Relationships between direct and indirect tensile strength of weak rock material

The tensile strength has an important place in rock mechanics applications such as failure criteria, stability of underground excavations, excavation and failure mechanics. However, it is used less considering the compressive strength in geotechnical designs. This parameter can be determined using direct and indirect methods. Despite the fact that the direct tensile test gives more reliable results, the difficulty of the testing requirements limits its extensive usage. The method is not widely used due to the difficulties like; difficulty of standard size sample preparation, the existing discontinuities in the sample, the requirement that the upper and lower surfaces of the sample should exactly be parallel to each other during the test, the sensitivity of the heads attached to the surface of the sample, the curing time of the epoxy and absence of pulling machine etc. For these reasons, indirect methods such as Brazilian, three- and four-point bending tensile and Loung tests, which are simpler and easier to carry out, are widely preferred. However, previous studies focused on the tensile strength showed that the tensile strength values determined using the indirect methods do not represent the true tensile strength of the rock material. Accordingly, it has significant importance to reveal the relations between the direct and indirect tensile strengths of the rock material. In this study, the relationship between the direct and Brazilian tensile strengths of weak rock material was investigated. For this purpose, the direct and Brazilian tensile tests were carried out on a total of 152 core samples collected from different lithologies, including andesites, granodiorites, tuffs, marls and clayey limestones. The results indicated that the direct tensile strength values of the rock material in general are lower than the Brazilian tensile strength. Likewise, the direct and indirect tensile strength values were evaluated by simple regression analysis and the relationship between the direct and Brazilian tensile strengths of the rock material was determined. According to the regression analysis results, there is a very strong positive correlation (r=0.89) between these two tensile strengths. With the equation expressing this correlation, the direct tensile strength of the rock material can be estimated by using the Brazilian tensile strength of the rock material with a tensile strength lower than about 12 MPa.