"SHIRASU" are the volcanic deposits, and are distributed widely throughout Kagoshima prefecture in Japan. The collapse of "SHIRASU" was caused by tensile strength from the results of investigation on in-situ shearing tests and in-situ compression tests. A standard triaxial compression test is testing method for determination of shearing resistance. Triaxial compression tests by acting partially longitudinal loads carried out in this report. The triaxial compression tests were analyzed in regard to mechanical balance between the wedge which shaped under longitudinal loads and the circumference of the wedge. The analysis was based on the criterion of tensile strength and shearing one, and showed good agreement with the experimenal results.