# Ground Deformation Associated with Deep Excavations in Beijing, China
## 1 Introduction  
The Beijing Metro is a world-class subway system that efficiently serves the city and surrounding suburbs. At the beginning of 2023, 54 cities in China had implemented urban rail transit, with 291 lines and 5609 stations in service. It is expected that by 2025, 65 cities in China will have opened or built urban rail transit lines, with a total mileage exceeding 13,000 km in operation. Over nearly half a century, numerous deep excavations have been carried out in subway stations across Beijing. With rapid economic growth, there is anincreasing demand for future urbanization. As the urban rail transit net expands, the engineering of deep foundation pits is developing on a larger scale, at deeper depths, and under more complex conditions. Deep excavations are often executed near existing buildings and infrastructures. And accidents may cause serious casualties, economic losses, and have a bad influence on society. The safety and stability of deep excavation has attracted much attention. Controlling ground deformation becomes even more challenging and it meets stringent requirements due to limited construction space. Since the 1990s, performance-based design concepts have been adopted [1]. Pit performance during deep excavation may be influenced by various intricate factors, including stratum conditions, stress levels, boundary conditions, support system stiffness, type of retaining wall, time effects, and workmanship. And they are difficult to access or quantify due to their interconnectedness. Therefore, the analysis of deformation with regard to deep excavation cannot comprehensively take into account all of these factors [2]. Predicting ground movement, which is necessary for the design and construction of deep excavations, is a practical challenge faced by engineers and researchers in megacities [1,3]. As a tangible reflection of the influence of diverse factors on excavation in actual projects, the performance of the foundation pit may inform future excavation predictions. The semi-empirical study of monitoring performance is a viable approach for deciphering associated deformations [4]. Around the globe, researchers have undertaken studies on deformation caused by deep excavation, primarily in clay deposits or sand [1,3,5, 6, 7, 8, 9, 10, 11, 12, 13]. In these studies, deformation data were usually categorized based on the ground conditions, and they were analyzed to study the effects of excavation depth, the type of support, and support system stiffness on the magnitude of ground settlement [2]. In 1969, [PERSON] [5] conducted a detailed study on ground settlements and excavations. The maximum ground settlement was approximately 1% of the excavation depth in soft clay, stiff clay, and sand [2,6]. A dimensionless curve of the relationship between the maximum settlement and the distance beyond the excavation that was normalized by the excavation depth was obtained. This work was well known and widely used in practice. Based on [PERSON]'s research, [PERSON] et al. [8] studied and analyzed the measured data of foundation pits in stiff clay, sandy clay [9], residual soil [10], and relatively stiff soil [13], respectively. The magnitude of the ground settlement is reduced in order to advance in the design and construction of support systems, and it ranges from 0.1 to 0.3% of the excavation depth. Maximum ground settlements that are less than 0.2% of the excavation depth have been found in mixed grounds [14,15]. [PERSON] et al. [2] studied the characteristics and influencing factors of foundation pit deformations in Taipei [3], Singapore [16], Hong Kong, and the Yangtze River delta in China [11]. The results of hundreds of pits with different lengths, widths, and excavation depths were analyzed. The surface settlement is affected by excavation geometry [17] (i.e., length, width, and depth of excavation), and it regularly changes with the width of the foundation pit [18]. A small length-width ratio produces little ground movement [19].