ABSTRACT

In the south of the Island of Gran Canaria (Spain) there appear layers of swelling clays inside volcanic formations of a fonolitic type, down to depths as low as 500 m, with thicknesses of 1 to 3 m. Two tunnels (each of length 1,500 m) have been built in these zones (ignimbrites and swelling clays), work on which started in the '70s. The construction has been influenced by phenomena of expansivity and decompression. Presented below are the problems that arose, the field and indoor studies conducted and the numerical optimisation of the lining.

ZUSAMMENFASSUNG

Im Sueden der Insel Gran Canaria (Spanien) kommen im Inneren vulkanischer Formationen phonolithischen Typs in Tiefen von bis zu 500 Metern 1 bis 3 Meter dicke Blahtonschichten vor. In diesem Gelande aus Schmelztuff und Blahton begann man zu Beginn der siebziger Jahre, zwei Tunnel von jeweils 1.500 Metern Lange anzulegen. Die Bauarbeiten standen unter dem Einfluss von Dehnungs- und Dekompressionserscheinungen. Der folgende Text enthalt eine Darstellung der aufgetretenen Probleme, der durchgefuehrten Feld- und Laborstudien und der numerischen Optimierung der Tunnelauskleidung.

RESUME

Dans le sud de l'Ille du Gran Canaria (Espagne) on a detecte des couches d'argile gonflante dans l'interieur des formations de roches volcaniques fonolitiques, à la profondeur autom des 500 m, avec epaisseur de 1 à 3 m. On a perfore deux tunnels (de longueur 1.500 m chaque un) dans cettes zones (igniumbrites et argiles gonflantes), chantier qui comenza pendant les 70'. La construction on a influence pour des phenomènes d'expansivite et de decompression. Dans cette communication on presente les problèmes presentes, les etudes de chantier et theoriques realices et l'optimisation numerique developpe pour dimensionner le revêtement.

Introduction

The Trasvasur Canal forms part of a hydraulic plan for linking up various reservoirs in the south of the Island of Gran Canaria (Spain) with the Tirajana Gully. The construction of this work started in the '70s and includes the construction of two tunnels (No. 4 and No. 5), each about 3,000 m in length. From the beginning, the drilling of these two tunnels presented serious problems due to the presence of a layer of swelling clays inside the volcanic rock (fonolite) in which the excavation had to be done. This layer, from 1.5 to 3 m thick, appears pseudo-horizontally, affecting the tunnels either in the crown, or in the sidewalls, or in the floor. The tunnels initially had a horse-shoe section, with a height of 3.50 m and a width of 2.70 m.

In the first phase of construction of the tunnel (in the ´70s) signs of expansitivty and extrusion of that layer started to show themselves when the tunnel was being excavated, depending on the damage of the relative layer/tunnel position. This situation has been continuated during twenty years ago (Fig. 1). In the case of tunnel No. 4 the layer was in the upper half of the tunnel, which brought it close to the sidewalls of the tunnel, in spite of the concrete lining that was placed (without floor).

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