Mechanical analysis of floor failure mechanism and floor failure depth of bottom coal roadway

Failure mechanism and analysis of roadway floor

Failure depth of roadway floor and bottom coal

Example verification

Numerical simulation study on the thickness of bottom coal in thick coal seam roadway

Numerical simulation model establishment and scheme

Engineering geological conditions

The no. 8302 working face of Xinjulong Coal Mine is the second working face in the eighth mining area of the second level. The coal face is located in the monoclinic structure area of Bigai anticline and Huangtang syncline structure. The dip length is 270 ~ 278 m, the strike length is 1180 m, the buried depth is about 900 m, the average dip angle of coal seam is 5°, and the average coal thickness is 8.97 m.

Establishment of numerical simulation model

Based on the geological data of no. 8302 coal face, a numerical simulation model was established. The size of the model was 400 m × 200 m × 150 m, with a total of 4,000,000 nodes and 390,000 grids. The thickness of the coal seam was 8 m, the size of the chamber was 3 m × 4 m, the length of the working face was 240 m, and the mining height was 8 m. The simplified model of numerical simulation was shown in Fig. 5.

The boundary conditions of the model were that X direction constrained the left and right boundary displacement, Y direction constrained the front and back boundary displacement, Z direction constrained the lower boundary displacement. The upper boundary of the model was applied with 20 MPa self-weight stress of the overlying strata.

Mohr–Coulomb yield criterion was used to judge the evolution law of stress, displacement and energy of composite floor of roadway.

Simulation scheme

In the mining of thick coal seam working face, if the direct floor of roadway is weak or soft lithology, the floor heave problem of this kind of roadway is more serious. Therefore, most of such roadway excavation methods are bottom coal excavation, so that the bottom coal plays a role in protecting and buffering the direct bottom^14,15,16.

It can be seen that the floor strength is an important factor affecting the stability of the roadway floor and the setting of the bottom coal, especially the relationship between the floor rock strength σ_RC and the bottom coal strength σ_MC. Therefore, three relationships between floor rock strength σ_RC and bottom coal strength σ_MC were set: the floor rock strength σ_RC was greater than, equal to or less than bottom coal strength σ_MC, namely, σ_RC > σ_MC, σ_RC = σ_MC, σ_RC < σ_MC. The selection of floor rock parameters referred to the ‘MT 553-1996 Classification standard of working face floor in gently inclined coal seam’, etc. The specific floor lithology parameters were shown in Table 2^35,36.

As one of the most directly measurable properties of bottom coal in roadway, the thickness of bottom coal has an important influence on the mechanism of bottom coal. The thickness of bottom coal will directly affect the proportion of coal-rock structure of roadway floor, and then affect the construction of floor impact model and the formation process of impact initiation. Therefore, the bottom coal thickness m₁ of the roadway was set to 0 m, 0.5 m, 1 m, 1.5 m, 2 m, 2.5 m and 3 m. After the bottom coal is retained, the roadway floor forms a coal-rock composite floor composed of bottom coal and floor rock. Considering the proportion of coal-rock structure of roadway floor and the existing drilling detection data, the thickness m of coal-rock composite floor was 4 m. According to the thickness of the bottom coal m₁ and the thickness of the composite floor m, the thickness of the floor rock m₂ is set, that is, m = m₁ + m₂. As shown in Fig. 6.

Considering the floor strength and the bottom coal thickness, 21 numerical simulation schemes were designed. The specific numerical simulation scheme was shown in Fig. 7.

Analysis of numerical simulation results

Variation law of vertical stress of mining roadway floor

The stress of surrounding rock is an important factor to determine the stability of surrounding rock and the safety of chamber. The stress state of surrounding rock directly affects the stability of roadway. As the strength of roadway floor strata changed from weak to strong, the stress of mining roadway floor was fitted with the thickness of bottom coal and floor strata. The result is shown in Fig. 8.

It can be seen from Fig. 8 that the floor stress increased first and then decreased with the increase of bottom coal thickness and floor rock thickness. The maximum values of floor stress were 10.22 MPa, 11.01 MPa and 12.25 MPa when the bottom coal thickness was the same as the bottom rock thickness. The relationship between floor stress and bottom coal thickness and floor rock thickness was almost quadratic parabolic surface, and the fitting degree was greater than 95%.

Analyzing the relationship between the floor strata strength and the bottom coal strength, it can be seen that with the gradual increase of floor strength, floor stress also gradually increased. However, when σ_RC = σ_MC, the increase of floor stress with the change of bottom coal thickness was the smallest, which was 34.29%. When σ_RC < σ_MC, the increase was the largest, which was 36.19%.

From the perspective of stress, when the thickness of bottom coal was the same as that of floor rock, the stress of composite floor was the largest. When the thickness of the bottom coal was 0.5 m, the stress of the composite floor was the smallest. The stress of composite floor increased with the increase of floor strength. It can be seen that when the thickness of bottom coal was small and the strength of bottom strata was low, the stress of bottom strata was smaller.

