Recent experiments by Finno et al. (1996) suggest that the evolution of localized zones in granular materials can be complex and is strongly influenced by local fluid flow even when conditions are globally undrained. A simple model of the drained response in terms of the shear stress and porosity dependence on strain is used to calculate the corresponding undrained response. The calculations suggest that relatively small variations in the porosity response can dramatically affect the undrained shear stress response. The effect of nonuniformity is examined by including a small layer with slightly different properties embedded in an infinite body and assuming that fluid mass exchange is proportional to differences in pore pressure.
Des resultats recents obtenus par Finno et al. (1996) suggèrent que l'evolution des zones localisees dans les milieux granulaires peut être complexe avec notamment une influence importante de I'ecoulement local du fluide interstitiel, même en conditions globalement non-drainees. On presente une etude utilisant une loi simple pour Ie comportement draine, formulee en termes de contrainte de cisaillement et de porosite en fonction de la deformation. Les resultats indiquent que de petites variations dans la relation porosite-deforrnation peuvent induire des changements importants dans la reponse en contrainte pour Ie materiau non-draine. L'eflet de la non uniformite est etudie en incluant une petite couche de proprietes mecaniques legèrement differentes dans un milieu infini, avec I'hypothèse que la masse de fluide echangee est proportionnelle à la difference de pression interstitielle.
Neuere Experimente von Finno et al. (1996) zeigen, daß die Formation lokalisierter Zonen in granularen Materialien komplex sein kann und stark von lokalen Strömungen beeinflußt wird, selbst wenn global Erhaltung der Fluessigkeitsmenge vorliegt. Ein einfaches Modell der Abhangigkeit von Schubspannung und Porösitat von der Dehnung fuer konstantem Porendruck wird verwendet, um das zugehörige Verhalten bei Erhaltung del' Fluessigkeitsmenge zu bestimmen. Die Berechnungen legen die Schlußfolgerung nahe, daß vergleichsweise geringe Anderungen der Porösitatseigenschaften das Schubspannungsverhalten bei Erhaltung der Fluessigkeitsmenge drastisch beeinflussen können. Es werden die Auswirkungen ungleichmaßiger Materialeigenschaften untersucht, indem von einer duennen Schicht mit leicht geanderten Materialeigenschaften in einem Körper unendlicher Ausdehnung ausgegangen wird. abel wird angenommen, daß der Fluessigkeitsaustausch proportional zur Differenz des Porendrucks ist.
Recent experiments on the shearing of simulated fault gouge (Marone et al., 1990; Marone and Kilgore, 1993) have indicated bulk response of gouge material depends on the thickness of the zone of active shear. This zone, which may be less thick than the gouge zone, evolves with ongoing shear and depends on the processes of shear localization within the gouge zone. These experiments also reveal a dependence of volumetric strain in the gouge (dilation or compaction) on the rate of shear. If the gouge zone is fluid saturated, dilation or compaction can after the local pore fluid pressure and the effective compressive stress and, hence, the resistance to shear. Several recent models of fault systems are based on regions of near lithostatic pore pressure in deep, hydraulically isolated zones (Sibson, 1982; Byerlee, 1990, 1993; Rice, 1992; Sleep and Blanpied, 1992). Thus, the development of localized deformation and, hence, the overall response of fault gouge material may be affected by the coupling of deformation and fluid flow during undrained conditions. Rice (1975) has studied the role of coupling of fluid flow with dilatant volume deformation on the development of localized deformation accompanying inelastic shear. For a simple shear deformation state, Rice (1975) showed that the reduction of pore pressure caused by dilatancy during undrained shearing deformation strengthens the shear response. Homogeneous undrained deformation becomes unstable, in the sense that infinitesimal spatial nonuniformities grow exponentially in time, when the conditions for localization are met in terms of the underlying drained response. For the simple shear deformation state considered by Rice (1975), the localization condition is met at the peak of the shear stress versus shear strain curve, but for arbitrary deformation states, the condition can be met before or after the peak depending on the constitutive relations for homogeneous deformation (Rudnicki and Rice, 1975). Rudnicki (1983) showed that the conclusions of Rice (1975) pertain to arbitrary deformation states for a wide class of solids.