A STUDY ON THE ORIGIN OF SCALE EFFECT OF ROCK STRENGTH BASED ON MULTI-SCALE SHALE BRAZILIAN SPLITTING TEST¹⁾

In order to explore the scale effect of the shale tensile strength and the origin of the scale effect of the rock strength in general, the outcrop of the continental shale of Chang 7 of Yanchang Formation in Ordos Basin is collected to make rock samples of 50 mm in diameter and 15.32 mm, 20.39 mm, 25.46 mm and 30.28 mm in length for the Brazilian splitting test. On the basis of analyzing the rock tensile strength, a new understanding of the origin of the scale effect is proposed. It is shown that the maximum load on the shale and the number of rupture surfaces increase with the increase of the sample size, while the tensile strength decreases with the increase of the sample size, and the correlation between the fracture time and the sample size is poor. The heterogeneity and the end friction effect are the main causes of the scale effect of the rock strength. The obvious scale effect of the tensile strength in the Brazilian shale splitting test results indicates that the scale effect of the rock strength is not entirely rooted in the end friction between the specimen and the indenter. The scale effect of the rock strength is formed by the interaction of the heterogeneity and the end friction effect. For small scale rock samples, the end friction effect plays the dominant role, and for large scale rock samples, the heterogeneity plays the dominant role. For various types of rocks, the size of rock samples may be neither "small" nor "large". The study of the scale effect and its origin for the rock strength is helpful to improve the accuracy of the rock strength prediction and for revealing the rock failure mechanism.