# Instability Risk Assessment for Deep Excavation of Soil-Rock Combinations Containing Groundwater
## 1 Introduction  
In recent years, to meet the demands of urban expansion, there has been a notable surge in the construction of deep and extensive excavation projects within densely developed areas. Nevertheless, the excavation process is fraught with the potential for substantial deformations, partial failures, and structural support breakdowns. These issues can be attributed to a range of factors, such as geological conditions, the stability of the support system, and the surrounding environment. Consequently, these challenges not only pose a significant risk to safety, leading to potential casualties, but also contribute to significant delays in the overall construction timeline [1; 2; 3; 4]. For example, on 15 November 2008, an excavation collapse occurred in the North 2 excavation of Xianghu Station of Hangzhou Metro Line 1, which caused 21 deaths and a direct economic loss of 49.61 million yuan. Another example, on 15 June 2021, a part of the north side of the excavation of the Bank of Nanjing Science and Education Innovation Park Phase II project in the Nanjing High-Tech Zone experienced a partial collapse accident, which involved casualties and 9.8973 million yuan in direct economic losses. On the other hand, excavation exerts an impact on the surrounding environment that cannot be ignored, especially in urban densely built-up areas, and is prone to triggering problems such as cracking of the surrounding buildings and deformation of road surfaces [5; 6; 7; 8]. In addition, on 21 March 2021, the interior of a building excavation in Hangzhou produced an inclined collapse, which caused the destruction of the supporting structure and, meanwhile, contributed to the collapse and displacementof the adjacent road, where the ground cracks were more than 30 m in length. Although monitoring is commonly employed in engineering to ensure excavation safety, it sometimes lags in its reflection of the safety status. As a result, this has received extensive attention and discussion among researchers, to make a correct assessment of the stability of an excavation during the construction process and to judge the safety status [9; 10; 11; 12]. Research about the risk analysis of excavations began in the 1960s, initially focusing on exploring theories, while analyzing the field data of actual projects in certain areas. [PERSON] et al. [13] presented a method of predicting the degree of risk based on the cracks of surrounding buildings to analyze the impact of excavation on the surrounding buildings and used this method to predict the potential risk. [PERSON] and [PERSON] [14] classified influencing factors by combining the results of physical modeling and numerical simulation with deformation monitoring data during the actual excavation process, so as to predict the safety of the excavation and the damage to the surrounding buildings. These analyses provide the basis for subsequent risk assessment studies. Additionally, numerous researchers have introduced mathematical models for the analysis of excavation risk and obtained more accurate results [15; 16; 17]. [PERSON] et al. [18] presented an underground engineering risk analysis method based on an uncertainty model with fuzzy concepts for analyzing the reliability of excavation projects, which was successfully applied in a Korean subway project. [PERSON] and [PERSON] [19] established a fuzzy comprehensive evaluation model using a Bayesian network that could qualitatively judge the safety status of excavations according to the characteristics of uncertainty and fuzziness. Meanwhile, they established a hybrid framework combining the ratio analysis method of stepwise weight assessment and the theory of complex proportion assessment. [PERSON] et al. [20] substituted different monitoring data and risk level monitoring during construction in a random forest model, on the basis of which an intelligent risk prediction model for subway deep excavation was established and successfully applied.