Construction of the A20 gateway near Le Locle

Since 2009, the canton of Neuchâtel has been planning the construction of a tunnel on the A20 motorway in order to relieve traffic congestion in the town of Le Locle by bypassing it to the north. The planned tunnel, 4 km long, consists of a bi-directional monotube with a diameter of 12 m (north tube) and a neighbouring safety gallery consisting of a narrower tube (south tube). The covered trenches at the ends of the bypass tunnel are connected to the various sections that link the routes currently in service. The western part of the tunnel is being excavated using a tunnel boring machine (TBM), while the eastern side of the tunnel is being excavated using traditional shotcrete techniques. Due to the geometric conditions, the use of a TBM is not feasible here. Three distinct sectors can therefore be described for this bypass: the Le Locle-Ouest sector, the tunnel proper and the Le Locle-Est sector. (source: https: //a20-contournementdulocle.ofrou.ch/vue-d-ensemble )

The east sector is located between the east portal of the future tunnel and the Crêt-du-Locle locality. In order to make the connection between the existing motorway at Crêt-du-Locle and the future tunnel, where a viaduct is planned, various geotechnical structures are required (retaining walls, nailed walls, pile foundations). The eastern sector also contains an excavation for the north and south tubes. This consists of a nailed wall and a Berlin wall supported by anchor bolts. A few metres behind this is a wall of secant piles, reinforced by a jet-grouting block, marking the start of the North tube. It is from this wall that the tunnel will be excavated by traditional driving. Further to the south and set back from the front of the north tube, there is a second screen, also made of secant piles and reinforced with a jet-grouting block at the rear. This marks the start of the second tube, the South Tube. In order to take up the loads exerted by the earth pressure on the walls of the secant piles, it was decided to build a covered trench, which also had to ensure the stability of the portal and take up the forces resulting from the asymmetrical topography (source: https: //a20-contournementdulocle.ofrou.ch/le-tunnel ).

The particularity of this project lies in the presence of the CFF line (Le Locle - Les Brenets) located directly above the east portal. The tunnel route is such that this same line will be intersected at a depth of around seven metres by the bypass tunnel. As this line is the only rail link between Le Locle and the rest of Switzerland, it is vital that its smooth operation is not hampered by the construction of the tunnel. One of the main objectives of the planning phase was therefore to determine the impact of the tunnel on the railway structure. (source: https: //a20-contournementdulocle.ofrou.ch/le-tunnel )

The services provided by Gruner SA's geotechnical department include :

• acting as expert engineer for the east portal of the bypass tunnel.
• Verification of the static calculations for the retaining walls (secant piles, nailed walls) and the entire east portal.
• Verification of the static calculations and design of the cut-and-cover tunnel.
• using comparative calculations (3D finite elements) to assess the settlement/displacement behaviour of the new structures (portal, cut-and-cover, tunnel) and existing structures (SBB track), as well as the existing ground.

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