A series of field and laboratory tests was conducted on an Ariake clay found in Kyushu Island of Japan. Undrained strength values obtained from KOconsolidated triaxial compression and extension, direct shear, field vane, unconfined compression and cone penetrometer tests are first presented together with the relationships among these strengths. Consolidation yielding stress determined by two types of oedometer tests are also presented and discussed, and a new procedure of determining consolidation yielding stress of this clay is proposed. Undrained strength for stability analysis determined with considerations both on strength anisotropy and strain rate effect is presented and related to various undrianed strengths and consolidation yielding stress. Finally, change of compressibility through loading in the normally consolidated state is investigated. It is revealed that the change of compression index in the normally consolidated state reaches several times and this change should be taken into account in evaluating settlement behaviour.