Cyclic Shear Behavior of Unsaturated Volcanic Sandy Soil under Various Suction Conditions

Mudflow type failure of a gentle fill slope occurred at Dateshita in Tsukidate town during the 2003 Sanriku-Minami earthquake in Japan. The flowed fill material was pyroclastic sediment that was classified as a volcanic sandy soil with pumice. This study reports the results of cyclic shear triaxial tests of unsaturated volcanic sandy soil considering suction. Tests were conducted for specimens with different initial suctions and the same dry density under undrained conditions for both air and water. Volcanic sandy soil specimens lost their effective stress under cyclic shear loading even if the degree of saturation is about 70 percent. If the initial suction is lower than the air entry value (AEV) of the soil-water characteristic curve, the effective stress of the specimens became zero or an extremely small value, and stiffness decreased. On the other hand, stiffness did not decrease in cases where the initial suction was higher than the AEV of the soil-water characteristic curve. It is considered that the variation of air distribution pattern in specimens below under the AEV and above the AEV causes the apparent difference in cyclic behaviors.