Jacked piles as mitigation measure of surface settlements due to tunnelling in clay

Tunnelling in highly urbanized areas very often leads to ground movements and subsequent po- tential risks of damage to adjacent structures and infrastructures. Intensive soil strengthening interventions ahead of and on the excavation face are usually carried out, but they are not always sufficient to keep soil movements within acceptable limits. The paper describes a well documented case-history of a shallow tunnel in clay in which, after the execution of a preliminary support by a pipe-roofing technique, jacked piles were eventually installed in order to stiffen the foundation soil under the base of the ribs and to further reduce tun- nel-induced surface settlements. The reported case-history is a novel application of jacked piles as settlement reducers. The pile installation sequence during the tunnel excavation is described herein. A set of installed piles were provided with extensometers: records of monitoring data show the evolution of the load transfer mechanism from the tunnel structure to the piled foundation during excavation as function of the subsequent construction steps.