Shear strength characteristics of a volcanic cohesive soil, Daisen Kurayoshi Pumice (DKP), which is a representative problematic soil in Japan, are complicated because of its partial saturation and inhomogeneous nature. A direct shear box testing machine was newly developed to measure the suction in both stages of consolidation and shearing. Direct shear tests were carried out under a normal stress lower than the consolidation yield stress pc. The results showed that: (1) shear strengths were different between specimens although they were tested under a particular normal stress; (2) however, the difference was rationally explained by using the variables of the suction and the normal stress measured below the base of the shear box that is fixed in the vertical direction; and (3) the relationship between the shear strength and the suction was non-linear
