The surrounding rock response of the fully mechanized gob-side roadway is usually directly related to the surrounding rock conditions at the excavation site [26]. By theoretically calculating the width of the limit equilibrium area in the CSNG before GED, the surrounding rock state of the roadway during driving can be determined to predict the deformation trend of the surrounding rock during GED and preliminarily explore the reasons for the difference in surrounding rock deformation caused by the change in coal seam thickness.

When calculating the limit equilibrium area of the CSNG, the limit equilibrium theory is usually used [27]. After the coal seam of the previous working face has been mined, the CSNG gradually undergoes plastic failure from its edge to the inside. The vertical bearing capacity of the CSNG is greatly reduced, and the bearing pressure is continuously transferred to the deep part of the coal seam. When the coal state changes from plasticity to elasticity, the bearing pressure in the coal seam reaches its peak value, as shown in Fig. 7. The coal seam in the range of \({x}_{1}\) is in the limit equilibrium state, and the coal seam in the range of \({x}_{0}\) is in the stress reduction area. According to the limit equilibrium theory, the expression of the bearing pressure \({\sigma }_{z}\) at the top of the coal seam within the limit equilibrium area can be determined:

where *M* is the thickness of the coal seam, m; \(\lambda\) is the pressure coefficient inside the limit equilibrium zone; \({\phi }_{0}\) is the friction angle in the coal seam (°); \({C}_{0}\) is the coal cohesion force, MPa; \({P}_{x}\) is the horizontal binding force of the gob on the coal seam, MPa; \({x}_{1}\) is the limit equilibrium area width, m. According to the geological production conditions of the 8204 working face and measured results of rock pressure, *M* = 6 ~ 24 m, \(\lambda\)=0.42, \({\phi }_{0}\)=27°, \({C}_{0}\)=1.1 MPa, and \({P}_{x}\)=0.25 MPa. These parameters can be input into Eq. (1) to obtain the exponential function curve of \({\sigma }_{z}\) with respect to x in the limit equilibrium area for different coal seam thicknesses, as shown in Fig. 8.

When \(x\)*=*\({x}_{1}\), \({\sigma }_{z}\) reaches the peak value; at this time,

where *K* is the stress concentration factor, which is taken as 2.5; \(\gamma\) is the average weight of the overlying rock, which is taken as 0.025 MN/m3; *H* is the burial depth of the roadway, which is taken as 450 m. These parameters were brought into Eq. (2) to calculate \({\sigma }_{zmax}\)=28.12 MPa. Figure 8 shows that with the increase in thickness of the coal seam, the width of the limit equilibrium area in the CSNG gradually increases. When the coal seam is 6 m, 9 m, 12 m, 18 m and 24 m thick, the width \({x}_{1}\) of the limit equilibrium area is 5.9 m, 8.9 m, 11.8 m, 17.7 m and 23.7 m, respectively, while the width \({x}_{0}\) of the stress reduction area is 3.9 m, 5.9 m, 7.8 m, 11.7 m and 15.6 m, respectively.

When identifying a location for GED, the stress reduction area is usually selected [28, 29]. According to the theoretical calculation results, when M ≥ 18 m, the roadway driving position is completely in the stress reduction area. Therefore, the surrounding rock state of roadway excavation should be better when M = 18 m than when M = 12 m. However, the results of in situ measurement are completely opposite because the limit equilibrium area and stress reduction area widen with increasing coal seam thickness. The main reason for the stress reduction is that the coal seam enters the plastic state. When M ≥ 18 m, the roadway driving position is also completely in the limit equilibrium area. At this time, the stress level of the coal seam is low, but the degree of plastic damage is high, the overall strength is low, and the stability is poor. As a result, the roof and two sides are more likely to collapse during roadway construction. In particular, when the coal seam is thicker, the roadway is in a larger range of damaged surrounding rock, and the control of surrounding rock stability will be more difficult.