Layer | Fill | Outline |
---|
Map layers
Theme | Visible | Selectable | Appearance | Zoom Range (now: 0) |
---|
Fill | Stroke |
---|---|
Collaborating Authors
Results
SYNOPSIS: A brief review is presented of the importance of dilatancy prior to rock failure and especially of its fundamental role in earthquake prediction. Constitutive equations for the visco-elastic-dilatant behaviour of rocks are presented. The dilatancy parameters are determined from various triaxial tests. The process of dilatancy prior to earthquakes is discussed on the basis of this hypothetical model. RESUME: Cette communication examine le rôle important des dilatations survenant avant la rupture de roches et plus particulièrement leur rôle fondamental dans la prediction des tremblements de terre. On presente des equations pour l'etude du comportement visco-elastique des roches et on determine les paramètres de dilatation pour divers essais triaxiaux. C'est sur la base de ce modàle hypothetique qu'on analyse le processus de dilatation avant les tremblements de terre. ZUSAMMENFASSUNG: Die Arbeit gibt einen kurzen Überblick ueber die Bedeutung der Dilatanz vor dem Bruch des Gesteins und besonders ihre grundlegende Rolle in der Voraussage der Erdbeben. Die Ausgangsgleichungen fuer das elasto-visko-dilatante Verhalten der Gesteine werden aufgestellt und die Dilatations-Parameter fuer verschiedene Triaxial-Versuche bestimmt. Die Dilatations- Vorgange vor den Erdbeben werden aufgrund dieses hypothetischen Modells behandelt. The concept of dilatancy is well known in rheology and soil mechanics. Its origin is in the observation that wet sands dilates under the action of shearing stresses (Reynolds1885). Dilatancy in granular media is associated with the overall decrease of packing density due to relative movements of groups of grains; it is a geometrical necessity in the deformation process. Dilatancy of rocks was first observed by Bridgman (1949); Handin et al (l963) measured dilatancy of rock samples of Berea sandstone at low confining pressures, while decrease in porosity (volume hardening)at high confining pressures. Dilatancy under deviatoric stresses is observed as a time dependent volume increase; a dilatancy model is shown in Fig.1; later many experimental results and their analysis have been published from laboratory testing by Paterson (l963), Edmond and Paterson (1972), Brace, Paulding and Scholz (1966), Bienawski (1967), Crouch (1970), Zobackand Byerlee (1975), Perkins, Green and Friedman (1970), Rummel (1974), Mogi (1977),Tan (1964) and others which are not listed here. The results of these investigators will be summarised and analysed in the next paragraphs. In Rock-engineering the phenomenon of dilatancy in rocks is often observed; although it is a fundamental factor in the stability of underground structures it has not yet got the attention it deserves. In another paper to this Congress (Tan 1983) it is emphasized that dilatant volume increase in rock masses plays a crucial role in the stability of potentially swelling rocks. Rock dilatancy has gained worldwide attention since it has been observed that the earthcrust dilates prior to earthquakes; direct evidence of dilatancy have been observed in the San Andreas Fault near Parkfield (Cherry and Savage1972); a very clear evidence have been reported from the bulging of the earthcrust prior to the large Haycheng Earthquake (1975). This has been one of the crucial scientific materials for the prediction of this earthquake. The Haycheng earthquake was the first earthquake, which was predicted correctly in site, magnitude and time (Fig.7A,B,C). Indirect evidence for crustal dilatancy has been reported from the lowering of deep water wells situated in a radius of 100 km around the epicentre of the large Tangshan Earthquake (Tan, He1982); a further indirect indication is the steady decrease in earth resistivity of the upper layer of the crust (Fig.2a-b).· The changes in vp/vs ratio (velocities of normal wave to shear waves) which is considered to be directly related to dilatancy, have been frequently measured in our country (Feng et al 1976; Duan et al 1976). Such changes in this vp/vs ratios have been earlier reported by Semenov(1969), Aggarwal (l973). Some U.S. scientists belief that fluid inflow during dilatancy is a crucial factor leading to earthquakes as it is accompanied by the generation of water pressure (i.e. decrease of the effective normal stress, thus strength) and the lubrication of fissures. A physico rheological model for Earthquake fore runners has been recently suggested (Tan 1982); it is based on the fundamental assumption that the earthcrust is a rheological dilatant body traversed by a network of planes of easier glide, the seismic belts. Time dependent dilatancy is an important problem, which has not yet been explored extensively. It is generally believed that this is due to "creep" but it is not clearly specified what is understood under "creep". As it will be discussed next, creep is due to the continuous compatible straining of grains which is increasing with the time with decreasing rate. This is the case as far as the deviatoric stresses remain below an upper yield limit f3(f3 for shear and f = 3f3 for compression). As soon as this upper limit is exceeded, then non compatible anelastic deformations will occur leading to void and crack formation and opening of inborn cracks.
- Asia (0.69)
- North America > United States > West Virginia (0.24)
- North America > United States > Pennsylvania (0.24)
- (2 more...)
