# Case Study and Risk Assessment of Water Inrush Disaster in Qingdao Metro Line 4
## 1 Introduction
Although a numerical model can calculate problems at the engineering scale, results are deficient and ignore the heterogeneity of surrounding rocks. On-site monitoring can reflect the dynamic changes in excavation engineering and the working state of surrounding rock and support [18; 19; 20]. Therefore, scholars have also striven to explain the water inrush mechanism based on on-site monitoring data such as surface deformation [21], lining stress [22], water inflow [23; 24], and microseismic signals [25]. Previous research has revealed the relationship between disasters and various factors [26; 27; 28; 29] that play an important role in reducing the risk during excavation. Water inrush risk assessment is an important tactic to reduce the occurrence of disaster. The study of tunneling risks started in the 1970s, and [PERSON] [30] first introduced the idea of risk assessment into the field of tunnelling in 1983. [PERSON] et al. [31] established a risk assessment structure chart and applied it to a cross-harbour tunnel, based on which [PERSON] et al. [32] further improved the risk assessment system and applied it to the metro. Multiple methods have been applied to water inrush risk assessment, including the analytic hierarchy process-AHP [33], cloudy theory [34], Bayesian network [35; 36], neural network [37], fuzzy theory [38; 39; 40], and other mathematical models. Meanwhile, scholars have continuously improved the original method to obtain more accurate results. For instance, [PERSON] et al. [41] improved the analytic hierarchy process using the Delphi survey technique, and established a probabilistic tunnel collapse risk evaluation model. [PERSON] et al. [42] proposed a risk evaluation model for water inrush combining the analytic hierarchy process and fuzzy Delphi method, helping to design a more suitable drainage system, drilling method, and support. [PERSON] et al. [43] applied an analytic hierarchy process and fuzzy theory and proposed an interval risk assessment method of water inflow and inrush in karst tunnel. The above-mentioned methods have been effective in reducing engineering risks. Previous research on the mechanism and risk assessment methods of water inrush have been mostly focused on mountainous and karst areas, while there has been less research on water inrush in the upper soft and lower hard composite strata. Furthermore, for most of the above methods, for the selection of evaluation indexes is it has been difficult eliminate subjectivity. Hence, this study aimed to clarify the water inrush mechanism in composite strata by a case of water inrush and ground collapse in Qingdao Metro Line IV, and proposed a water inrush assessment method based on characteristics of composite strata. First, the case of water inrush and ground collapse is considered. Then, the disaster-causing factors of the water inrush were analyzed comprehensively, and the water inrush mechanism are discussed with respect to a numerical model. Finally, the water inrush risk of the tunnel is evaluated using the improved method. This study has value in reducing the occurrence of water inrush in a composite strata tunnel.