The construction of NABETACHI-YAMA tunnel will make typical records about "Squeezing Rock" that usually seen at Green-Tuff Zone in Japan. we had been applying a new method, so called "NATM", to cope with high pressure and large displacements of bedrock that sometimes over 100cm, and this new method, using shot-crete and rock-bolts,(and sealing immediately)had got rather successful consequences. Through the difficult constructive conditions, we had been making two efforts; one is clearing the activity of bedrock when excavated, to examine the mechanics of displacements, and the other is finding out any indexes available to presuppose "Squeezing Rock". We will mention some about excavating data, and conclude some relations between the expanding rock and its natural properties.
(Figure in full paper)
NABETACHI-YAMA Tunnel is single-track railway tunnel (area÷30m2) located in middle of Japan, NIIGATA Prefecture~ and being constructed by Japan Railway Construction Public Corporation. The length of tunnel is 9117m and overburden is almost 250m hight at maximum. The geologic compositions are mostly formed of sandy shale 9r mudstone belonging the tertiary era, and so it is well known as Gass-producting strata in this districts, that there is a good deal of methane gass according to excavation of the tunnel usually. At the nearly middle of the tunnel, it exsists the TERADOMARI stratum,(the lbwest-oldest in this neighbor) along the GIMYO Anticline extending south to north. SHIIYA and NISIYAMA--the newer strata, are distributed symmetrically in each sides (east and west) of the axle according to the natural row itself.
The planned formation level was settled to pass through these three strata, therefore it had been foreseen some bedrock would make bad condition and in fact the progress, of works changed obviously by the layer, mentioning the details, there exists following two anticline; one is steeply folded GIMYO Anticline and another one GAMOU Anticline has rather gentle slope in SHIIYA Stratum. But we met heavy troubles at the neighborhood of both anticlines and we had to change the tunnel design from horse-shoe shape to circular section so as to cope with huge earth pressure and displacement.
The typical excavating method we used was step short bench cuttinq, S but we tried other various methods, e.g. long bench cutting, mini bench cutting, centre heading etc. and those have got different result in convergences. It was the most important concernings not only to decrease convergence but also to keep excavated face intact, and we used some subsidiary aids of forepoling, face rockbolting (or piling), 1nterim invert and so on.
Measuring the displacements of the facing (axial direction), we could confirm the effect of face rockbolting so it became to be adopted as routine work The interim invert clos with shot-crete showed striking efforts to hold the radial displacements at any time, but when the strength of rock itself diminished, the displacements of axial direction concentrated only to the facing and increased rapidly.
