## 2 Engineering Background
### Engineering Impact and Status Evaluation  
Based on the findings from the 3D laser detection, Figure 3 illustrates the plan view of the abandoned mining area adjacent to the highway bridge foundations. The mining method employed here was room-and-pillar mining, a technique involving the excavation of \"rooms\" into the rock mass while leaving behind \"pillars\" of unexcavated material to support the roof. This creates a grid-like pattern of extracted areas and supporting structures, balancing the need for resource extraction with maintaining the stability of the surrounding environment. The primary mine shaft runs in a north-south direction. The ore body is present as stratoid beds with an average mining thickness of around 3 m. In this specific case of room-and-pillar mining, the pillars and remaining structures were not sufficiently robust to prevent some level of collapse and instability, which is indicative of the challenges and considerations that come with this mining method. Among the mining rooms, only a few have been backfilled with waste slag, while the majority remain unfilled, lacking significant lateral constraints. The exposed area of the abandoned  
Figure 2: 3D laser scanning detection and results.  
region, near the bridge foundation, is extensive, accounting for roughly 84% of the total ore body area. The mining rooms are predominantly elongated, with their orientation approximately orthogonal to the highway central line. The maximum length of the mining room is about 150 m, which is much longer than the room width. The roof depth varies between 40 and 60 m, and the mining rooms span from 14 to 24 m, with an average span of 16 m, maintaining an equal width on both sides. It is worth noting that the average width of the side walls and pillars is only 3 m, and many pillars were severely overmined, and some even collapsed. This highlights another aspect of room-and-pillar mining, where the size and integrity of the pillars are crucial to the long-term stability of the mined areas. There is a recurring pattern of interconnected mining rooms, and a significant portion of these rooms exhibit signs of roof collapse, underscoring the importance of carefully planned and executed room-and-pillar mining to mitigate such risks.  
The detection results indicate that several pillars within the mining area have already experienced subsidence. Moreover, the volume and extent of the abandoned mining area raise concerns. Consequently, the stability of the supporting system becomes increasingly challenging, especially in the face of long-term pressure transfer and water-induced softening. The enduring stability of the abandoned mining area is primarily contingent upon the stability of the roof and the rib. When the exposed areas of the roof and rib become extensive, instability becomes almost certain. Under these circumstances, there is an urgent need to conduct a comprehensive stability analysis of the abandoned mining area. This analysis is vital to ensuring the highway's safety and averting potential catastrophic accidents.  
According to the results of the geological survey and 3D laser detection, although the gypsum mine has left roof pillars during the mining process, due to the large volume of the empty area, some of the pillars have already collapsed. It will be difficult to maintain the stability of the pillars under the influence of long-term stratum pressure transfer and softening of the water seepage. The long-term stability of the mining area will be mainly controlled by the stability of the roof and sidewalls. When the exposed area of the roof plate and sidewalls of the mining area is too large, the roof plate and sidewalls will be destabilized, which may result in the room collapsing, or even in large subsidence. In order to ensure the safety of the upper highway construction and prevent catastrophic accidents induced by the destabilization of the mining area, it is urgent to analyze the stability of the abandoned mining area.  
Figure 3: Plan view of the abandoned mining area.