Assessing twin tunnel-induced ground settlements in alluvial deposits by InSAR data

Tunnel excavation in densely urbanised environments presents significant geotechnical challenges. Reliable design predictions, precise monitoring, and a thorough understanding of these challenges are crucial for ensuring the safety and stability of tunnels and surface structures. This study investigates the analysis of ground deformations caused by twin tunnel excavation using an EPB-TBM in alluvial deposits. The analysis is based on data collected through the European Ground Motion Service (EGMS) InSAR and on-site measurements, incorporating both empirical and analytical methods, and time series decomposition techniques (Principal Component Analysis, PCA; Independent Component Analysis, ICA). A key focus of this research is the development of a novel approach for blind identification of TBM activities along the tunnel alignment, followed by a supervised settlement analysis using a modified Gaussian function. By using a consistent number of measurement points for each analysed tunnel cross section, and considering uncertainty by a Leave One Out Cross Validation, we support asymmetric ground settlement resulting from excavation of a second tunnel. The back analysed mean volume loss was found to be 0.33% (st.dev. 0.18), and parameter k was 0.44 (st.dev. 0.23). PCA/ICA techniques were employed to isolate and quantify various deformation components, such as thermal effects, noise, and actual long-term settlements. This enhances the accuracy of settlement predictions, improves the understanding of the deformation patterns and the back-estimation of soil geotechnical parameters, and demonstrates the potential of InSAR data for monitoring tunnel-induced ground deformation, and for attaining more accurate deformation predictions and better infrastructure planning. Finally, the advantages and limitations associated with the use of InSAR data are discussed.