Abstract

Compressive strength of hard rocks is one of the most widely used parameters in rock engineering applications such as comminution and rock breakage. Compressive strength of hard rocks is controlled, among other factors (such as shape and size of the tested specimens, strain rate, porosity, grain size, etc.), by microcrack population, whether it be natural (which is present in all rocks and comes from their geological history) or loading induced (which is caused for example by excavation activities). Therefore, evaluation of microcracks influence on compressive strength of rock is of fundamental importance.

The aim of this study is to evaluate numerically the effect of natural microcrack populations on the compressive strength of heterogeneous hard rock specimens. Heterogeneity is taken into account by representing rock mineral mesostructure as random clusters of polygonal cells and by assigning to each cell different mechanical properties. The rock constitutive model employs a (strong) embedded discontinuity finite element formulation to describe cracks in rock material. Crack initiation follows Rankine criterion. According to this, a crack is introduced in the element when the first principal stress exceeds the tensile strength. In polycrystalline rocks, such as granite, fracture is generally of mode I type. Tensile microcracks induced by tensile stresses grow and coalesce to form macrocracks, leading eventually to axial splitting failure mode of the specimen.

The model performance is tested first in numerical simulations of uniaxial compression on idealized, numerical, heterogeneous granite-like rock specimens having different percentages of initial microcracks. The results are then compared to the ones coming from intact specimens (without initial microcracks) where only heterogeneity is considered.

1 Introduction

Conventional breakage of rocks, like crushing and grinding, requires high energy amounts. Therefore, other breakage methods based on the use of chemical or physical agents to replace and/or assist the mechanical breakage are being searched at present time.

Thermal shock pre-treatment is a method for weakening of mechanical properties of rocks. It consists of applying an external heat flux at the rock surface, up to high temperatures (Wang and Konietzky 2019). Rapid application of high intensity heat flux induces a high thermal gradient, which as a consequence causes the build-up of a compressive stresses in a thin layer of rock in proximity to the surface. This can generate new cracks, due to the mismatch in thermal and elastic properties of the composing minerals. It can also advance further the already existing cracks that are favourably oriented, e.g. by the wing crack mechanism. As a result, depending on flux intensity and heating time, cracks may take place inside the rock body or locally, close to the heated surface (spallation phenomenon).

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