## 1 Introduction  
The construction of underground structures has developed rapidly in recent years and plays an important role in the development of the Chinese national economy [1]. As an important feature of infrastructure, it is of great significance to estimate the stability of concrete structures, especially for underground construction. Complicated environmental factors make underground constructions more vulnerable to external interference and can induce disasters [2, 3]. Previous studies have demonstrated that the stress concentration occurs around damaged positions and induces leakage disasters [4, 5]. Thus, the present study focuses on the mechanical response of a concrete structure under the effect of local damage. A typical underwater concrete structure, the Nanjing Yangtze River tunnel, was selected as a case study. Field investigation showed that damage induced by grouting hole leakage is an important factor influencing structural stability. However, there has been limited research on the detection of local damage induced by grouting holes and its influence on the overall stability of structures [6]. Numerical simulation in light of field investigation is vitally important for disaster prevention. It is of great significance to combine field investigation with mechanical response analysis [2; 3; 7]. Based on the current research, this work developed an integrated framework to detect the damage conditions of the concrete structure and analyze the influence of local damage on structural mechanical performance through nondestructive testing (NDT) technology and numerical models. Our framework consists of three modules--accurate testing in the field, numerical analysis, and reliable evaluation. Accurate testing provides fundamental information for numerical modeling. The existing methods, presented in the past few years, that are used to detect the status of structures can be divided into two categories--manual testing and automatic monitoring [8; 9]. Manual testing is a comprehensive method, but the testing process takes a long time. In recent years, automatic detection methods, such as sensor monitoring and automatic photographic equipment, have developed as a reliable technology to maintain the stability of tunnels [10; 11]. These methods have been widely used in tunnel engineering for the detection of damage conditions [12; 13]. However, these studies have mainly focused on the detection of lining cracks and segmental joint openings [6], and commonly ignore the influence of grouting holes. Based on the status of the research and the complicated field conditions, nondestructive testing (NDT) technology has been carried out to determine the local damage conditions of tunnel structures. The common application of this method is to reflect the internal status of an object without causing any damage [14; 15]. According to the detection information obtained from NDT results, numerical simulation is then carried out to study the impact of local damage on structural mechanical performance. As it is one of the most important components of the framework, various numerical models have been developed in recent years. Most of these numerical models based on discrete element theory, finite element theory, and the coupling of these two theories [7; 16]. The combined application of field detection results with numerical simulation is of great significance to stability assessment, which has remained an underlying problem in previous studies [17]. Along this line, in the present study, a numerical model developed on the basis of finite element theory is applied to study the structural mechanical response to local damage. In addition, the influence of local damage is discussed under different boundary conditions, including various water pressure and damage positions. The study was structured as follows. Firstly, the methodology of the framework used for the structural stability assessment was devised. Then, the devised integrated method was employed in an underwater shield tunnel as a study case.