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ABSTRACT: The paper deals with a TBM excavation in the Hong Kong granite; the Tolo Effluent Export Tunnel which is 3.56m in diameter and 7.4 km long. The performance of the machine, which is a double shield tunnel borer, is examined with special attention to the influence of the lithotype strength and the quality of the rock mass. A comparison with the results obtained in another tunnel (Hong Kong Cable Tunnel), which was excavated with an open borer at about the same time is presented.
RESUME: On examine la performance d'un tunnelier à double bouclier pour I'excavation dans Ie granite de Hong Kong du Tunnel "Tolo Effluent Export" qui a un diamètre de 3,56m et 7,4 km de longueur, en mettant I'accent en particuIier sur l'influence de la resistance du lithotype et la qualite du rocher. On presente aussi les resultats obtenus dans un autre tunnel (Hong Kong Cable Tunnel) où les activites d'excavation ont ete menes en meme temps mais en utilisant un tunnelier overt.
ZUSAMMENFASSUNG: Eine Bohrung im Hong-Kongischen Granit wird beschrieben, die fuer den Bau des Tolo Effluent Export Tunnel (Durchmesser, 3,56 m; Laenge, 7,4 km) mittels eines Doppelschildtunnelbohrer (TBM) durchgefuehrt wurde. Die Autoren haben die Betriebsleistung des Fraesers untersucht und besonders die Einwirkungen von des Festigkeit des Lithotyps und von der Qualitat der Felsmasse bestimmt. Ein anderer Tunnel (Hong Kong Cable Tunnel) wurde gleichzeitig mittels eines offenen TBM gebaut, und die davon erzielten Ergebnisse werden verglichen.
1. INTRODUCTION The Tolo Effluent Export Scheme was designed to reduce the pollution levels of the Tolo Harbour by diverting the sewage effluent of the Sha Tin and Tai Po treatment plants toward Victoria Harbour which has a greater capacity in accommodating sewage effluents. The scheme includes a 7.4 km long effluent tunnel, running from the Sha Tin portal to an 18 m deep shaft at Diamond HilI(fig. 1). Constraints on tunnelling included a high pressure aqueduct tunnel located at only 3.5 m below the Tolo Effluent Export (T.E.E.) tunnel with a limitation on blasting vibrations of 13 mm/s; besides, the T.E.E. tunnel runs 12 m below the twin tunnels of the Tate Cairns Highway. Tenderers were allowed to adopt either a TBM option or a drill-and-blast excavation method; for the latter option an intermediate access point was available. It is to be noted that there had been no previous experience of mechanised excavation in the Hong Kong territory. The joint venture between the Italian Contractors Vianini and Seli produced the lower bid and were awarded the contract in 1990. The excavation of the tunnel was completed in 1992; the technical results of the project are discussed in this paper. Roughly at the same time, another tunnel, the 5.4 km long Hong Kong Cable Tunnel (HKC), was being mechanically excavated by Nishimatsu Constructor Company; it is therefore interesting to present a comparison between the performances of the two machines.
2. GEOLOGICAL SETTING A comprehensive site investigation was carried out during the design stage to characterise the rock mass; 20 exploration boreholes were driven, most of which near the portals as well as in a zone where intermediate access had been planned should a drill and blast excavation method be adopted; seven boreholes were designed to cover the overall length of the tunnel. A 300 m long sub-horizontal borehole was driven to explore the zone at the Sha Tin tunnel portal. The tunnel runs exclusively through intrusive igneous rocks of Mesozoic age which are crossed by some dykes of hypoabyssal rocks.
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