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.
It is necessary for measurment of moisture content and porosity to define the dry condition of rock samples. Nevertheless various coditions of drying are actually adopted by many peoples. In this paper, the weight of samples, velocities of elastic waves and uniaxial complessive strength of rock specimens were measured at seven different condition of drying which are 50, 75, 105, 125, 150°C constant temperature for 48 hours, kept in desicator contained silicagel for a month, and dried naturall in the room. Sandstone and andestite have a different tendency of change in measured values. Results obtained in this paper are summarized as follows: (1) In regard to the change of weight, a remarkable effect of drying is showed at 50°C constant temperature dring for andesite which have higher porosity, and showed barely a constant weightat drying over 105°C for sandstone which has lower porosity. The drying in desicator contained silicagel has a similar effect at 50°C oven drying for andesite and 75°C for sandstone. (2) Dillatational waves of specimen show a maximum velocity at 50, 75°C drying forandesite, at 75, 105°C and silicagel drying for sandstone. Increase of the velocities for both rocks become dull at drying over 125°C Velocities of longitudinal vibration show a similar tendency of the dilatational waves for andesite, and show a maximum value at 105°C and silicagel drying. (3) Compressive strength of both rocks shows an increase in the region from room temperature to 150°C drying. Compressive strength for andesite fairly increases at 50°C drying, and that for sandstone shows a remarkable increment at 105°C drying. Compressive strength at silicagel drying shows a similar tendency of velocities.
The Hino diorite mass, intruded into the Paleozoic rocks and Mesozoic volcanic rocks, are exposed within restricted area of western part of Tottori Prefecture. This dioritic rock is chiefly composed of plagioclase, hornblende, biotite and quartz together with few magnetite, sphane, orthopyroxene and epidote. In the earlier stage of the weathering, biotite is rapidly altered to secondary minerals such as montmorillonitevermiculite mixed layer mineral, biotite-montmorillonite mixed layer mineral and halloysite. So that, at this stage, the change of chemical composition of biotite differs markedly from that of the fresh rock. Approaching toward the later stage, biotite is altered to biotite-vermiculite mixed layer mineral and halloysite. But, at this process, the change of chemical composition of biotite is little. On the other hand, as weathering proceeds, plagioclase and hornblende are gradually elliminated and halloysite appear in stead. The change of chemical composition of rock in the weathering process is controlled by biotite at earlier stage and by plagioclase at middle stage and by bornblende at later stage.