SUMMARY:

The Clusette Road Tunnel is one of the few three-lane tunnels of Switzerland. Situated on the TIO Trunk Road, nearly midway between Neuchatel and Verrières, it has a length of 1 kilometre, base width at the road level of 12.9 m and an excavation section of about 120 m2, the longitudinal slope of the tunnel is of 6 %. The ventilation is of the semi-transversal type. The design and construction of this tunnel which was put into service in November 1975 posed a number of problems related to the nature of rock encountered, limestones and marls of the Jura range and the presence of karstic zones. The present article gives a brief review of these problems as well as of those related to the exploitation and maintenance.

RESUME:

Le tunnel de la Clusette est l'un des rares tunnels de la Suisse à trois voies de circulation. Situe sur la TIO à mi-distance entre Neuchatel et Les Verrières, il a une longueur de 1 kilomètre, une largeur au niveau de la chaussee de 12,9 m, une section d'excavation de l'ordre de 120 m2 et une pente longitudinale de 6 %. Le type de ventilation adopte est celui du semi-transversal. Les etudes et les travaux de construction de cet ouvrage, qui a ete mis en service en novembre 1975 ont pose certains problèmes lies avec la nature des formations rocheuses traversees, calcaires et marnes de la chaine du Jura et la presence des zones karstiques. Ils sont evoques brièvement dans le present article qui traite egalement de l'exploitation et de l'entretien.

ZUSAMMENFASSUNG:

Der Clusette-Tunnel ist einer der seltenen dreispurigen Verkehrstunnel der Schweiz. Auf halber Strecke der TIO zwischen Neuchatel und Les Verrières betragt er eine Lange von 1 Kilometer, eine Breite von 12,9 m ab Fahrbahnoberkante, und eine Grubenstrecke in der Grösse von 120 m2. Das Langsgefalle des Tunnels ist 6 %. Die Ventilation ist nach Halbquerlueftung konzipiert gemacht worden. Den Studien und Bauarbeiten dieses Bauwerkes das in November 1975 in Dienst gesetzt wurde, sind gewisse Probleme begegnet, deren Art durch felsige Uberquerungen verbunden waren, die vom Kalk und Mergel der Jurakette her stammen, sowie durch Anwesenheit von karstischen Schichten. Auf diese Probleme wird in diesem Abschnitt kurz hingewiesen, sowie ueber Betrieb und Unterhalt.

1. GENERALITIES

The passage of The Clusette is situated approximately midway between the frontier village of Verrières and the city of Neuchatel on the TIO trunk road which forms the shortest road between Pontarlier and Derne. Constructed about 160 years ago, it was known to be dangerous because of frequent landslides and stone falls. In fact, the valley of "Travers" ends abruptly, in the East, at the Village of Noiraigue because of the presence of the Clusette mountain. The river Areuse enters here in a narrow canyon which excludes a road connection through it with the plain. The rocky slope of the Clusette is formed by a succession of layers dipping towards the North-North West which manifest a certain instability because of the presence of soft Layer's at its base on the one hand and of the errosive action of the Areuse river at its foot on the otherhand. The behaviour of the Clusette mountain can be characterised by an alternance of short active periods, in which landslide and stone falls are frequent and new crevices in the slope are formed, followed by relatively long periods of apparent inactivity. The last important landslide occured in Autumn 1968 due to which the road was closed for a long period. After this incident, the "Grand Conseil" (Legislative Assembly of Neuchatel State) demanded the Government to give priority to the investigations and studies leading to the realisation of a definite solution to the problem of security and to improve winter servicibility (the old road had a width of 5.5 m and a maximum slope of 9.4 %). Three types of solutions were examined in parallel for carrying out a correction extending over 2.83 km of the old road:

  • new alignment by-passing the Noiraigue village by the South and connecting the Brot- Dessous village with the aid of important bridges and viaducts, correction of the old road with incorporation of galleries of protection against landslides and stone falls,

  • partial correction of the old road incorporating a tunnel of 1.0 km length to by-pass the unstable zone of the Clusette mountain.

The choice of the final solution was determined above all by the results of the geological studies done by the Institut of Geology of the University of Neuchatel.

The conclusions of these studies are summarized below:

  • the soil and rock formations situated on the right bank of the River Areuse are of the same nature as those of the left bank,

  • for 150 to 200 m length, the old road is situated in the unstable zone.

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