Effects of Cohesion and Granularity on Deformational Behavior of Anisotropic Rock

Abstract

Triaxial compression tests were run on seven orientations of three rocks with pervasive planar anisotropy-slate, phyllite, and schist-to evaluate the effects of cohesion and granularity on anisotropic strength variation and deformational mode. The rocks show a gradation in the percentage of platy minerals, and in the size, shape, and percentage of quartz grains present. Experiments were carried out at confining pressures of 500, 1,000, and 2,000 bars, and all tests were duplicated. The tests on slate were duplicated in two different apparatuses, with and without specimen end lubrication, to evaluate the effects of testing conditions on the experimental results.The strength data obtained were analyzed in terms of cohesive strength and internal friction. Cohesive strength increases with increasing granularity, but the coefficient of internal friction becomes the principal contributor to the strength variation. The coefficient of internal friction does not appear to change significantly with increased granularity; moreover, it appears to be less sensitive to material variability than does the cohesive strength. Cohesive strength becomes the more significant contributor to strength variation with increased percentage of platy minerals. Cohesion can have a pronounced effect on the mode of deformation.Curves of differential stress at failure versus anisotropy orientation are concave upward with pronounced strength minima for maximum compression at 30° to 45° to the anisotropy, depending upon the granularity and percentage of platy minerals. Maximum strength occurs in the 90° or 0° orientation, also as a function 95 on July 20, 2015 memoirs.gsapubs.org Downloaded from 96 F. A. DONATH of material characteristics. The observed strength variation among different orientations is in excellent accord with the extended variable cohesive strength theory of Jaeger (1960). Strength for a given orientation increases linearly for phyllite at pressures up to 2,000 bars; the increase is nonlinear for slate above 1,000 bars.Significant shear stress developed on the ends of specimens under certain testing conditions that, for one orientation, resulted in the rotation of the maximum principal stress from the vertical loading direction by as much as 30°. As a consequence, for these test conditions measured values of strength for this orientation can be about 25 percent lower than the true strength. Lubrication of specimen ends and a rigid loading piston are believed to give the most accurate results. CONTENTSAbstract 95

Effects of bedding and foliation on mechanical anisotropy, damage evolution and failure mode

“…8a: Donath's 1972 data on phyllite; Fig. 8b: Niandou et al 1997 data for shale), has been observed in relatively compact rocks such as slate, phyllite, and schist (Donath 1972;Nasseri et al 1997), and gneiss and amphibolite (Vernik et al 1992a), as well as a porous rock such as shale (McLamore & Gray 1967;Niandou et al 1997).…”

“…(a) Peak stress as a function of bedding angle /3 for selected data on phyllite fromDonath (1972). (b) Peak stress as a function of bedding angle/3 for selected data on shale from Niandou et al(1997).…”

On the nucleation of non-Andersonian faults along phyllosilicate-rich mylonite belts

“…4); this must therefore be considered an absolute weakening mechanism. In order to assess the level of mechanical anisotropy that may be expected due to a foliation marked by phyllosilicates, we have reviewed data published in several papers reporting on triaxial tests (Donath 1961(Donath , 1972Walsh & Brace 1964;Attewell & Sandford 1974;McCabe & Koerner 1975;Bell & Coulthard 1997;Duveau et al 1998) and direct shear tests (Collettini et al 2009). Triaxial tests were performed changing systematically the orientation of foliation with respect to the imposed s 1 axis from 08 to 908.…”

Contractional kink bands formed by stress deflection along pre-existing anisotropies? Examples from the Anglo-Brabant Deformation Belt (Belgium) and the North Dobrogea Orogen (Romania)

“…Even if a direct link between kink band geometry and palaeostress is assumed, the question can be raised whether the pre-existing local anisotropy may have influenced the (far-field) stress directions (cf. Zandvliet, 1960;Ramsay, 1962;Cobbold et al, 1971;Donath, 1972;Murphy, 1988;Konopasek et al, 2001). This is examined by means of an analysis of contractional kink bands from two slate belts in which the orientation of the main, kinked anisotropy gradually changes on a scale exceeding that of the kink bands.…”