Abstract:

Migmatite from the Skalka region (Czech Republic) was chosen as an experimental rock material. It has a macroscopically visible plane-parallel structure (foliation). The foliation was caused mainly by biotite grain arrangement. Four cylindrical specimens of migmatite with sub-horizontal, sub-vertical and oblique foliation were uniaxially loaded up to failure. A network of 8 broadband sensors was employed for acoustic emission monitoring and ultrasonic sounding. A grid search method with an anisotropic velocity model was used for AE hypocenter localization. The source types of successfully localized events were determined from the average first arrival amplitude. Structural anisotropy of the tested rock material caused the anisotropy of its mechanical properties (peak strength, Young's modulus) as well as a different fracturing in dependence on the angle between the axial stress and the foliation plane. The combination of tension and shear microcracking together with sliding in biotite basal planes was found to control the failure of specimens with sub-horizontal foliation. Shearing and sliding were dominant in the failure of specimens with oblique foliation. With greater dip of foliation, the role of sliding increased at the expense of shearing. Due to the favorably oriented system of microcracks already present, the shearing and splitting was at the same level during fracturing of specimens with sub-vertical foliation before nucleation began.

Introduction

The process of failure of low-porosity rocks depends on their mechanical properties and actual stress and temperature conditions. At low pressure and low temperature, brittle failure is most common. This is a progressive process requiring the initiation, growth and coalescence of cracks (Lockner, 1993). Stress strain behavior of low-porosity crystalline rocks during laboratory compression experiments is divided into four characteristic stages: crack closure, elastic region, stable crack growth and unstable crack growth which leads to brittle failure, (Brace et al., 1966; Bieniawski, 1967; Lajtai, 1974).

The fracturing process of stressed rock begins with crack initiation (sci), which for low-porosity rocks occurs approximately at 40-50% of peak strength (sp) (Cai et al., 2004; Nicksiar and Martin, 2013). After sci, dilatancy begins and stable crack growth follows up to the crack damage threshold (scd), which is approximately at 80% of sp (Cai et al., 2004). After crossing the crack damage level, there is unstable crack growth accompanied with nucleation of the fault plane (sn) at 97-100% of sp (Rao et al., 2011). The stress drop accompanied with the formation of a macro-scale shear failure plane follows after peak stress is crossed.

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