DILATANCY AND PRECURSORY BULGING ALONG INCIPIENT FRACTURE ZONES IN UNIAXIALLY COMPRESSED WESTERLY GRANITE

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. Because of the large amplitude of the strain inhomogeneity prior to failure recorded by the slit diffraction method, the faster method of recording without magnification by a motion picture camera was then tried. 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. 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.