## 2 Evolution Characteristics of the Roof Structure in Gob-Side Entry Retaining by Roof Cutting
### Analysis of Roof Structure Characteristics in Different Stages  
Due to the effect of pre-splitting roof cutting, the roadway can be divided into pre-cutting and post-cutting stages. During the mining of the working face, the GERRC can be divided into five stages: excavation, pre-splitting cutting, primary mining retention, retaining stability, and secondary mining impact, as shown in Figure 4. Because of the large subsidence space of the roof during the retaining period, the roof movement is intense, which is a critical stage for ensuring secondary safety reuse. Figure 4: Stages of gob-side entry retaining by roof cutting. Figure 3: Design parameters of pre-splitting drilling in roof cutting roadway: (**a**) Plan view of roof cutting parameters. (**b**) Profile view of roof cutting parameters. Figure 2: Roadway layout and geologic histogram of working face: (**a**) Geologic histogram. (**b**) Roadway layout plan. The structural evolution of the roadway roof at different stages leads to changes in the stress characteristics of the direct roof. The deformation of the roadway roof is characterized by the direct roof, bearing not only the supporting pressure imposed by the main roof but also the supporting force from the roadway internal support structure. The support of the roadway roof by bolts and cables is an internal force of the roof. Therefore, when analyzing the stress characteristics of the immediate roof, the support forces from bolts and cables are ignored. As gob-side entry retaining by roof cutting progresses, the roadway roof undergoes structural changes at different stages. The evolution characteristics of the roof structure in each stage are as follows:  
1. Roadway excavation stage: The additional stress caused by excavation disturbance is relatively small, and the direct roof is supported by solid coal at both ends, with limited expansion of internal cracks. It remains stable under the support of the solid coal at both ends. In this stage, the immediate roof can be simplified as a beam structure with both ends fixed, as shown in Figure 5a. 2. Pre-splitting roof cutting stage: In this stage, the advanced working face implements pre-splitting cutting on the roadway roof, severing the mechanical connection between the roadway roof and the working face roof. The pre-splitting roof cutting holes are generally at an angle of 70-80\({}^{\circ}\) with the roadway roof [23; 24]. Under the action of the self-weight of the overlying strata and the mining support pressure, the roadway roof at both ends remains supported by solid coal after the advanced working face pre-splitting roof cutting. This stage generally focuses on controlling the stability of the roof and increasing temporary support within the roadway. Temporary support often uses single-leg supports and stacking supports, which can simplify the roadway roof structure into a cantilever beam with one end fixed and the other end simply supported, as shown in Figure 5b. 3. Primary mining retention stage: Due to the influence of the working face mining support stress and the large turning subsidence space of the roadway roof, the movement of the roadway roof is intense and prone to significant deformation, which is a critical support stage during the retention process. To control the subsidence of the roadway roof, temporary support is usually increased within the roadway. This often involves the use of multiple rows of single-leg supports or two rows of stacking supports. Therefore, the roadway roof structure in this stage can be simplified as a cantilever beam with one end fixed (solid coal), as shown in Figure 5c. 4. Retaining stability stage and secondary mining advanced impact stage: After the roadway is stabilized, the caved gangue fills the gob area, providing support for the roadway roof on the cutting side until the secondary reuse of the roadway ends.