A Comprehensive Approach for Conventional Tunneling Optimization in Urban Areas

Conventional tunnelling, CT, in densely populated urban areas requires ensuring that both serviceability and failure limit states are satisfied throughout construction, to avoid excessive ground deformations and catastrophic failures, while maintaining optimum support and excavation lengths. To achieve both goals, continuous monitoring of the tunnel-ground system behavior and adaptation of the tunneling process through calibrated numerical models becomes critical. This paper documents the field performance and construction optimization of a 4.5 km long tunnel excavated by CT in the stiff soils of the northwestern Mexico City region. A five-step monitoring and back-analyses procedure is introduced for risk reduction and optimization of the tunneling process during CT. The monitoring information, adopted support types, and excavation lengths demonstrate that construction times can be shortened through the proposed approach, enhancing construction processes with the corresponding cost reduction. Both three-dimensional numerical models and geotechnical instrumentation, including convergences, surface topographic references, extensometers, and pressure cells, were implemented throughout construction. The numerical models were continuously calibrated against field measurements to increase their predictive capability, accounting for actual subsoil conditions, tunnel geometries, and construction procedures. From the results gathered here, the benefits of using this integral approach to ensure good tunnel performance during excavation are established, in particular when the tunnel is excavated below densely populated areas in brittle cemented fine-grained soils