Dilatancy and precursory bulging along incipient fracture zones in uniaxially compressed westerly granite

Abstract

The existence of precursory bulge along an incipient fracture zone in a uniaxially compressed Westerly granite sample has been investigated by two optical methods. The first method is the method of slit diffraction. The cross section of a cylindrical rock sample is monitored by bringing two straightedges next to the rock sample to form two slits (each slit being formed between one straightedge and one side of the rock sample) and by illuminating alternately the two slits with a collimated laser beam. The FraunhoFer diffraction pattern is recorded on film in the direction perpendicular to the straightedge and can be interpreted as the absolute value squared of the one-dimensional spatial Fourier transform of the slit under certain conditions, thereby providing a simple method of magnification of the rock surface geometry. The conditions under which the Fraunhofer diffraction pattern can be interpreted as the absolute value squared of a one-dimensional Fourier transform are related to the radius of the rock sample, width of the slit, position of the recording film plane, and nature of deformation of the rock surface and are presented in this paper. The films are digitized by a microdensitometer. The data are analyzed by digital filtering and interpolation techniques to give a strain resolution of 10-•. During a test with the slit diffraction method, strain inhomogeneities in terms of local bulges indicative of incipient failure zones were found to develop at -92% of the uniaxial compressive strength, and their propagation is traced at 2.66-s intervals until failure. Local strains in the incipient failure zones are of the order of 10-• before failure takes place. Because of the large amplitude of the strain inhomogeneity prior to failure recorded by the slit diffraction method, we then tried the faster method of recording without magnification by a motion picture camera. In the second test a precursory bulge in the middle of the sample first appeared at -3.75 s prior to failure at a load of >99.7% of the uniaxial compressive strength. The bulge developed rapidly in successive frames until eventually a failure plane passed through this sharp bulge. The results from both tests demonstrate the formation of a concentrated weak zone as a result of the interaction and coalescence among the microcracks in the final stage of the test, which then develop into fracture zones. The bulging is the result of accentuated deformation in the weak zone because of its reduced deformation moduli. It is considered that the local bulge and orientation of the fracture zones in the first test were controlled by the stress concentration at the sample-load block interface, whereas those in the second test were controlled mainly by the inhomogeneity in the material properties within the sample. The precursor times of both tests do not fit into the empirical relationship between precursor time and fault dimension as derived from earthquakes and mine rock bursts. The precursor times of these tests are too long by 3 orders of magnitude in compa...

The Soviet-American exchange in earthquake prediction

“…They identified a region ahead of a propagating fault (macrocrack) that is composed of many smaller and often differently oriented cracks. The region was observed to be very localized, a finding that is in agreement with our holographic observations on Westerly Granite [Spetzler and Martin, 1974;Spetzler et al, 1977] and the slit diffraction observations of Liu and Livanos [1976], also on Westerly Granite. In addition, the detailed crack studies ofHallbauer et al [1973] also indicate that crack development and hence dilatancy are very localized.…”

Fracturing process of granite inferred from measurements of spatial and temporal variations in velocity during triaxial deformations

“…Incipient growth of damaged regions due to extreme concentrations of cracks near the surface around peak stress was found in granite during testing in a stiff testing machine [Wawersik and Fairhurst, 1970]. Inhomogeneous deformation and local bulges of the surface just before failure have been observed in Westerly granite using some optical methods, for example, slit diffraction [Liu, 1976]…”

“…To better understand the fundamental problems of earthquake occurrence, we have clarified the localization behavior during the deformation of rocks. Local strain changes on the surface of samples have been observed by both laser holography and optical slit diffraction to assess in detail the behavior before failure [Spetzler and Martin, 1974;Liu, 1976;Sobolev et al, 1978;Spetzler et al, 1981;Kurita et al, 1983]. The determination of source locations of acoustic emission has been considered a useful method for detecting concentrations of microcracks in rocks [Mogi, 1968;Scholz, 1968;Byerlee, 1977a, b, 1980;Yanagidani et al, 1985;Nishizawa et al, 1985].…”

Earthquake Prediction-Related Research at the Seismological Laboratory, California Institute of Technology, 1974-1976

“…The existence of a precursory bulge along an incipient fracture zone in a uniaxially compressed Westerly Granite sample has been investigated by two optical methods, slit diffraction and direct observation on film (LIU and LIVANOS, 1976). Both tests demonstrate the formation of bulging as a result of accentuated deformation in a pre-fracture weak zone with reduced deformation moduli.…”