ABSTRACT: This paper deals with an experimental analysis of natural ductile shear zones (NDSZ) in a granodiorite. Attempt is made to quantify implications of NDSZ on behaviour of rocks samples and conditions of appearance of mechanical strain localization. Samples were cut outside and inside the NDSZ (respectively undeformed material UDM and foliated material FM) and in an intermediate zone (intermediate material, IM). Compressive loadings of samples permit to show anisotropy of response (related to the presence of NDSZ) which increases progressively from UDM to FM. Macroscopic observations on mechanical shear bands (MSB) show, in the different materials, en echelon structure which is highly influenced by the foliation induced by natural deformations. At the microscopic scale it is observed that this echelon structure is form by microbands (structure of axial microcracks).
RESUME Cet article concerne une etude experimentale en laboratoire des bandes de cisaillement ductiles naturelles dans un granitoide. On s'est, dans un premier temps, interesse aux consequences de celles-ci sur la reponse mecanique et les conditions d'apparition des bandes mecaniques dans des echantillons prelevees dans et en dehors des zones de cisaillement naturelles. Les essais de compression isotrope montrent une anisotropie du comportement mecanique qui augmente progressivement lorsque l'on passe du materiau non deforme (UDM) au materiau folie (FM) par la deformation naturelle. Les observations macroscopiques, realisees sur l'ensemble des ehantillons testes, mettent en evidence (dans les differents materiaux) des structures en echelon fortement influencees par la foliation. A l'echelle microscopique on montre que ces structures sont consiituees de microbandes (structures de microfissures axiales).
ZUSAMMENFASSUNG: Diese Arbeit befasst sich mit einer experimentellen Analyse von natuerlich duktilen Sherzonen in eminent granodiorite. Ein Vorschlag ward rematch um die Einwirkung von NDSZ auf das Verhalten Von Felsproben und Bedingungen fuer die Erscheinung von mechanischen Dehnungslokalisierungen quantifizieren. Proben wurden von der Außenflache ales ouch vom Inner der NDSZ entombment (unverformtes Material UDM und Folienmaterial FM) und in einer Übergangszone, uebergangsmaterial (lag), Druekversuche an Proben zelgen progressives anisotropes Verhalten (betoken auf die Anwesenheit von NDSZ) beim Übergang von UDM in FM. Makroskopische geometrisehe Beobachtungen von mechanischen Scherbandern (MSB) Eigen in versehiedenen Werkstoffen eminent echelon welche stark von der Folien beinflußt ward, die durch natuerliche Verformiungen induziert werden. Auf der mikroskopischen Ebene Zeist sich, day diese Echelon- Refuge Dutch Mikrobander entstehen.
1 INTRODUCTION As first described by Teall (1885), shear zones are planar or curvilinear zones of high deformation which are long relative to their width', surrounded by under formed rocks. In geological area, the first comprehensive study of these zones (Ramsay and Graham, 1970) includes a description of the main types (from brittle to ductile). Since them, many investigators have studied shear zones on various geological scales (Carreras et aI., 1980; Simpson, 1983, Ingles, 1985; Ramsay and. Huber, 1987; Lamouroux et aI., 1991). In civil and rock engineering strain localization in bands is a common feature and has focused attention of many researchers in the last decades. Several experimental works have been done on strain localization in soil and rock mechanics (Wong, 1982; Desrues, 1984; Vardoulakis and Graf, 1985; Santarelli, 1991; Ord et aI., 1991; Viggiani et aI, 1993, Haied et al, 1993; Wawersik et aI., 1993 etc …). In spite of these interesting studies, few attempts have been made to join experiments on natural shear bands and those performed at laboratory scale by rock mechanicians (Evans and Wong. 1985). This paper is concerned by an experimental analysis of natural ductile zones (NDSZ) under mechanical loadings.