Mechanical damage of a crystalline rock having experienced ultra high deviatoric stress up to 1.7 GPa

Abstract

1. IntroductionDamage induced by high mechanical stress often leads todrastic changes in the physical and mechanical properties of rocks.In the last decades, damage mechanisms have been extensivelyinvestigated under different loading conditions [1–4]. Theconsequences of damage on rock properties, such as strengthand permeability, have also been analyzed in several papers [5,6].Special attention has been paid to the damage developed inultra high stressed rock masses in relation to different situations,such as faulting and earthquakes [7], mining at great depth [8,9]and rock masses subjected to a burst or to a blast [10].In a porous weak rock, damage is mainly related to porecollapse and compaction [11,12], whereas damage mechanisms inhard mafic rock subjected to high stresses are less clear and notwell documented. This scarcity of results is mainly due totechnical limitations for applying the ultra high stresses thatcould damage these types of rocks.The present research is part of an ongoing project to study thefeasibility of earthquake prediction. Within this project, anexperimental program has been carried out to investigate themechanical and thermal damage of hard crystalline rocks [13].This paper focuses on the mechanical damage of a gabbroinduced by an ultra high deviatoric stress. For this purpose, twotypes of triaxial tests were performed following a compressionstress path. One test with a medium confining pressure (200 MPa)was undertaken to determine the mechanical strength of therock and another one, with a high level of confining pressure(650 MPa), was performed to understand the mechanical damagethat occurred in the rock specimen.Damage to a rock specimen tested at 650 MPa confiningpressure was confirmed by subsequent measurements; the changesin the elastic wave velocity as well as in the acoustic emissionswere recorded during a uniaxial (unconfined) compressive testperformed on the pre-damaged specimen. Additionally,microscopicinvestigations were undertaken to highlight, at the crystal scale, thedeformation mechanism responsible for the observed damage.2. Experimental device and testing procedure2.1. The high-capacity Giga triaxial pressUltra-high confining pressure tests were carried out using the‘‘Giga press’’. This high-capacity triaxial vessel was originallydesigned in the University of Grenoble to test concrete specimensunder ultra high pressure. The maximum capacity of the vessel is850 MPafor the confining pressure (minor principal stress,