Single layer caving mining method ground pressure control

    I. Overview

When mining horizontal or gently inclined sufficiently stable thin layers of roof ore, the use of single caving method. The method of managing the roof of the method is to advance forward with the mining work, periodically cut off the direct roof, and fill the goaf by the collapsed roof rock to ensure the stability of the ore near the mining face. After topping, the pressure acting on the roof rock above the working face and the pressure above the falling rock are wavy (Fig. 1).

Fig.1 Pressure distribution in the roof rock layer of single-layer caving mining method

I-stress reduction zone (free pressure zone); II-stress rise zone (support pressure zone); III-primary rock stress zone

As the mining work advances forward, the pressure waves in the roof rock layer also move forward. Formed in the roof rock layer in the front and rear of the working space: stress reduction zone (I) stress rise zone (II) and original rock stress zone (III). The stress value and distribution range of the stress rise zone are determined by the mechanical properties of the roof rock rock, the roof management method, and the mining depth. Laboratory and on-site observations show that the greater the strength of the rock in the roof rock, the deeper the mining depth, and the higher the stress peak.

    Second, reasonable determination of the maximum ceiling distance

When using single-layer caving mining, in order to have a safe section near the working face, the maximum suspended roof distance should be correctly selected according to the mechanical properties of the roof rock. The ceiling distance l is equal to the top distance (II) and the top distance (I) (Fig. 2).

Figure 2 Schematic diagram of the working face of the single-layer caving method

I-mounting distance; II-controlled top distance; l-overhanging distance

Applying the theory of elastic foundation beam, the side of the ore body is regarded as the elastic foundation, and the roof behind the working face is regarded as the cantilever beam which is placed on the elastic foundation. The mechanical model is shown in Fig. 3.

Figure 3 Single-layer caving mechanics model

According to the theory of elastic foundation beam, it is deduced that P _y should be used in the roof rock layer:

In the formula:

L-cantilever length, m;

q _o - original rock stress, q _o = γH, Pa;

a = π / L, m ^-1 ;

L-pressure wave half-wave length, m.

When x=0, the value of P _y is the largest

P _ymax =q _o (1+al) ²

When x=∞, the P _y value is equal to the original rock stress, P _y =q _o

It can be seen from the formula that when q _o and L are constant, the pressure P _{y is} mainly related to l. As the work advances forward, the length of the cantilever beam increases continuously, increasing the pressure P _{y in the} roof rock formation, and periodically cutting off the cantilever roof rock layer, which can lower the pressure P _y and ensure the safety of the mining work.

The stress concentration factor K near the working face is:

The stress concentration factor K near the working face is related to the pressure wave wavelength (2L) and the suspended roof distance l. The magnitude of the pressure wavelength (2L) is related to the mechanical properties of the rock in the roof rock. For example, if the roof rock layer is thick and high elastic modulus rock, and the ore is soft and the elastic modulus is low, the pressure wave wavelength is large. On the contrary, the elastic modulus of the roof rock is low, and the elastic modulus of the ore is high, the pressure wave wavelength is short. General coal mine pressure wave wavelength changes to 12 ~ 60m, some up to 60m or more.

According to the site determination, the stress concentration factor near the working surface changes between 3 and 11. The stress rise zone ranges from 20 to 30 m and sometimes up to 50 m. The survey data of several coal mines in China are listed in Table 1.

Table 1 Range of stress rise zone in front of the working face

    Third, topping work

From the previous theoretical analysis, under certain mining depth conditions, the pressure value acting above the working surface is mainly determined by the length l of the cantilever beam, that is, the size of the suspended roof. Therefore, in order to ensure the safety of the mining work, the pressure value acting on the upper surface of the working face is small, and must be carried out with the work of the mining work, the work is advanced forward by a certain distance (suspension roof distance), and a dense cutting column is set at the control top distance. Carry out the topping. The roof rock stratum collapses along the chopping column and fills the goaf. When the roof rock layer is relatively hard and the dense roof pillar is difficult to cut the roof, it needs to be topped by means of blasting.

After the topping, the cantilever length of the roof rock layer is shortened, and the pressure above the working surface is lowered, so that the mining work is in a safe state. The roof rock layer with a collapsed roof often cannot fill the goaf and make the old roof dangling. When the exposed length of the exposed old top reaches a certain limit, the veteran will collapse (Fig. 4), causing a sharp increase in the pressure in the roof rock formation above the working face, and a secondary pressure. In order to prevent the secondary pressure caused by the sudden collapse of the old roof to damage the mining face, it is necessary to grasp the law of secondary pressure and take timely measures to ensure the safety of the mining work.

Figure 4 Single-layer caving mining method

When there are layers, joints and faults in the roof slab, which weaken the strength of the roof slab, causing the cantilever length of the roof slab to not reach the limit, it will fall. Therefore, when laying the roof, the relationship between the layout of the working face and the weak surface of the geological structure should be considered.

For example, if the roof rock layer is hard and the roof is difficult to be placed, the working face mineral wall should be arranged in parallel with the joint direction. This helps the roof rock formation to collapse. On the contrary, if the roof rock layer is weak and difficult to maintain, the working face ore wall should be arranged at a certain angle with the joint. This prevents the top plate from falling ahead. It is safer to recover under the conditions of the recumbent joint (Fig. 5b) than under the condition of the suspension joint (Fig. 5b).

Figure 5 Relationship between geological weak surface occurrence and working surface layout

A-volt joint; b-suspension joint