## 3 The Study on the Evolution of Overlying Strata Fractures
### Result and Discussion
#### 3.2.3 Characteristics of Fracture Development in Dip Direction  
The gob-side support method using filling structures affects the failure modes of the overlying strata along the dip direction, particularly the rotation, subsidence, fracturing, and collapse of the lateral roof above the retaining roadway. Therefore, numerical simulation were conducted to analyze the failure patterns of overlying strata under gob-side entry retaining with roadside filling and traditional coal pillar retaining conditions. The fracture distribution was zoned, as shown in Figure 7.  
In general, the ranges of the caving zone and the fracture zone are consistent under both filling and non-pillar conditions. The caving zone extends from 0 to 14.4 m, and the fracture zone extends from 14.4 to 40.7 m, which is consistent with the results in Section 3.2.2. Comparative analysis shows that under traditional coal pillar conditions (Figure 7a), the overall fracture distribution is symmetrical. Under gob-side entry retaining with roadside filling (Figure 7b), the side without entry retaining exhibits a pattern similar to that of the traditional condition. Both exhibit similar distribution characteristics in the lateral fracture area and separation zone, with a collapse angle of 76\({}^{\circ}\). The primary difference between the two lies in the roadway-retaining side. Under roadside filling conditions, the fracture distribution of the overlying strata on this side is more complex. A moderately fractured zone exists between the well-developed separation zone and the poorly developed lateral fracture area. This occurs because under traditional coal pillar conditions, the overlying strata breaks and completely collapses with the coal pillar as the fulcrum. In contrast, under gob-side entry retaining conditions with roadside filling, the roadway roof breaks at the coal-side fulcrum outside the roadway, but the filling body restricts roof displacement, thus preventing collapse. As a result, collapse initiates at the filling body instead. The area between the breaking line and the collapse line is defined as the secondary breaking area. The secondary breaking area is an inverted trapezoid, with a narrow lower part and a wider upper part. The breaking angle is equal to the collapse angle on the non-entry-retaining side, both measuring 76\({}^{\circ}\), and the collapse angle on the roadway-retaining side is 52\({}^{\circ}\).  
In addition, under the condition of gob-side entry retaining with roadside filling, changes in rock damage and the formation of the secondary breaking zone increase the span of the separation zone from 24 m to 30 m compared to traditional coal pillar conditions. The larger span of the separation zone and the smaller collapse angle jointly result in a larger space of the separation zone. This result shows that the roadside filling body can  
Figure 6: The fracture development characteristics of the working face after mining 105 m along the strike of the working face.  
delay the subsidence rate of the overlying strata, expand the range of the separation zone and cause it to shift toward the goaf.  
The formation of a secondary breaking zone above the backfill, as well as the expansion of the separation zone span and the reduction in the caving angle on the side of gob-side entry retaining with roadside filling, are considered unique characteristics under the conditions of gob-side entry retaining with roadside filling. The specific numerical variations should be determined according to the actual conditions of the mine.