In Japan, a significant volume of municipal solid waste incinerator ash (MSWIA), slag and soil are disposed in coastal landfill sites located in Tokyo and Osaka bay. Future reclamation of these final disposal sites is an important goal. Thus, it is relevant to understand the shear strength properties of the waste layers in coastal landfill sites to utilize the land after closure. However, the particle size distribution of the waste ranges from fine-grained to coarse-grained (gravel, glass, etc.) affecting the estimation of the shear strength properties of the waste sample in coastal landfill sites. There is no available research on the effects of oversize particle on the shear strength of waste samples in coastal landfill sites. Therefore, this paper presents the results of larger triaxial test (150 mm x 300 mm) for waste samples. In the large triaxial test, the pore water pressure generated is higher compared to the results obtained for small samples. The smaller values of shear strength for large samples are also related to the crushability of large particle size. Thus, the frictional angle of large specimens is slightly smaller than that of small ones. Moreover, to calculate the stability of coastal landfill sites, if the strength parameters results from large triaxial test are used, the safety factor might decrease due to the lower of friction angle of large triaxial test and it would be more precise compared to the value obtained from a small triaxial test.