CAUSE OF THE ACCIDENTS DUE TO OXYGEN DEFICIENCY IN UNDERGROUND WORKS IN TOKYO AREA

Abstract

About 55 dead cases due to oxygen deficiency have been reported in underground construction works in Japan from 1960 to 1968. In Tokyo area, where dead cases amount to 20, these accidents mostly occurred in pneumatic caisson arriving at subsurface Pleistocene gravels. The cause of oxygen deficiency was studied geologically, mineral-ogically, and geochemically.Accidents have occurred in gravel layers in central part and in sand layer in southern part of Tokyo. The former belongs to lower gravel layer of the Upper Tokyo Forma-tion and distributes around 20 m under ground surface, while the latter belongs to the Upper Tokyo Formation and extends about 10 m below the surface. These gravel and sand layers are covered with impermeable silt or clayey sediments. Water table and pore-water pressure in these layers have decreased remarkably by excessive pumping for industrial use year by year. The excavation for construction has to arrive at the gravel layer since the gravel layer is selected as the strata on which the basements of all constructions are established. Under these conditions, the compressed air for pneumatic foundation can pass easily through these gravel layers, and oxygen in it is consumed to oxidize the ferrous ion in pore water to ferric ion. Although ferrous ion content of pore water in gravel layers is small in the absolute value, total volume of iron in them is remarkably large as compared with the volume of the compress-ed air sent artificially, because gravel layers distribute widely in the ground.Accidents of this type have happened, or will possibly happen, in other Japanese populous cities along the coastal area, because they have similar geological environ-ments and are suffering from lowering of underound water table.