Effects of mica content on rock foliation strength

Abstract

Foliation is a planar geological structure characterized by the alignment of minerals, including mica grains. Foliated metamorphic rocks exhibit anisotropic mechanical behaviors, wherein the weakest properties are usually oriented parallel to the foliation planes. Rock mechanics research on this subject has been focused on characterizing the full anisotropic behaviors of rocks with stresses applied parallel, oblique and perpendicular to the anisotropy direction (foliation planes). However, less effort has been given to characterizing the mechanical behaviors of isolated foliation planes and describing the influences of mineralogical properties on the weaknesses associated with such geological structures. This paper presents a laboratory testing program, including direct shear tests and pull-off tests, to characterize the influences of mica content on the shear and tensile strengths of foliation planes of the biotite gneiss (which is basically composed of biotite, quartz and feldspar). Direct shear tests were performed under constant normal load and constant normal stiffness boundary conditions, wherein the foliation planes were isolated in 1 cm-thick shear zones. The mica content was estimated by a practical digital image analysis approach that was capable of quantifying the percentage of biotite area on the foliation surfaces that failed in shear and tension. The results showed that the mica (e.g., biotite) content had substantial effects on the mechanical behaviors of isolated foliation planes, especially on the peak shear and tensile strengths. A practical 3D strength envelope was proposed that included the mica content as an independent variable, which can be quantified from foliation surfaces on rock blocks and outcrops. The results provided new insights into the influences of mica-rich foliation planes on the mechanical behaviors of metamorphic rocks, which is directly applicable to rock engineering problems.