# Research on Risk Assessment Method for Land Subsidence in Tangshan Based on Vulnerability Zoning
## 3 Hazard Assessment Results for the Study Area
### Construction of Hazard Assessment Index System for Ground Subsidence
#### 3.1.1 Natural Factors  
The natural factors significantly contributing to the ground subsidence hazard encompass the lithology, quaternary groundwater, geological faults, and proximity-to-water systems within the study area. Figure 9: Evaluation index system of urban ground collapse risk. (1) Lithology  
Drawing upon engineering geotechnical investigation reports, the study area was segmented into seven engineering geological strata, namely, miscellaneous fill, silty clay, loess, loam, sandy soil, gravely soil, and carbonate rock. These strata were integrated with the 1:200,000 geological map of Tangshan City, resulting in a four-class categorization of the lithology, as detailed in Table 7. The specific classification is visually represented in Figure 10.  
(2) Quaternary Groundwater  
A statistical analysis of the correlation between the thickness of the quaternary cover layer and the frequency of collapses reveals a significant influence of the former on the latter. Collapses are predominantly observed in areas where the thicknesses range from 15 to 30 m, while areas with thicknesses exceeding 30 m witness fewer collapses. This can be attributed to the fact that, in areas where the thicknesses surpass 30 m, the soil voids that form are sufficiently supported by the thickness of the overlying soil, thereby enhancing the resistance to subsidence. Conversely, in areas where the thicknesses are less than 15 m, the pore water is typically drained, reducing the likelihood of subsurface erosion. The groundwater levels in the study area were classified into three categories based on thickness: less than 15 m, 15-30 m, and exceeding 30 m. Table 8 presents the groundwater monitoring data for the study area. Utilizing well-logging data, the inverse-distance weighting method was employed to classify the groundwater in the study area, as depicted in Figure 11. \begin{table}
\begin{tabular}{c c} \hline \hline
**Stratigraphic-Rock-Type Classification** & **Value** \\ \hline Carbonate Soil, Gravely Soil & 1 \\ Silty Clay, Loess & 2 \\ Loess, Colluvium & 3 \\ Sandy Soil & 4 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Stratigraphic lithology classification values in the study area. Figure 10: Lithology of the study area. (**a**) Lithology distribution. (**b**) Lithology classification. The propensity for ground subsidence increases with a decline in the groundwater level. Human activities, such as groundwater extraction, can precipitate a rapid decrease in the water table, thereby exacerbating the risk of subsidence incidents. The groundwater level monitoring data for the study area, represented by latitude and longitude coordinates, were imported into ArcGIS. The inverse-distance weighting method was then applied to conduct an interpolation analysis of the quaternary groundwater levels. The resulting distribution map of the quaternary groundwater levels in the study area is illustrated in Figure 11, with corresponding categorization. Areas with thicknesses less than 15 m were assigned a value of 1, those with thicknesses of 15-30 m were assigned a value of 3, and those with thicknesses exceeding 30 m were assigned a value of 2. (3) Geological Faults  
Geological faults include the following. (i) The Douhe Fault (F1), depicted in Figure 12, which is a predominantly NE-dipping normal fault: Notably, during the 1976 Tangshan earthquake, this topographic scarp was the site of sand ejection and water eruption phenomena. However, surface investigations yielded no direct evidence of faulting. (ii) The Weishan-Changshan-Fengnan Fault (F2), which is bifurcated into two segments due to the presence of the Dachengshan and Fenghuangshan mountains within the urban area, the Weishan-Changshan South Slope Fault (F2-1) and the Tangshan-Fengnan Fault (F2-2): The F2-1 fault is characterized by angular gravel fill, and the fault surface exhibits clear evidence  
Figure 11: Groundwater of the study area. (**a**) Groundwater distribution. (**b**) Groundwater classification.