ABSTRACT:

Following an overview of the project and methodology for the construction of the Serrières Tunnel on Highway N° 5 in Neuchâtel (Switzerland), the authors will detail a few chosen aspects to illustrate the great complexity of major infrastructure works in urban areas. Descriptions will be provided in particular of the construction constraints arising from the very thin cover under the built environment and from the structure's position, which is partly below the level of the lake aquifer.

1 INTRODUCTION
1.1 Serrières tunnel

The Serrières Tunnel is located on Highway No 5 in the Canton of Neuchâtel (Switzerland). It represents the last section to be built to complete the underground motorway crossing of the City of Neuchâtel. The structure includes two motorway tubes, about 1,100meach, of which 800munderground and 150m in cut-and-cover at each tunnel mouth.

1.2 Specific framework

The construction presents several challenges as it faces many major constraints. In order to integrate the work into the existing context, the chosen route was subject to advanced optimisation. First, the dense urban environment imposes constraints with respect to noise and vibration levels for both open and underground structures. The adjacent tram line and extremely busy city access road involve setting up relatively complex traffic diversion measures. The proximity of the lake and its surrounding leisure areas adds to the external constraints. The longitudinal section of the chosen tunnel passes not deep under the built environment and is partly located under the aquifer level of the Lake of Neuchâtel close by.

2 GEOLOGY

The rock mass through which the underground tunnel is being built is made up of Cretacean limestone of the Upper Hauterivian complex. The relatively massive rock shows strata of varying thickness with monotone subhorizontal stratifications.

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