# Analysis of the Impact of the New Two-Lane Shield Tunnel Underpass on the Existing Tunnels
## 1 Introduction  
As urban surface space becomes increasingly saturated, the development and utilization of underground space have garnered growing attention. The construction of new metro tunnels beneath existing ones is becoming more common, as urban space constraints necessitate such projects. However, the construction of new tunnels beneath existing ones inevitably induces deformation in the existing tunnels. Metro operations impose stringent requirements for controlling tunnel deformation. Under-passing construction presents a significant safety risk to existing metro lines and is widely regarded as one of the highest-risk aspects of metro construction. Academics have extensively studied the settlement effects of new tunnel shield construction on existing tunnels. The research methods primarily include theoretical analyses, field measurements, model tests, and numerical simulations. [PERSON] et al. integrated sink theory into elastic mechanics by treating the strata as an elastic semi-infinite body, thereby determining the impact of strata loss on surface soil displacement [1]. [PERSON] et al. combined the random medium theory and cumulative probability curve to calculate the vertical displacement caused by soil loss and estimated the vertical displacement of the existing tunnel by rotating the staggered platform cooperative deformation model [2]. [PERSON] et al., [PERSON] et al., and [PERSON] et al. proposed a theoreticalanalytical model by using a two-stage analysis method to predict the impact of the construction of a new shield tunnel on the settlement of an existing tunnel [3; 4; 5]. [PERSON] et al. established an expression for the proximity influence degree and divided the zoning control criteria based on the structural displacement discrimination criterion [6]. [PERSON] et al. and [PERSON] et al. studied the deformation characteristics of existing tunnels and pipelines induced by shield construction through on-site monitoring [7; 8]. [PERSON] et al. pointed out that the vertical spacing between two tunnels also affects the deformation of existing tunnels based on the monitoring of construction displacement [9]. [PERSON] et al., [PERSON] et al., and [PERSON] et al. analyzed the disturbance effect on the adjacent existing tunnels during the construction of new tunnels with the help of model tests, and clarified the settlement and deformation law during the process of tunnel penetration [10; 11; 12], while [PERSON] et al., [PERSON] et al. and [PERSON] et al. used finite element and discrete element methods to model and deeply analyze the tunnel excavation for different regional characteristics and construction techniques, revealing the mechanisms and characteristics of ground and surface settlement during construction [13; 14; 15]. Using numerical tests, [PERSON] et al. constructed an impact zoning model based on the settlement of existing tunnels [16]. [PERSON] et al. investigated the specific impact of shield under-passing on the structural deformation of existing tunnels using FLAC3D v6.0 finite element software combined with field data [17]. Overall, research on underpass construction's impact on existing tunnels has yielded valuable insights, contributing to a deeper understanding of the issues associated with double-line underpass projects. However, the complexity and regional variability of underground tunnel crossings limits the applicability of current findings. This study employs theoretical calculations, numerical simulations, and on-site measurements to examine the impact of the interval project between Dongfang Road Station and Suoyuwan Station on Dalian Metro Line 4. It analyzes the vertical displacement and additional stress changes in the existing tunnel caused by the proximity of the new tunnel. The study reveals the settlement characteristics of the existing tunnel due to the excavation of the new two-lane tunnel. The accuracy of the numerical simulation model is verified through comparative analysis.