Tunnels for the new Barcelona Metro Line 9 are being constructed in a densely built urban area, close to sensitive structures. The paper presents a case history involving tunnelling in difficult mixed geological conditions belowan existing Metro Line (Line 4).Tunnelling was performed with a dual EPB boring machine and the closed mode of operation was prescribed beneath all sensitive structures. Despite this original prescription, an open mode excavation (with a partially filled pressure chamber) had to be adopted to avoid the interruption of the boring process in the area below the existing Metro Line 4. The paper describes the geological conditions of the site, presents some of the ground movement observations prior to the change of excavation mode and discusses the rationale that allowed the change in the prescribed tunnelling mode as well as the results obtained. Observations of displacements during the excavation in open mode confirmed that no soil inrush into the head chamber occurred and that in addition, with the new excavation procedure, ground movements were reduced significantly.