BRI Research Paper


Geotechnical Properties of Tokyo Subsoils.

Y.Ohsaki; January, 1961. 19p.


As seen from the geological map shown in Fig.1, the topography of the Tokyo Metropolis may be divided into two areas of geohistorically different characteristics,the upland and the lowland. The eastern side of the upland is distinctly bounded by a line connecting Akabane, Ueno, the Imperial Palace, Shinagawa and Omori. The ele-vation of the upland is about 20m at the eastern edge, gradually rises toward the west and becomes about 60m at Kichijoji.

The upland, furthermore, comprises two parts with slightly different elevations.One, in turn, consists of a land of triangular shape extending over the Imperial Palace, Shinjuku, Mataubara and Shinagawa, and a strip-shaped land at north-west side of Omori. These lands are grouped together under the name Yodobashi Terrace. The remaining parts of the upland are grouped as Toshima Terrace, which is of relatively lower height than Yodobashi Terrace.

Within the upland a large number of rivers are developed, most of which flow from west to east and enter the lowland. The greater part of the lowland is 1 to 3m above the sea level, gradually rising toward the margin of the upland. However, around the mouth of the Arakawa Canal the ground surface is lower than the sea level.

These configurations have resulted from the difference in geohistorical process by which the respective parts were geologically formed. In general, the lowland is covered by alluvial deposits, whereas the upland is overlain by diluvial deposition of volcanic ash origin known as Kanto-loam, except in valleys developed by rivers flowing through the upland.

The geological structure of Tokyo area is shown in Table 2. or in the cross section of Fig.2, which is drawn along a chain line in E-W direction shown in Fig.1. In Yadobashi Terrace, as will be seen from Table 1, Kanto-loam with the depth of 7 to 1Om rests directly upon Shibuya Clay which geohistonically belongs to the Tokyo Formation. In Toshima Terrace, however, Shibuya Clay and upper parts of the Tokyo

Formation were eroded away by the eastward advance of the Tame River due to the relative lowering of the sea level, and are replaced by younger river-terrace deposits. These deposits are simply called Terrace Bed, which consists of clayey portion called Itabashi Clay and gravelly one called Yamate Gravel Bed.

Alluvial deposits are widely developed covering the entire surface of the lowland and are also distributed in a complicated branching pattern, filling the valleys which are cut into the upland. The thickness varies as shown by contour lines in Fig.1; near the mouth of the Arakawa Canal the alluvial deposit attains a thickness more than 50m.

The Tokyo Formation formed in Pleistocene epoch may be divided into three parts, Upper Tokyo Formation, Tokyo Gravel Bed and Lower Tokyo Formation, in descending order .Tokyo Gravel Bed was deposited almost uniformly, although locally discontinuous, throughout the entire region of Tokyo and its vicinity. Tokyo Gravel Bed is an important formation from the standpoint of foundation engineering since it is the only and most reliable bearing base for heavy structures in the lowland, The thickness of this bed is generally 5 to 1Om, occasionally becoming as thick as 20m.

Lower Tokyo Formation is predominantly sandy and locally intercalated by clayey or silty materials. In general, the sandy part is of extremely high strength, occasionally forming a well concreted tuffaceous sand-pan, and the clayey or silty part forms a well consolidated tuffaceous hard-pan. The facies of Upper Tokyo Formation range from sandy to clayey; at the same time, its strength also varies from that of sand-pan or hard-pan to the softness of alluvial deposits.

The Tokyo Formation is underlain by Tertiary Bed of Miura Group which was formed in the closing period of the Pliocene. Little is known about the beds older than the Miura Group. It should be noted that whole sediments younger than the Miura Group are thinner to the west and thicker to the east in Tokyo area as shown in Fig.2.

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