Triaxial strengths are difficult to obtain when the rock is very heterogeneous with respect to particle size and particle lithology and the samples are difficult to obtain and prepare. A fault breccia with clasts weaker than the matrix was investigated. It was found that its strength was related to the proportion of the blocks and the size of the specimen. The volumetric block proportion and texture coefficient were evaluated by core scanning and image processing. Triaxial compressive strength tests were conducted on different sized cores and different proportions of blocks. The most accurate estimations of the lower bound strength were obtained using larger diameter cores and samples with a higher volumetric block proportion.