## 2 Twin-Link Stepping Temporary Support Mechanism
### Comparison of Construction Techniques  
Due to the complex conditions in underground operations, a significant portion of comprehensive excavation tunnel construction currently depends heavily on manual labor for serial operations. The process primarily comprises six key steps: mobilization of excavation, cutting and removal of coal, loading and transportation of coal, installation of anchor nets, and pre-tensioning of anchor rods. The schematic diagram illustrating the traditional serial operations for comprehensive excavation roadways can be seen in Figure 5.  
Figure 4: Timeline diagram illustrating the parallel process of excavation, support, and anchoring operations. In the conventional sequential operation process, the tunneling machine is required to retreat a specific distance after advancing to ensure roof control. Support workers utilize single anchor rod drilling machines to offer temporary or permanent anchor support in the face area. Tunnelling and anchor support activities must take turns and cannot be conducted simultaneously in the sequential operation mode. Due to the inaccessibility of the tunnel face by the drilling and anchoring vehicle, manual anchor support takes a considerable amount of time, resulting in low efficiency. Furthermore, during manual support operations, if the roof falls and there are currently no effective means to protect miners' safety, it poses a significant threat to their lives. In comparison to the traditional process, this paper presents a novel excavation-support-anchor technique named the twin-link stepping temporary support, which operates in parallel. The excavation area is partitioned into three zones: tunneling, temporary support, and permanent support. The temporary support function acts as a bridge, connecting the spatial aspects of excavation and permanent support activities. This enables the simultaneous and non-interfering progression of tunneling and permanent support work, ensuring a high level of operational coordination. Additionally, in the event of a roof fall, the duplex stepped temporary support can employ a safety relief valve to gradually release pressure, ensuring that the roadway does not experience abrupt collapse. The new parallel excavation-support-anchor technique incorporates highly automated excavation, support, and anchoring equipment. Through parallel operations, it enables intelligent utilization of the six principal construction process equipment in comprehensive excavation roadways, resulting in increased excavation speed and reduced personnel involvement. Figure 6 illustrates the comparison of time efficiency between the parallel excavation-support-anchor technique and conventional excavation construction methods. The ratio of time dedicated to excavation and support was reduced from the original 1:2.1 to 1:0.89, leading to an almost threefold improvement in support efficiency. The overall efficiency of excavation and support can be enhanced by approximately twofold. By following a collaborative parallel operation approach of \"support ahead and anchor behind\" during excavation, the parallel excavation-support-anchor construction technology significantly reduces the likelihood of roof cave-ins and rib accidents in coal mines, thereby enhancing mining efficiency. The twin-link stepping temporary support system designed in this paper significantly enhances the safety and efficiency of the excavation work face. However, during the advancement of the roadheader and the movement of the twin-link stepping temporary support unit towards the excavation face, the support in the same roof area must be repeatedly removed and repositioned. This repetitive support activity during the movement process can potentially exacerbate hazards such as roof collapse, delamination, and roof fall, particularly if the roof stability is compromised. Therefore, ensuring a certain level of  
Figure 5: The presentation of traditional serial operations for comprehensive excavation roadways.