Experimental and numerical modelling of stress redistribution due to construction of cross passages of shallow tunnels

In tunnel practice, the planning and execution of cross passages is crucial for successful building operations. The tunnel area in proximity of the cross passages requires additional support measures. Tunnel designers often rely on complex three-dimensional numerical calculations whose validation is scarce. The general objective of this research is to increase the understanding of stress redistribution due to cross passage opening. Due to the drainage effect of the running tunnel prior to the opening of the cross passage, the soil in the cross passage is partially saturated. This study aims to include the effect of partial saturation in the stability calculations of cross passages. The stress redistribution in the area of the cross passages is investigated both experimentally in the geotechnical centrifuge and numerically. In the centrifuge, tests are performed with model soil at varying ratios of overburden to diameter. For the numerical calculations, the hypoplastic constitutive model is extended to partially saturated soils under consideration of the correlation between degree of saturation and preconsolidation pressure. Numerical calculations are performed on model tunnels to estimate the sensitivity of the results to changes in input parameters. This sensitivity analysis is used to develop design aids for the tunnel construction practice.

This study focuses on understanding the impact of constructing cross passages in tunnels, which are essential for emergency access, maintenance, and equipment storage. Cross passages are small tunnels that connect larger running tunnels. Despite their importance, the effects of their construction on the main tunnels are not well understood. This research aims to fill that gap by examining how stress is redistributed around these cross passages. To investigate this, researchers conducted experiments using a scaled model of a tunnel at 50 times the normal gravitational force (50g). They installed strain gauges near the top (crown) and sides (springlines) of the cross-passage opening to measure changes in stress. These experimental results were then compared with computer simulations using three-dimensional finite element analysis. The strain gauges proved to be a reliable method for estimating changes in stress in both the hoop (circular) and longitudinal (lengthwise) directions. The experiments showed that the hoop force near the cross-passage opening increased significantly-by about 2.68 times for a ratio of cross passage diameter to tunnel diameter (C/D) of 0.5, and by 1.72 times for a C/D of 1.0. While there was some agreement between the experimental results and the computer simulations, there were also notable differences. Specifically, the bending moments (forces causing the tunnel to bend) showed irregular results in the experiments, likely due to interference from the model's end supports. Understanding the stress changes caused by cross passage construction can help engineers design safer and more efficient tunnels. This knowledge is crucial for ensuring the structural integrity of tunnels, which has significant implications for public safety and infrastructure maintenance. The study identified some limitations in the current experimental setup, such as the interference from end supports. Future research will aim to address these issues to improve the accuracy of the results. The findings from this study can be used to enhance the design and construction of tunnels, making them safer for emergency access and maintenance. This has potential benefits for various sectors, including transportation, urban development, and public safety.