Abstract
The authors tested characteristics of brown and white hard clay developed in state of vein in tuff brecciaand tufferous andesite in Yabakei district, Oita Prefecture. The brown hard clay is natural unit weight γt=1.89 g/cm3, moisture content ω=34.9%, velocity of elastic wave Vp=2.2 km/sec, Vs= 0.8 km/sec, and unconfinedcompressive strength qu=14.5 kg/cm2, and it has characteristics similar to tertiary muddy soft rocks.But hair cracks are caused to dry it. The dried brown hard clay break down small pieces, 2 or 3 mm in dia., in a moment as it is soaked in water, and it occurs high expansive pressure.
The hard clay is consisted of montmorillonite, halloysite, feldspar and other clay minerals according to Xrayanalysis. It is considered that the hard clay is consisted of bad crystalline clay minerals, thinking of otheranalyses with X-ray analysis.
The authors consider that the hard clay is producted by hydrothermal alternation under 50°C.
