## 2 Study Area  
Recently, a dual-direction, four-lane expressway, designed for speeds of 100 km/h, is planned to be constructed between Yuanling County and Chenxi County. This expressway will pass through a long-abandoned refractory clay mining area in Chenxi County. The mine is located to the east of Chenxi County in Huaihua City, Hunan Province. The mine is situated near the Yuanjiang River, approximately 1.2 km upstream from the Second Bridge of Yuanjiang River, to the north of the county seat (Figure 1). The Yuan-Chen Expressway spans the mining area from K32+580 to K33+440, covering a length of approximately 860 m.  
Based on the collected data, it can be confirmed that the terrain of the study area is predominantly hilly. The region experiences a subtropical monsoon humid climate with abundant rainfall, with an annual precipitation of approximately 1300 mm. The highest elevation is 200 m, while the lowest is 120 m, resulting in a relative elevation difference of 80 m. The study area is located on the southeastern edge of the Northwestern Hunan block, with the main fault being the Pengjiawan Fault, which strikes northeast (Figure 2). Figure 1: (**a**) Map of study area, (**b**) location of ERT survey lines and boreholes. Black rectangle: study area. Yellow line: expressway planning route. White dot: borehole points. Red line: ERT survey lines, L1–L5 from west to east in (**b**). Figure 2: Geological map of the study area. White line: expressway planning route. Black rectangle: study area. Before conducting the ERT field survey, the relevant personnel carried out an engineering geological investigation of the study area and performed borehole exploration at the site. Based on the collected data, an engineering geological map of the study area was created (Figure 3). Figure 3 indicates that the exposed strata in the study area consist, in sequence, of Quaternary silty clay, Permian quartz sandstone, mudstone and sandy shale, and Carboniferous limestone. The refractory clay mine is located within the mudstone strata and is associated with the overlying quartz sandstone and the underlying sandy shale. Laboratory tests on drill samples reveal that the Permian quartz sandstone, mudstone, and sandy shale are fragmented with poor physical and mechanical properties, while the Carboniferous limestone is intact and exhibits strong mechanical properties (Tables 1 and 2). The tests also show that the Permian quartz sandstone, mudstone, and sandy shale have low uniaxial compressive strength due to their fragility. The presence of underground abandoned mining tunnels suggests that the expressway foundation may encounter insufficient bearing capacity in this area. Between K32+580 and K32+900 m, the refractory clay mine is relatively shallow, with open-pit mining as the primary method, supplemented by tunnel mining (ZONE-1 in Figure 1). From K32+900 to K33+440 m, tunnel mining serves as the primary extraction method (ZONE-2 in Figure 1). The mine, operational since the 1990s, has primarily been mined by private enterprises and local residents. Mining activities gradually ceased after 2015, but illegal mining continued even after the official cessation, resulting in a more complex network of internal passages. As mentioned earlier, the study area experiences abundant rainfall, and a large \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample ID** & **MC \%** & **Sp. Gravity** & **Liquid Limit \%** & **Plastic Limit \%** & **Plasticity Index** & **DD (g/cm3)** & **VR** \\ \hline
1-1 & 21.9 & 2.72 & 36.4 & 20.3 & 16.1 & 1.63 & 0.67 \\
1-2 & 22.1 & 2.73 & 32.8 & 21.9 & 10.9 & 1.67 & 0.59 \\
1-3 & 21.8 & 2.69 & 33.7 & 20.8 & 12.3 & 1.64 & 0.63 \\
1-4 & 21.6 & 2.67 & 34.1 & 20.5 & 14.2 & 1.66 & 0.64 \\ \hline Average & 21.85 & 2.70 & 34.25 & 20.9 & 13.38 & 16.50 & 0.63 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical parameters of soil mass in study area. Figure 3: Engineering geological profile of the study area. Red circle: mudstone formation.