ABSTRACT
Groundwater control is a key issue in the construction of a conventional tunnel under water level. Authors analyze different challenges faced during detailed design and construction of Venice Tunnel, main tunnel of Venice Airport Railway Link Project in Venice (Italy), realized by top-down methodology with temporary support of sheet piles, permanent diaphragm walls and reinforced concrete top slab and a jet grouting plug under bottom slab. During detail design phase, additional geological, geotechnical and hydrogeological surveys highlighted very complex conditions, marked by heterogeneous mixed-ground conditions, high permeability values in sandy geological layers and presence of gas (CH4; SO2). Evidence has been corroborated by in-situ test results and experienced during in-situ trail tests (e.g., diaphragms, piles, jet grouting). All these aspects have been analyzed with a risk-management-based approach, starting from the definition of an updated geotechnical and hydrogeological model, in compliance with in-situ test results, and arriving at evaluation and choice of mitigation measures for both construction and long-term perspective. The result is a systematic methodology driven by risk management that will lead to outstanding benefits not only during construction but along the whole tunnel life cycle. The methodology is a simple but powerful tool to mitigate risk level up to an acceptable level and plan possible countermeasures, preserving the project planning and budget.
