Assessing effect of mode of failure on uniaxial compressive strength of rock samples

Determining the UCS of rock samples as one of the most important parameters of rock engineering is so difficult because of occurrence of so many problems. One of the most important problems in this regard is the scattering of the results of USC tests and lack of specific criteria for choosing the data which represents the strength of intact rock. Investigating the form and the mechanism of samples failure through the USC tests shows the significant impact of micro-fractures and microscopic discontinuities on the failure mode and also on test results. According to the research results, that the maximum ratio of UCS to tensile strength and consequently maximum rate of USC occurs in the simple extension failure which indicates no impact or so weak impact of micro-fractures on the sample failure of this type unlike other types. In case of other extreme, the minimum USC ratio to tensile strength and the USC belongs to the samples with simple shearing failure and in this case the failures is totally impacted by micro-fractures. So regarding the ratio of USC to tensile strength and also considering the samples fracture mode, the real data representing the USC of intact rock could be chosen. The suitable criteria for choosing the correct data of the UCS tests is submitted in this research.