CONSTRUCTION OF UNDERGROUND CAVERNS FOR PETROLEUM STORAGE IN OROGENICAREAS - GEOLOGICAL STABILITY

Japan is located at the boundary between three continental and oceanic plates, and there is a great variety and complexity of geological fea tures as a result of tectonic activities. The possibility of excavatin g large underground tunnels in the seemingly ''fragile'' rocks in orog enic areas was considered as a national project for the underground st orage of petroleum. One hundred and seventy sites in 57 areas were sel ected based on a comprehensive survey over the whole country. After a successful experiment with a test tunnel, 112 m long and 250 m2 in cro ss-section area, excavated in Cretaceous granite, three sites in grani tes and pyroclastic rocks were selected for construction. The largest of these storage tunnels is 550 m in length and 20 x 30 m in cross-sec tion. Before construction, a detailed survey of the subsurface geology was carried out, taking into consideration the structural geology, th e mechanical properties of the rocks and the hydrogeology. It was conc luded that even in active orogenic areas, there are many suitable site s for large underground excavations and that these can be detected by detailed, planned surveys.