Diaphragm Walls and Ground Improvement on the Open-Cut Excavation of Bologna HS Railway Station-Paper

Paper

Pdf Summary

Bologna’s underground High-Speed railway station is a 640 m-long, 56 m-wide open-cut structure excavated to approximately 25 m below ground beside the existing central station. Its design had to limit interference with groundwater and protect the surrounding urban area.<br /><br />The excavation’s retaining system uses two innovative types of reinforced-concrete diaphragm wall. Arched “Vaults” resist ground and water pressure mainly through compression, transferring loads to T-shaped “Buttresses,” which resist them through bending and shear. Steel struts and temporary cross walls supported the system during excavation; the base slab and internal floors provided permanent support. The walls’ large, irregular cross-sections and Bologna’s variable, water-bearing soils made excavation stability a major challenge.<br /><br />Cutter Soil Mixing (CSM) was used to improve the soil around the diaphragm panels. The method mills and mixes soil with cement grout to form controlled, overlapping panels, helping prevent collapse and discontinuities during construction. Testing of approximately 260 samples found an average compressive strength of 3.02 MPa.<br /><br />An automated monitoring programme tracked geotechnical, structural and topographic conditions, including nearby buildings and active railway infrastructure. It managed roughly 3,200 instruments and more than 11 million measurements, with real-time data validation and alarms. Monitoring recorded settlement of about 4.5 cm during construction of the preliminary retaining wall, associated with tie-rod drilling. During the main excavation, the ground instead swelled slightly, reaching a maximum of about 1.5 cm near the centre of the station box. These effects were compatible with the serviceability of neighbouring buildings and railway tracks.<br /><br />The project demonstrated how specialized retaining structures, ground improvement and extensive monitoring could enable a very large urban excavation while limiting impacts on groundwater and the surrounding city.

Keywords

Bologna underground station

high-speed railway

diaphragm wall

cutter soil mixing

groundwater control

urban excavation

retaining structure

geotechnical monitoring

reinforced concrete

ground improvement