Ground Improvement Operations for the Construction of the ``Via Gramsci Railway Underpass''

The project involves the construction of a natural tunnel under Via Gramsci to connect the new high capacity railway line between Bologna and Florence with the Florence Railway Station located in the city centre. The ground involved belongs to the geological Formation of the Bacino di Firenze, consisting of silty sands and sandy silts with a varying content of clay and with lithic elements varying in size from the millimetric to centimetric. It is below the water table. Located between two stretches of artificial tunnel and diaphragms consisting of columns of ground reinforced by jet-grouting, the tunnel is approximately 50 m. in length and has an overburden of just 5 m. Excavation is performed according to "ADECO-RS" principles which consists of reinforcement of the face-core using fibreglass structural elements injected with expansive mortars and improvement of the ground around the tunnel using horizontal jet-grouting technology. A field test was carried out to fine tune the operating parameters of the ground improvement works. The results obtained are shown in the paper.