Volcanic ash soils show peculiar engineering properties owing to their abundant moisture contents. So there are many problems in connection with land reclamation of these soils. Engineering properties of these soils were examined with emphasis on the phenomenon of softening and hardening.
The following results were obtained.
(1) The dry density of compaction soil samples were effected more by soil genesis and moisture contents than by external force. Younger tephra (Tachikawa Loam Formation) showed a low degree of dry density.
(2) The degree of softening and hardening depended on the age of tephra. Younger tephras (post-Tachikawa Loam, Tachikawa Loam and Musashino Loam Formations) showed low degrees of softening with much recovery of strength.
(3) The same tephra, containing much moisture, softened to a large degree, with less hardening. With little moisture, the degree of softening was low with more hardening. This phenomenon could be explained by the relation between soil moisture and structure.
(4) At the soil fill, the permeability fell by compaction. So it is necessary to set up surface and subsurface drainage, pipe drain and sedimentation tank.
(5) Volcanic ash soils with humic materials are not suitable for construction from the viewpoints of strength and permeability, but for the green tract of land.