Gob-side entry retaining plays an important role in the sustainable exploitation of coal resources and has been widely used in coal mining. However; some disadvantages, such as high labour intensity and low efficiency of wall construction, have gravely restricted the application of this technique. To solve this issue, this paper developed a new gob-side entry retaining approach including a lightweight and high-strength foam concrete and a mortise-and-tenon structure hollow-block wall. The experiment results show that the preparation of foam concrete with a density of 1200 kg/m 3 ; with uniaxial compressive strength greater than 15 MPa, can be realized by the chemical-foaming method. The method is performed by adding silica fume, slag and fly ash to a system of "cement and sand" structure collocation and by optimizing the proportioning of the materials. The density of foam concrete is reduced by 50% relative to that of common concrete which can greatly reduce the weight of the blocks. Compared with the conventional gob-side entry-retaining wall the new wall structure has many advantages. For example, it can improve stability by increasing the width-to-height ratio of the wall and save 44.44% of the material, and at the same time; it can save manpower and improve labour efficiency. Finally, the new approach was tested in the 3203 gob-side entry-retaining working face in a Chinese coal mine and the application effect proves that the new approach is well suited for entry retaining. The wall structure and parameters are reasonable. This work can contribute to the sustainable and efficient exploitation of underground coal resources.

A large area of room mining goaf has been left throughout the history of coal mining worldwide. However, because the pillars cannot perpetually retain stability, the sudden collapse of a large area of goaf can occur. The existing large areas of room mining goaf have become a potential source of geological risks in mining areas with room mining goafs worldwide. To elucidate the distribution and state of the goafs, a survey of the characteristics of goafs was carried out by geophysical exploration and drilling methods. From this complete survey of room mining goafs, this paper has outlined eight common goaf risks, emphasizing the types of goaf risks that require precautions, and the formation mechanisms of each goaf risk has been analyzed. Considering that the occurrence of a risk may lead to several other risks and there is a certain interaction between the risks, more specific room mining goaf risk forms, causes and scopes of influence have been listed. It is advantageous to use a support vector machine, which is useful for small-sample problems, to assess the goafs. A support vector machine assessment model of room mining goafs was established, and sample training and verification were carried out. Comparing the risk assessment results with those of the field investigation, it can be seen that the evaluation model has a high accuracy. This work can greatly contribute to the prediction and prevention of risks induced by room mining goaf.

Abstract:Backfill mining is an effective way to control ground subsidence and govern gangue. To solve the problem of thin coal seam mining under villages, a new thin seam backfill mining technology was proposed. This paper investigated a reasonable proportion of filling materials by experiments, designed the filling system and introduced key technologies for thin seam working face filling. Finally, an industrial test of thin seam backfill mining technology was carried out in the C1661 working face, Beixu Coal Mine. The results show that the developed filling material meets both the pumping liquidity and strength requirements of the filling body during the early and late stages. The design and equipment selection of the paste filling system were reasonable. By using the key technologies for thin seam working face filling, the time needed for working face filling, the connection and disconnection of the filling pipeline and gob-side entry retaining were all greatly shortened. The labor intensity of the workers was reduced, and the mechanization level of the mine was improved. A fill mining length of 480 m was successfully completed. With effective roof subsidence control, the ground subsidence can be reduced, and good results can be achieved. This study can contribute to the development of backfill mining in thin coal seams.

In Chinese coal mines, gob-side entry retaining, an efficient technique for coal mining, has been widely used. However, severe roadway deformation and slow retaining speed have gravely restricted the popularization and its application. Hence, in order to solve the existing problems, the deformation mechanism of gob-side entry was studied. Then, a new approach for gob-side entry retaining technique (GERT) was proposed to increase the speed of gob-side entry retaining. Finally, the application effect of the new GERT method was tested and analysed. The results show that the rotation and subsidence of roof key block B lead to severe deformation of roadway. And the proposed gob-side entry stability control technology can affectively resist the severe roadway deformation. Compared with the conventional support method for gob-side entry retaining, GERT has completed the complex wall construction work in advance, thereby simplifying the process of gob-side entry retaining and increasing the speed of gob-side entry retaining. When retaining the entry in panel 183U04 with the new support method, the entry retaining speed significantly improved from 0.25 m/h to 1.0 m/h compared with the traditional method, and then the problem of gob-side entry severe deformation was solved.

No abstract