The characteristics of deformation of elastic sedimentary rocks are closely related to the porosity which is reduced during the process of geological compaction. Various kinds of Tertiary sedimentary rocks in Japan have been deformed experimentally under confining pressures. Porous sedimentary rocks show the transition from brittle behavior to ductile behavior at low confining pressures. The critical pressure for transition from normal ductile behavior to visco-ductile behavior is related to the effective pressure which the rock suffered during geological compaction. Natural and artificial over-pressure may cause flow-like deformation of soft sedimentary rocks.
Les caracteristiques de la deformation des roches sedimentaires clastiques sont en relation avec la porosite qui se diminue dans le processus de la compaction geologique. Des diverses roches sedimentaires tertiaires au Japon ont ete deformees en experimentations triaxiales. Les roches sedimentaires poreuses font la transition de la condition fragile à la deformation ductile à la basse pression laterale. La pression critique pour la transition de la condition ductile normale à la condition visco-ductile est en relation avec la pression efficace que la roche a souffert dans Ie processus de la compaction geologique. La surpression naturelle et artificielle peut causer la deformation fluide des roches sedimentaires tendres.
Die Verformungseigenshaften dersedimentaren Truemmergesteine bezieht sich auf die Porositat, die sich unter dem geologischen Verdichtungsprozess vermindern. Die Verformung der verschiedenen Arten von tertiaren Sedimentgesteinen wurde unter triaxialem Druck untersucht. Poröse Sedimentgesteine zeigen die Umwandlung vom fragilen bis zurndehnbaren Verhalten unter niedrigem Seitendruck. Der kritische Druck fuer die Umwandlung vom normalen dehnbaren bis zum viskodehnbaren Verhalten bezieht sich auf den wirksamen Druck, den die Gesteine wahrend der gelogischen Verdichtung erfahren haben. Es wird angenornrnen,dass durch natuerlicher und kuenstlicher Überdruck die fluessigen Verformung der weichen Sedimentgesteine verursacht wird.
A great variety of mechanical properties of elastic sedimentary rocks multiply the difficulty of estimation of mechanical stability of rock mass. Often, it is impossible to test every variety of rocks which participate in deformation. During the process of compaction, sedimentary rocks continuously transform from unconsolidated sediments into hard rocks. The overburden pressure and tectonic stress gradually reduce the porosity of sedimentary rocks. One of the authors reported the relation between porosity and strength of clastic sedimentary rocks(Hoshino, 1974). The chemical composition, mineral assembly and texture of rocks also change during the process of compaction. In case of porous sedimentary rocks, the porosity is the most important factor which indicates the degree of compaction and which controls the characteristics of deformation. In this paper, characteristics of deformation of sedimentary rocks under confining pressure are discussed in terms of geological compaction. The apparatus and method of experiments have been described in Hoshino et al.(L972).
The porosity of rock is gradually reduced by compaction. The older rock is generally less porous than younger rocks under similar sedimentary and tectonic environments.
The behavior of deformation of rocks is classified based on the stress-strain relation as in Fig. 3. The brittle behavior shows abrupt fracturing with no or very slight inelastic deformation. The ductile behavior is characterized by large inelastic deformation over the yield point without fracturing. Rocks often show work hardening at the ductile condition. The transitional behavior shows inelastic deformation but is accompanied with partial stress drop by faulting. The behavior of deformation of rock is transformed from brittle failure through transitional mode into ductile flow with the increase of confining pressure. Soft sedimentary rocks are transformed into ductile state at very low pressure. However, the transition of behavior of soft rocks follows similar simple law as the one for hard rocks which was found by Mogi(1966) and Byerlee (1968). The fields of brittle failure and of transitional and ductile deformation are divided by a boundary line (R) in the confining pressure-strength diagram(Fig.4).
A phase of extremely ductile flow has been found by deformation experiments of soft sedimentary rocks under confining pressures(Koide et al., 1971, Hoshino et al., 1972). This flow phase was temporarily named as visco-ductile behavior. The visco-ductile behavior is characterized by a monotonical gentle slope of stress-strain curve(E in the Fig. 3). Test data of core samples indicate that the critical confining pressure for transition into visco-ductile flow becomes higher with increase of sampled depth. The critical pressure is almost appropriate to the overburden pressure(Fig.6). The transition into visco-ductile flow is a kind of thixotropic weakening. A structural framework of porous sedimentary rock is broken by higher confining pressure than the effective precompaction pressure. Porous sedimentary rocks are liquefied by high effective confining pressure. The transition into visco-ductile flow from normal ductile deformation occurs along the boundary line(S) in the Fig.4. The equivalent viscosity after the definition by Griggs(1939) is very low for porous sedimentary rocks(Fig.7). The low equivalent viscosity indicates the viscous liquid-like behavior of porous sedimentary rocks.
