In Hokkaido, landslides are hitherto believed to occur not so frequently when Compared with the other' part of Japan. However, in recent years, landslides and allied phenomena were observed at many places, as human activities such as railroads, highway or other construction works go further into the undeveloped areas of this island. They occur at various geologic horizons from the so-called Paleozoic group up to the present deposits and igneous rocks from basic to acidic nature, Tha most abundant accidents occur in Miocene. marine sediments which Consist of mudstone with intercalation of tuffite that easily decompose into clay, and. also in Cretaceous marine sediments consist of mudstone with frequent thin alternation of sandstone, Serpentinite bodies which intruded into Crotaceous formations slide very often in various parts of central Hokkaido when they suffered from weathering. From the view point of geologic structure, they appear in large extent in faulted zones, particularly when accompanied by intensely faulted belt or high-throw fault. They also occur in area of homoclinal structure even in gentle dip when faced toward downslope. This is paticularly true when rivers or seawaves cut foot of the slidable area. Various other common factors in lndslide, i.e. lithologic, structural and topographic, are also recognized in Hokkaido, and most of them are combined together to Cause slide. Among them, in usual case, Climatic factors, particularly seasonal or unusual precipitation take important role in landslide.
A new tunnel was excavated parallel to the old Tanna Tunnel from which it is removed for 50m to the north. The new tunnel passes through a thick section of older Miocene, Pliocene, and older Pleistocene volcanic rocks interbedded occasionally with thin layers of marine and lacustrine sediments. The Miocene Yugasima Group, Hudo Tunnel Basalt Group, Atami Tuffs, and Ohira Andesite Group were laid down under the sea, resulting in the prevalence of autobrecciated lava flows and water-quenched basaltic and andesitic glass breccias. The Pliocene arid Pleistocene volcanic rocks were largely accumulated subaerially, and therefore the lavas are frequently interbedded with unconsolidated ash layers. Many accidents and difficulties in the engineering proce sses were encountered during the excavation of the old tunnel.The study of the geology of the new tunnel revealed that these accidents and engineering difficulties arose where solfataric clay produced from the pyroclastic rocks and glass breccias of the Yugasima Group and also from the lava of the Pliocene Ainohara Andesite Group, fault breccias such as found in the Ohira Andesite Groupand also along the Tanna Fault cutting the pyroclastic rockes of the Pleistocene Taga Volcano at the center of the tunnel, and the unconsolidated ash layers are soaked with much underground water. During the excavation of the new tunnel, no great accidents nor engineering difficulties occ ur, largely owing to the fact that a great amount of the underground water is being drained by special tunnel which has been constructed along the old tunnel. It is emphasized that more detailed geological works than have done in the past, especially the observations on the mode of emplacement and deposition and also on the nature of alteration of volcanic materials, should be done before and during the engineering programs for any tunnels traversing volcanic rocks.