A Combination of Artificial Ground Freezing and Grouting for the Excavation of a Large Size Tunnel Below Groundwater

Within the Metro Line 1 extension project in Napoli (Italy), the service tunnel at Toledo station was one of the most critical sections of the project. The 13m wide, 17m high, 40m long tunnel is located downtown under a deeply urbanized area. It was excavated starting from an access shaft and from this tunnel four platform tunnels are under excavation. The geology of the site is characterized by volcanic soils: loose pozzolana silty sand and tuff. The underground works were carried out under a hydrostatic head of 27 m. In order to allow a safe and substantially dry excavation, soil/rock improvements were carried out by a combination of two different methods: cement and chemical grouting at the sides and invert (in tuff) and Artificial Ground Freezing (AGF) at the crown (in pozzolana). The treatments were carried out both from a drift located approximately 10 m above the crown and from the access shaft. The grouting process was controlled and monitored by a computerized system. To check the development and the efficacy of the freezing process, suitable sensors and automatic data acquisition and recording systems were installed. The variation of the temperature within the soil and the cooling fluids were monitored during the entire freezing operation