INTRODUCTION

The No.1 Takaseyama Tunnel was constructed In Tochigi-Prefecture, 120 km north of Tokyo as part of the Yagan Railway Line, It is a Single-track No.1-type tunnel, and has a length of 346m and a sectional area of 30m2.Both the drilling of rock debris to the tunnel and the excavation of the tunnel itself were carried out by applying NATM method.

Actual examples of the application of NATM in the excavation of the whole tunnel length have, up to now, been few. However, drilling of the rock debris bas almost always been carried out with conventional excavation methods which use the Side driff method. This is particularly true at situations where the tunnel entrance has been covered with a deep layer of rock debris. In many cases however, use of conventional methods has resulted in problems such as landslides caused by loosening of the bedrock.

The use of the NATM method right from the drilling of the tock debr' of the tunnel entrance on the No.1 Takaseyama tunnel resulted in favourable conditions and avoided such Problems as loosening of the bedrock. it is believed that these results will greatly widen the application of the NATM method and that they moreover confirm its surperiority to other conventional methods.

the following report contains a summary of the excavation work and the results of measurements. Particular attention is given throughout the excavations involved in the drilling of the rock debris of the tunnel entrance.

CHARACTERISTICS OF TOPOGRAPHY AND GEOLOGY

The Takaseyama tunnel is located approximately 120 km north of Tokyo in the region where the Ojika River, which flows south from the border of Fukushima-Prefecture, changes its course from south to east. (See Fig. 1)

(Figure in full paper)

Geologically speaking, the region is composed of a Mesozoic sill of granitic diorite which is covered with a discordant layer of Neocene tuffaceous psephite. For approximately 150m from the north entrance to the No.1 tunnel, the principal component of the excavated material was granitic diorite, whereas the remainder was tuffaceous psephite. After large quantities of granitic diorite had been extracted from the first 40mat the north entrance of the tunnel (the area of excavation which is the subject of this report), there successively appeared later intrusions of quartz porphyry at right angles to the tunnel shaft.

The area, then, is composed of geologically complex zones including regions of dense faulting. Furthermore, it was detected at the earlier stage of the construction that the rock debris layer at the tunnel entrance might extend to a depth of some 20m along the shaft of the tunnel.

This rock debris layer composed of accumulations of weathered granite. Coarse and fine grained material, as well as gravel, are packed loosely into the crevice between the irregular agglomerations of angularly shaped rocks. The natural slope of the rock debris at the tunnel entrance was rather steep ranging 35–45 degrees.

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