Fracture characterization of chalk in uniaxial and triaxial tests by rock mechanics, fractographic and electromagnetic radiation methods Dov Bahat a, * , Avinoam Rabinovitch b , Vladimir Frid a a Department of Geological and Environmental Science and The Deichmann Rock Mechanics Laboratory of the Negev, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel b Physics Department and The Deichmann Rock Mechanics Laboratory of the Negev, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel Received 27 July 2000; revised 25 January 2001; accepted 30 January 2001 Abstract This study relates to fracture characterization of chalk in uniaxial and triaxial tests by combined application of rock mechanic, fracto- graphic and electromagnetic radiation EMR) methods. We found high strength results s 1 2 s 3 ) that ranged from 37.7 to 52.6 MPa. We identi®ed several `key surface morphologies' that were distinct from each other, and expanded on their characterization according to tensile and shear categories. Tensile features include plumes or striae) and stairs. Shear features include a sugary texture, ridges and grooves and steps. Fracture surfaces induced by tension and shear show distinct morphologies. Longitudinal splitting domains can occur continuously on an axial, tensile plane or adjacent parallel planes. On the other hand, shear failure into meso-fractures up to about 10 cm in length) is a process that combines a series of distinct domains on separate surfaces that slightly differ in curvature and orientation, and are inclined to the sample axis. Four fracture sequences that initiated either in tensile or in shear surfaces were distinguished in the failed samples. Results obtained by rock mechanics and by EMR methods are well correlated for microcracks and for meso-fractures and may vary over ®ve orders of magnitude in size, and this correlation is partially supported by fractography. In particular, a very good linear relationship was obtained between the ratio of the time interval to reach EMR pulse envelope maximum and the pulse frequency T 0 /v ) and the largest fractures formed. Extrapolating this result enabled us to calculate fracture sizes when the value of T 0 /v was known. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Uniaxial; Triaxial; Tensile; Shear; Fractography; Electromagnetic radiation; Domains; Sub-domains 1. Introduction Rock mechanics of chalk are of great importance in plan- ning and carrying out engineering, hydrological, environ- mental and oil production in chalk provinces throughout the world e.g. Burland, 1990). Fractography has turned out to be a useful tool in studying joints cutting chalks in outcrop scale Bahat, 1991) but was not really explored in laboratory scales. The inducement of electromagnetic radiation EMR) by brittle fracture was successfully applied to determine crack dimensions e.g. Rabinovitch et al., 1998). We aim in this study to experimentally characterize the failure of chalk under both uniaxial and triaxial loadings, by com- bining together rock mechanics, EMR and fractographic methods, with the expectation of widening the horizon of each of these individual methods. The new observations, when compared with previous experimental results, will enable a fresh evaluation of certain geological problems. 1.1. Rock mechanical tests Two experimental techniques are used to test fracture in rock. They differ by the form of the initial sample: some investigators use `virgin' samples that are tested without introducing any discontinuities to the rock e.g. Jaeger and Cook, 1979), while others prefer `pre-cracked' samples into which two initial ¯aws are introduced e.g. Bobet, 2000). The failure of virgin rock samples better reveals the behavior of the rock in nature and the in¯uence of sample boundaries on fracture initiation. On the other hand, the arti®cial pre-cracked samples help to investigate the growth, interaction and coalescence of fractures that grow from the early ¯aws. In this study, we concentrate on virgin samples, but compare our results with those obtained from samples tested by both techniques. Scott and Nielsen 1991) compared results obtained by triaxial tests on sandsone Journal of Structural Geology 23 2001) 1531±1547 0191-8141/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0191-814101)00018-9 www.elsevier.com/locate/jstrugeo * Corresponding author. Tel.: 1972-7-6472-623; fax: 1972-7-6472-997. E-mail address: bahat@bgumail.bgu.ac.il D. Bahat).