Rock brittleness is one of the most significant properties of rock having a major impact not only on the failure process of intact rock but also on the response of rock mass to rock excavation. In fact, the brittleness is a combination of rock properties including not only the uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) but also density and porosity of rocks. Due to that, the brittleness should be examined very carefully for any excavation projects, i.e., mechanized excavation, drilling and blasting. The aim of this paper is to compare the strength-based brittleness indices with both the rock brittleness index (BIo), directly obtained via punch penetration test (PPT) and also estimated via Yagiz’s approach (BI1) as a function of strengths and density of rocks. For the aim, database including more than 45 tunnel cases are used to compute common rock brittleness indices (BI2, BI3, BI4), different combination of UCS and BTS. Further, these indices are compared with both BIo and BI1 as well as each other. It is found that the BIo and BI1 have a significant relations (ranging of determination coefficients (r2) from 0.69 to 0.88 with strength-based brittleness indices commonly used in practice. Also, based on findings, several rock brittleness classifications are also revised herein.
Brittleness is a key rock characteristic; it is pertinent to predicting rock fragmentation behavior, energy consumption in cutting rock, and the selection of proper cutting geometry considerations in mechanical excavation [1]. While brittleness is typically understood as a concept, there is no universally accepted measure for this rock characteristic; often a combination of rock properties is used define brittleness rather than a single test to make a direct measurement. Brittleness is a material property describing the material’s loss of carrying capacity with a small deformation [2]. The brittleness is defined as a lack of ductility [3-4]. Ramsey [5] defined brittleness as the breakage of the internal cohesion of rocks. Obert and Duvall [6] defined brittleness as the fracture of materials at or only slightly beyond the yield stress. Hucka and Das [7] defined brittleness as follows: "with higher brittleness, the following facts are observed: low values of elongation, fracture failure, formation of fines, higher ratio of compressive to tensile strength, higher resilience, higher angle of internal friction, formation of cracks during indentation" [3-6].