Effect of soil improvement on ground movements induced by conventional tunnelling

Line C of Rome underground will cross the city from southeast to northwest, with a total length of about 25 km, passing through the historical city centre. This is a difficult environment due to many archaeological finds and pre-existing buildings of great historical value. Along stretch T3 of the line two conventional tunnels connect the TBMs launching pit to the new San Giovanni station. They run for a length of 140 m at a depth of about 25 m and reach the station passing at a short distance from the ancient Aurelian Walls (3rd century CE). Excavation of these tunnels was carried out following a three-stage procedure: (i) excavation of two small diameter tunnels (D = 3 m) using a mini slurry shield machine; (ii) soil improvement via cement grouting using tubes à manchettes installed radially from the mini-tunnels; and (iii) conventional excavation of the two running tunnels in the improved soil. An extensive monitoring system was set to control ground movements induced throughout the excavation process. This paper presents the displacement measured at the ground surface during the construction activities, highlighting the effects induced by grouting. The effectiveness of a protective barrier, made by a line of piles, in reducing the movements induced by tunnelling in the Aurelian Walls is also assessed. A 2D FE back-analysis is finally presented, showing that a satisfactory description of the observed behaviour can only be obtained by simulating the volume changes induced by the cement grouting.