# Case Study and Risk Assessment of Water Inrush Disaster in Qingdao Metro Line 4
## 1 Introduction  
As metro construction increases, the pressure of urban surface transportation brought about by urban development and dense population has been greatly alleviated, and metro is the preferred transport mode of people [1, 2]. According to statistics from the National Bureau of Statistics, 51 cities in China had opened metro lines by the end of 2021, with a total operating distance of 8736 km. However, water inrush disasters often occur due to geological and groundwater during metro tunnel construction [3]. For example, on 6 May 2011, a water inrush accident occurred during the construction of Tianjin Metro Line 2, and caused the collapse of the road surface. Another example is that on 7 February 2018, in Foshan Metro Line 2, when a water inrush accident occurred and caused the tunnel and roadway to collapse. Water inrush disasters not only cause life and property loss, but also cause environmental problems such as surface collapse and groundwaterloss [4]. Therefore, reducing the occurrence of water inrush is of great significance for safe construction, property loss and environmental protection. Various methods, such modeling experiments, numerical simulation and field tests have been adopted to explain the mechanism of water inrush. In terms of modeling experiments, various types of models with different sizes and conditions have been proposed to study the process of water inrush. For instance, [PERSON] et al. [5] studied the stability of the surrounding rock using a true triaxial geological model test, revealed the internal connections between physical information such as displacement, stress and seepage pressure during tunnel excavation, and established the theoretical basis of the predicting of water inrush. [PERSON] et al. [6] proposed a 3D fluid-solid coupling model test to study the evolution process of water inrush caused by excavation and rainfall, and revealed the mechanism of water inrush. [PERSON] et al. [7] carried out a seepage instability water inrush test of a karst pipeline to study the evolution process of the filling material seepage instability water inrush and analyzed the changes of surrounding rock displacement, stress, and seepage pressure during the water inrush process. [PERSON] et al. [8] developed a visual experimental system of particle migration to study the mass loss evolution of broken porous rock induced by seepage failure, and discussed the influence of grain gradation on particle migration. However, experimental research is restricted by models without real geological conditions, resulting in difficulty to truly deduce the evolution process of water inrush disaster in engineering. With the development of computers, scholars are addressing this problem using numerical models [9; 10; 11; 12]. [PERSON] et al. [13] studied the water inrush mechanism in faults of a harbour tunnel by physical and numerical models, and discussed the causality between crown displacement and hydraulic pressure. [PERSON] et al. [14] revealed the mechanism of water sand inrush in water-rich sandy dolomite strata by a flow-solid coupling model, and analyzed the influence of different factors on the deformation value of the tunnel face. [PERSON] et al. [15] developed a numerical model combining the discontinuous deformation analysis method (DDA) and the smoothed particle hydrodynamics method (SPH) to clarify the water inrush mechanism and the critical thickness of a water-resistant rock mass in a jointed rock tunnel, and divided the process of water inrush into a mutation stage, a cataclysmic stage and a stable stage. [PERSON] et al. [16] studied the mechanism of the collapse of a funnel-shaped stratum through numerical simulation, and found that the essence of the collapse is the process of converting potential energy into kinetic energy. [PERSON] et al. [17] proposed a conceptual model which emphasized the evolution of fracture connectivity to investigate fluid flow through fractured rocks and study the water inrush mechanism, and discussed the influence of the connected fractures on water inflow.