Variation law of vertical displacement of mining roadway floor

The deformation of surrounding rock of roadway is one of the important indexes to measure the stability of roadway. It is an important reference for judging the existing support effect, maintaining the stability of roadway, guiding the design of on-site support and ensuring the safety production of coal mine. As the strength of the floor rock changed from weak to strong, the floor displacement of the mining roadway was fitted with the bottom coal thickness and the floor rock thickness. The result is shown in Fig. 9.

It can be seen from Fig. 9 that the displacement of roadway floor increased first and then decreased with the increase of bottom coal thickness and floor rock thickness. When the bottom coal thickness was 2.5 m, the floor displacement reached the maximum value, and the maximum displacement was 7.53 mm, 7.50 mm and 7.49 mm respectively. The relationship between floor displacement and bottom coal thickness and floor rock thickness was almost quadratic parabolic surface, and the fitting degree was greater than 95%.

Different from the floor stress, the floor displacement decreased gradually with the increase of the floor strength. When σ_RC = σ_MC, the increase of floor displacement with the change of bottom coal thickness was the smallest, which was 33.61%. When σ_RC < σ_MC, the increase was the largest, which was 36.66%.

According to the analysis of floor displacement, when the thickness of bottom coal was 2.5 m, the displacement of composite floor reached the maximum. When the thickness of the bottom coal is 0.5 m, the displacement of the composite floor was the smallest. As the strength of the floor rock increased, the displacement of the composite floor gradually decreased. It can be seen that when the thickness of the bottom coal was small and the strength of the floor rock was large, the displacement of the floor was smaller.

Variation law of elastic strain energy of surrounding rock of roadway

The accumulation and release of elastic strain energy are closely related to the failure process of rock. The elastic strain energy can be used to further analyze the mechanical response and energy transformation mechanism of surrounding rock. It provides scientific basis for the design of roadway construction, support and maintenance. At the same time, the elastic strain energy accumulated in coal and rock mass is the prerequisite for the occurrence of rock burst. Understanding the accumulation degree and release law of elastic energy can effectively predict the occurrence trend and potential risk of rock burst. As the strength of the floor rock changed from weak to strong, the elastic energy of the roadway floor was fitted with the bottom coal thickness and the floor rock thickness. The result is shown in Fig. 10.

It can be seen from Fig. 10 that the elastic strain energy of roadway floor gradually decreased with the increase of bottom coal thickness, and the minimum values were 2.54 × 10⁴ J, 2.52 × 10⁴ J and 2.49 × 10⁴ J, respectively. Moreover, with the increase of floor strength, the decrease of elastic strain energy with the change of bottom coal thickness became smaller. When σ_RC < σ_MC, the amplitude was the largest, which was 14.58%. When σ_RC > σ_MC, the amplitude was the smallest, which was 9.85%.

By analyzing the energy law of the floor, it could be found that when the bottom coal thickness was 3 m, the energy accumulated in the composite floor was the least. When the bottom coal thickness was 0.5 m, the energy accumulated in the composite floor was the most, but the energy accumulated in the floor was less than that when the bottom coal was 0 m. It can be seen that the retention of bottom coal could effectively reduce the accumulation of energy in the roadway floor and reduce the risk of rock burst.

Determination of reasonable bottom coal thickness in thick coal seam roadway

It can be seen from the stress, displacement and energy variation characteristics of the composite floor that as the floor strength increased, the composite floor stress increased, and the displacement and the energy decreased. Comparing and analyzing the stress, displacement and energy of the floor with and without the bottom coal, it could be seen that the stress, displacement and energy of the floor with the bottom coal was smaller than that without the bottom coal. This is because the bottom coal plays a buffering role here, which can effectively protect the floor from damage. Further analysis of the influence of bottom coal thickness and floor rock thickness on the composite floor showed that when the bottom coal thickness was the same as the floor rock thickness, the composite floor stress was the largest. When the bottom coal thickness was greater than the floor rock thickness, the composite floor displacement was the largest. Therefore, it can be considered that when the bottom coal thickness was less than the floor rock thickness, it was a more reasonable thickness of the bottom coal. In addition, the composite floor energy decreased with the increase of bottom coal thickness.

Therefore, considering the numerical simulation results of stress, displacement and energy, the safe thickness of bottom coal in thick coal seam mining is 1 m. Under this bottom coal thickness, the surrounding rock of the roadway, especially the floor, will be in a state of low stress and small deformation, and the surrounding rock of the roadway is relatively safe as a whole.

Safety evaluation process of bottom coal thickness in thick coal seam roadway

According to the above research and analysis, the thickness of the bottom coal in the thick coal seam roadway is related to the ultimate failure depth of the floor. On the basis of considering the safety factor and the stress of bottom coal, the calculation method of safe thickness of bottom coal in thick coal seam roadway was established. Combined with the numerical simulation method, the evaluation process of the safety thickness of the bottom coal in the thick coal seam was proposed, as shown in Fig. 11. The evaluation process of safety thickness of bottom coal in detail is as follows:

Firstly, the calculation results of the ultimate failure depth of the bottom coal roadway in thick coal seam are obtained.

Secondly, the force and safety factor of bottom coal are determined according to field measurement.

Thirdly, carry out numerical simulation analysis.

Fourthly, the safety thickness of bottom coal is given according to the results of numerical simulation.

Fifth, determine the construction technology and parameters of the bottom coal roadway.