Geotechnical Aspects of Underground Construction in Soft Ground –Yoo, Park, Kim & Ban (Eds) © 2014 Korean Geotechnical Society, Seoul, Korea, ISBN 978-1-138-02700-8 EPB tunnelling in mixed geological formations: A case study A. Di Mariano & A. Gens Department of Geotechnical Engineering and Geosciences, Universitat Politècnica de Catalunya, Barcelona, Spain ABSTRACT: 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 below an 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. 1 INTRODUCTION Tunnels for the new Barcelona Metro Line 9 are being constructed in a densely built urban area, close to sen- sitive structures and beneath existing Metro Lines and other services (Gens et al., 2006; Schwarz et al., 2006; Di Mariano et al., 2007; Di Mariano et al., 2009; Gens et al., 2009; Gens et al., 2011). Ground movement control has therefore always been a primary concern during both design and construction. The Line 9 (L9) tunnels connect the North side of Barcelona to the South and cross different geological formations. The variability of ground conditions has strongly influ- enced the mechanized excavation process in the L9 project, making at times difficult the optimization of machine design and operation parameters. In such cases, compromises have been necessary in order to achieve acceptable performances (Zhao et al., 2007). The paper presents a case history involving tun- nelling in difficult geological conditions below an existing Metro Line (Line 4). Tunnelling was per- formed using a 12 m diameter Earth Pressure Balance (EPB) boring machine capable of operating both in open and closed mode, depending on local ground con- ditions. In the area of this case study, geological con- ditions proved to be more difficult than expected, with the presence of very high-strength abrasive hard rocks (schists, quartzites and hornfels intrusions) within unstable loose formations below groundwater level. In those conditions, the initially prescribed closed mode of operation proved impossible to achieve in a satisfactory manner. Consequently, an open mode excavation (with a partially filled pressure chamber) had to be adopted in order to avoid interrupting the boring process especially in the area below the exist- ing Metro Line 4. The paper describes the geological conditions of the site, presents some of the ground movement observations prior to the change of the exca- vation mode and discusses the rationale that allowed the change in the prescribed tunnelling mode as well as the results obtained. 2 SITUATIONAND GROUND CONDITIONS The Guinardó area corresponds to the part of the L9 route in the vicinity of Guinardó Station, one of the fifty-two stations of L9 whose position along the L9 route is indicated in Figure 1. It belongs to Section 3 of L9 which runs through the high part of Barcelona city (indicated as Part 3 in Fig. 1). In the Guinardó area, the L9 excavation passes below an existing Metro Line, Line 4 (L4), whose service was not to be interrupted at any time (Figs. 2 and 3). The two subway tunnels run parallel, at different depths, for about 150 m after Figure 1. Full route of L9 and location of Guinardó Station. 539