Fundamental properties, especially specific surface of rock, are useful in order to understand their secondary properties. In boring samples, their fine soil particles are usually lost, therelore another fundamental property instead of specific surface is needed for site investigation. In this study, water absorption of rock, grain size distribution and compaction on volcaniclastic sedimentary soft-rock as called “Mizoguchi Tuff Brecca” were tested. The results obtained are as follows : 1. Water absorption of rock collected by boring does not change significantly t han that of test pit sample. 2. Higher value of water absorption of rock is observed in compaction as grain size distribution changes to fine side due to grain crushing. 3. As water absorption value of rock becomes higher, maximum dry density in compaction becomes lower. 4. Water absorption of rock is not only related to optimum water content but also maximum dry density in compaction, that is similar to specific surface.