SYNOPSIS: A characterization of swelling rocks is being presented. The mechanism of the swelling is described by means of the: 1) Physico-chemical effects, 2) Rheological effects: creep, the elastic recovery and dilatancy. The importance of mineralogical and physico-chemical tests is stressed. Some rheological testing methods are suggested. An analysis is given of the routine swelling test. Constitutive equations for dilatancy behaviour in two dimensions and a calculation of the dilatant zone around a tunnel are described. RESUME: Cette communication donne une description de roches gonflantes. Le mecanisme de gonflement est attribue: 1) aux effets physico-chimiques, 2) aux effets rheologiques - le fluage, la contraction elastique et la dilatation. Le plus souvent il y a une combinaison des deux phenomènes. On souligne l'importance des essais mineralogiques et physico-chimiques et propose quelques tests rheologiques. Par ailleurs, on explique la marche à suivre pour l'analyse des resultats d'essais d'expansion et les relations de comportement dilatants bi-dimensionnels, ainsi qu'un calcul de la zone dilatante autour du tunnel. ZUSAMMENFASSUNG: Diese Arbeit gibt eine Charakterisierung drueckender Gesteine. Der Mechanismus der Schwellung wird beschrieben an: 1) Physikalisch Chemischen Effekten 2) Rheologischen Effekten: das Kriechen, die Elastische Nachwirkung und Dilatanz. Gewöhnlich arbeiten diese Effekte zusammen. Die Wichtigkeit von mineralogischen und physikalisch-chemischen Versuchen ist hervorgehoben. Empfehlungen zur Durchfuehrung rheologischer Versuche werden gegeben und eine Basis fuer die Analyse von Schwellversuchen wird vorgestellt. Ausgangsgleichungen fuer das Dilatanz-Verhalten in zwei Dimensionen und eine Berechnung der dilatanten Zone um die Tunnel werden angegeben. Part I. SWELLING ROCK, CHARACTERATION AND TESTING METHODS The basic concept is accepted that a rock mass is a rock structure with discontinuity planes and that the deformation and strength of this mass are governed by the rheological properties of the discontinuities and its fillings (joints, interbedding clayey layers etc.) Due to the processes of rockgenesis and tectonic history further inherent properties of the rock mass are (1) presence of cracks also in the intact rocks (2) internal stresses (3) heterogeneity and (4) anisotropy. Swelling problems must be analysed on the basis of these * A major portion of Part I has been submitted to the ISRM Commission for Swelling Rocks fundamental concepts. The inwards motions of the boundary surface of tunnels and excavations, which in many cases are accompanied by the cracking of the linings and the structure of powerhouses, have been attributed by several authors to the causative swelling of rocks. Swelling is generally considered as a time dependent volume increase, which is related to mineralogical composition and physico-chemical effects. Such effects are very clear in the case of potentially swelling rocks, mudrocks, slates, marls. anhydrite rocks and sandstones. However, in general, time-dependent inwards motion is the complex result of the mutually strengthening interaction of two main factors: 1. Physico-Chemical swelling which is a (generally anisotropic) volume increase 2.Mechanical processes as the recovery of the rock-layers after (partial) stress relief and the volume dilatancy which is a volume increase due to deviatoric stresses exceeding a certain upper yield-value. A: CHARACTERIZATION OF SWELLING ROCKS A1. Phenomenological definition Swelling is the time dependent volume increase of a rockmass (i.e. an overall volume increase due to the swelling of the material of the intact rock, the material within the discontinuities and the separation of the discontinuity surfaces). Inwards motion of the surface boundaries of cavities, tunnels, excavations is the integrated result of the combination of volumetric strains (swelling) and shear strains, so inwards motion is only partly due to swelling. A2. Underlying mechanisms of swelling Swelling may be due to the following mechanisms: A2.1 Physico-chemical effects:water is taken up by clay-minerals of the montmorillonite-chlorite-illite groups; Anhydrite-gypsum transformations due to hydration further similar transformations leading to volume increase can be included. As these minerals are not homogeneously distributed, expanding kernels will be formed within the rock-mass (intact rock and materials of discontinuities), creating extra deviatoric or tensile stresses in their surroundings, which in turn will lead to fissures (voids) formation. So the physico-chemical swelling and the kernels will be accompanied by a mechanical swelling and deteriozation of the structure. Temperature ChangesFreezing; dilatation due to water-ice transformation, which as a result of heterogeneity also may lead to fissuring: then thawing will result into decrease in strength properties. Temperature increase; A2.2 Rheological effects: 1)Time-dependent dilatancy Dilatancy is the volume increase, increasing with the time, due to void formation under the action of deviatoric stresses. From triaxial and torsion experiment it is known that in the beginning reversible volume changes occur; but that this is followed by a volume increase as soon as the deviatoric stresses exceed an upper yield value (called f3). The volume increase can be observed after the stress differences exceed a critical limit f** which for many rocks is larger than 50% of the strength σ3 (see Table 1).
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Mineral > Silicate > Phyllosilicate (0.89)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (0.34)
- Reservoir Description and Dynamics > Reservoir Characterization > Exploration, development, structural geology (0.48)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (0.47)
- Well Drilling > Drilling Fluids and Materials > Drilling fluid selection and formulation (chemistry, properties) (0.34)