千葉県鴨川市嶺岡隧道の地質と膨脹性地圧について

抄録

The Mineoka highway tunnel constructed during 1970-1973, is situated in the southeastern area of Boso Peninsula, Japan.
The tunnel is 725m in length and about 80 m in maximum depth under the ground surface.
The route of tunnel intersects the so-called Mineoka Up-lift which is lithologically composed of serpentinite, basalt, sandstone and mudstone. The geological age of these rocks has been regarded as Tertiary.
All rocks encountered in this tunnel are so much sheared, crushed, foliated and intermingled with each other that in cases they can only be recognized by the different assemblages of such minerals as serpentine, chlorite, montmorillonite, talc, etc.
As a matter of fact, it was realized before the construction of the tunnel that the rocks along the tunnel route are of quite low from 1, 300-2, 600m/sec. in velocity of seismic refrection. Therefore, it was also assumed that large swelling earth pressure of rocks might be produced by the excavation of the tunnel.
Accordingly the side drift method was employed for the construction work of the Mineoka tunnel.
Actual costruction work turned up to be very difficult owing to very strong expansibility of rocks in the tunnel. As a result of this enormous earth pressure, many steel supports were bent, buckled and failed.
It is obvious that the swelling is due to less strength in mechanical property of the rocks, and is also supported from measured value of uniaxial compressive strength, which is about 2 ton/m² in average.
Expansion of the tunnel surface increased remarkably for about ten days after excavation, but it became rather constant afterwards. When the face of a pilot tunnel approached 5 to 10m to the proceeding one, the same kind of rock expansion as above mentioned took place. In this case, the swelling pressure nearly doubled what it was for a single tunnel. Such a phenomenon may be interpreted as a result of mutual interference between plastic domains of both tunnels. Also such a interference happens between up and down tunnels, and left and right, and it has a close relationship to distance between neighbouring tunnels and the rock property.