ABSTRACT:

As a part of a global research dealing with the validation of geomaterials' models (chalk in this case), torsion-compression tests were realized. These tests were made under drained conditions with methylic alcohol according two paths of stress: a isotropic compression and a deviatoric compression following by a torsional load. One can note the influence of these two paths of stress through parameters like the shear modulus or the angle of the failure's helix. The results obtained let us conclude to a localization of this chalk under a torsional load.

Resume:

Dans Ie cadre globale de recherches concernant la validation de modeles s'appliquant aux geornateriaux (une craie dans notre cas), des essais de torsion-compression ont ere realises. Ces essais ont ete effectues en conditions drainees so us alcohol mcthylique suivant deux chemins de sollicitations: une compression hydrostatique et une compression deviatorique suivies d'un couple de torsion. On peut alors noter l'influence de ees deux types de chargernent au travers de divers parametres tels que Ie module de cisaillement ou I'angle d'inclinaison de l'helice de rupture. Les resultats ainsi obtenus nous arneneru a conclure a une localisation de la craie pendant I'essai de torsion.

ZUSAMMENFASSUNG:

lm Rahmen des Gesamtforschung betreffend die Validierung der Modelle fur Geomaterialen wurden Torsions-Kompressions-Verusche durchgefuhrt. Diese Versuche fanden stall unter drainierten Bedingungen auf zwei Sannungsweisen: einer hydrostatischen und eincr abweichenden Kompression, gefolt von einem Drillmomenl. Mann kann dann den Einfluf dieser beiden Spannungstypen uber werschiedenc Parameter messen wie z.B. das schermodul. Die so erzielten Ergebnisse erlauben uns auf eine Lokalisierung der Kreide wahrend des Torsionsversuchs zu schliellen.

I - Introduction

Chalk is a geomaterial with a particular behaviour. It presents a larga variety of behaviours according to the level of loading applied and the loading type. The global response of chalk under classical loading paths is actually wellknowned [2], [3]. We presents, in this framework, a special test which permit to have rotation of principal axes of stress. II - Torque machine The tests were performed on hollow cylinder samples, on which, firstly an isotropic compression or a deviatoric compression are applied followed by a monotonic torque loading up to failure. To optimise the non-uniformity of stresses [2] and to take into account the current geomaterials characteristics (grain size for example). III - Basic properties of studied chalk The chalk was taken from the open quarrie stone of Haubourdin in the North of France. The geological location is Senonien. The principal physical and mechanical properties are given in Table 1. IV - Torsional loading tests IV.I - Hydrostatic tests - Comparison. In order to qualify the apparatus, we have made two hydrostatic tests. The first was made on a cylinder (36 mm for thediamater, 72 mm for the height) and the second on an hollow cylinder. These two tests were performed on two apparatus: the torsion's machine and a hydraulic press Instron. Figure 1 presents the two responses. IV.2 - Hydrostatic torsion compression tests The isotropic pressure Pc is taken before and after the Pore Collapse. They are equal to 0, 03, 08, 13 and 17 MPa. The strains measures are made with strain gages and inductive transducers. Figures 2 and 3 present the curves torque evolution vs angular deformation and volumetric deformation vs angular deformation. A notable difference is observed in the behaviour of chalk for the confining pressure before and after the pore collapse. One can note an important difference of the behaviour between tests made before and after the Pore-Collapse Po. For the tests realized with an isotropic pressure of 13 MPa, the angular deformation are more important than those made with an isotropic pressure of 08 MPa. For the tests made after the Pore-Collapse, the deviatoric stress have a small influence. The shear moduli are quasi-constant (Table 5).

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