out, as in sublevel-room mining systems, by equation (6).

Example II.

For a mining system with sublevel extraction of ore, determine the possible monthly output of a room for the development of preparatory and cutting workings with the following data: \(k=0.96\); \(\mu=0.94\); \(\gamma=3.5\ \text{t}/\text{m}^3\); \(m_z=40\ \text{m}\); \(s=15\ \text{m}\); \(c=7\ \text{m}\); \(h=80\ \text{m}\); \(a=14\ \text{m}\); \(k_0=1.25\); \(l_0=30\ \text{m}\); \(v_{\mathrm{sh}}=60\ \text{m/month}\); \(v_0=v_{\mathrm{op}}=v_v=v_{\mathrm{shch}}=50\ \text{m/month}\); \(\sin\alpha=0.9\); \(n=2\) groups; \(t''_n\) (according to item “g,” taking account of the system) \(=\)

\[ = \frac{n m_z}{v_{\mathrm{op}}}+\frac{h-a}{v_{\mathrm{sh}}\sin\alpha}=3.06\ \text{months};\quad N=2\ \text{rooms in the wing}. \]

The possible monthly output of the room (equation 6) is

\[ P_k= \frac{0.95\cdot 0.94\cdot 3.5\cdot 40\cdot 15\cdot 66} {1.25\left(\dfrac{110}{120}+\dfrac{70}{50}+\dfrac{80}{45}+3.06\right)} =13.8\ \text{thousand t}. \]

If it were necessary to mine the rooms more intensively, it would be necessary either to drive the workings faster or to change somewhat the sequence of cutting the room. Increasing the rates of driving the workings by 25% would make it possible to raise the output of the room to 17.4 thousand t per month, if such output could also be achieved in ore breaking and in drawing it from the rooms.

When thick deposits are mined with ore storage, breaking may be carried out by fan-shaped and parallel blastholes.

When preparing rooms, as shown in Fig. 4, for the case of ore breaking by fan-shaped blastholes, the time \(t'_n+t'''_n+t''_n\) is found

Fig. 4. Diagram of room preparation in mining with ore storage and breaking by fan-arranged blastholes.

also, as for mining systems with open stoped-out space, by equation (4). Only instead of the raise \(B_2\), driven in a time equal to \(\dfrac{h}{v_v \sin \alpha}\), in this case a raise \(B'_1\) in the pillar at the hanging wall is taken into account for the same duration. With respect to determining the time \(t''_n\), the difference is only that here, instead of a cut slot, the formation of a small compensation space has to be taken into account. Consequently, here the time \(t''_n\) will vary within the limits

from 0 to \(\dfrac{m_z}{v_{op}} + \dfrac{m_z}{v_k}\), or from 0 to \(m_z \left(\dfrac{1}{v_{op}} + \dfrac{1}{v_k}\right)\), where \(v_k\) is the rate of driving the compensation working. The corresponding values for \(t''_n\) are chosen as indicated in items “a”–“d”.

If ore breaking is carried out by parallel boreholes, then to drill these boreholes it is necessary to drive drilling crosscuts in the interchamber pillars or drilling drifts along the contacts of the ore bodies with the wall rocks. Preference should be given to drilling crosscuts, since they provide safer working conditions. In order not to weaken the interchamber pillars, it is advisable to drive the crosscuts in them in a staggered arrangement and to drill boreholes from them in both directions. With such an arrangement of the boreholes, there is no need to drive raises in the pillars at the hanging wall. Before stoping begins, at least one drilling crosscut must necessarily be present in each pillar. In this case, if \(v_{ob}\) is the rate of driving a drilling crosscut, the duration of preparation and opening-up of chambers is

\[ T_2 = \frac{N(c+s)}{v_{sh}} + \frac{l_0}{v_0} + \frac{c+s}{2v_{sh}} + \frac{h}{v_v \sin \alpha} + \frac{m_z}{v_{ob}} = \]

\[ = \frac{(c+s)(2N+1)}{2v_{sh}} + \frac{l_0}{v_0} + \frac{h}{v_v \sin \alpha} + \frac{m_z}{v_{ob}} . \tag{7} \]

Here it is understood that the other preparatory and opening-up works required before the start of stoping must be completed during the driving of the ore drift, the raise, and the drilling crosscuts. If a different sequence for driving the workings is planned, this must be correspondingly taken into account in determining the value of \(T_2\).

From equations (1), (2), and (7),

\[ P_k = \frac{k \mu \gamma m_z s (h-a)} {k_0 \left[ \frac{(c+s)(2N+1)}{2v_{sh}} + \frac{l_0}{v_0} + \frac{h}{v_v \sin \alpha} + \frac{m_z}{v_{ob}} \right]} . \tag{8} \]

Using equation (8), one can, as in the preceding cases, find the values of \(P_k\) for the specific conditions of application of the given mining system.

Conclusion

1. For rhythmic operation of a mine when extracting ore from thick ore deposits by room-and-pillar mining systems, it is necessary that the rates of preparation and opening-up of chambers correspond to the in-

the intensity of stoping operations in the rooms. In this connection, in production it is necessary constantly to regulate the rates and sequence of preparation and cutting of rooms in order to bring them into conformity with the sizes of room reserves, the rate of their extraction, and the production capacity of the enterprise.

1. This article has considered a method of calculations necessary for solving this important problem, as applied to the conditions of mining steeply dipping deposits when reserves are extracted by a straight advance.

2. The schemes of preparation and cutting of rooms presented in the article do not exhaust all possible cases and have been adopted as examples in considering the methodology for solving the questions posed.