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ABSTRACT: Test methods for evaluating the dynamic mechanical properties of a discontinuous rock mass have never previously been established. Therefore, we developed an in-situ triaxial test method for dynamic (cyclic) triaxial tests. Then, in-situ cyclic triaxial tests were carried out on discontinuous rock made of weathered sandstone. The strain level dependencies of the Young's modulus and damping ratio were evaluated based on the results of these in-situ tests. 1 INTRODUCTION In Japan, the design earthquake motions for nuclear power plant increased than ever before since the Great East Japan Earthquake. Therefore, a dynamic analysis in the stability evaluation of a rock foundation and surrounding rock slope at a plant has become increasingly important. To improve the credibility of the analysis results, it is necessary to adequately evaluate the dynamic (cyclic) deformation characteristics of the rock used for the dynamic analysis. Sedimentary soft rocks, in which discontinuities have less influence, do not have a significant scale effect (Ikemi et al. 1998). Therefore, dynamic deformation characteristics (strain-level dependencies ofYoung's modulus and hysteresis damping ratio), that is used for the dynamic analysis, are examined by elastic wave velocity logging in the field and cyclic triaxial tests in the laboratory, as well as the soil ground. On the other hand, the mechanical properties of a discontinuous rock mass are evaluated using in-situ rock tests of large-scale specimens. These tests include a plate loading test (PLT) and rock shear test (RST). In the past, several cyclic loading PLTs and RSTs for in-situ rock masses were attempted to evaluate the cyclic deformation and strength characteristics (JSCE 1989, Fujiwaraet al. 1984). In recent years, these tests have not been put into practical use because they are not element tests, and the interpretation of the test results is difficult. Therefore, we developed an in-situ cyclic loading triaxial test method that uses a large scale specimen, which is equivalent to an in-situ rock test. Then, two in-situ cyclic triaxial tests were carried out on a discontinuous rock mass of weathered sandstone.
- Geology > Rock Type (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.96)
- Geology > Geological Subdiscipline > Geomechanics (0.92)
ABSTRACT: The results of laboratory tests, and the consequent values of the mechanical parameters, could be appreciably influenced by the characteristics of the testing device and by the adopted experimental procedure. Some of these, effects, such as those related to the stiffness of the loading frame, are discussed here on the basis of a series of triaxial tests performed on samples of a soft volcanic rock (tuff) from central Italy. In particular, the influence is discussed of the "local" or "global" technique adopted for strain measurement on the overall shape of the stress-strain curve, and on the values of the elastic parameters determined before reaching the "peak" load level and in the "after peak" condition. RÉSUMÉ: La methode experimentale et les caracteristiques de l'appareil peuvent avoir une influence remarquable sur les resultats de l'essais en laboratoire et donc sur les valeurs des parametres mecaniques qui sont derivees, Cette etude presente les resultats d'essais de compression triaxial obtenus sur une roche volcanique ("tufo di Riano", Italie), en utilisant soi un systeme de mesures traditionnelles (mesures globales) soi un systeme avec une instrumentation placee à l'interieur de l'appareil (measures locales). Les valeurs des parametres mecaniques, obtenues des differents essais, ont ete comparees, En plus, des commentaires sont aussi presentes sur l'influence de la rigidite de l'appareil. ZUSAMMENFASSUNG: Das Pruefverfahren und die Eigenschaften der Pruefeinrichtungen (z.B. die Steifigkeit der Pruefmaschine und die Eigenschaften des Maßverfahrens) können einen nicht vernachlaassigbaren Einfluß auf die Testergebnisse haben. In dieser Arbeit werden die Testergebnisse von triaxialen Versuchen vorgestellt, welche einerseitss mit einem herköommlichen Messverfahren (global Messung) und andererseits mit einem Verfahren, bei dem die Messung innerhalb der Meßzelle durchgefuehrt wird (lokal Messung), gewonnen worden. Abschließend werden die Werte der gewonnenen mechanischen Parameter aus beiden Versuchsserien miteinander verglichen. INTRODUCTION The design of major civil engineering structures resting on soft rock formations requires an accurate estimation of the settlements of their foundations in order to avoid negative effects on the overall behaviour of their structural members. This, in turn, requires a proper evaluation of the deformability of the rock mass, as well as of the parameters governing its load bearing capacity (JARDINE et al., 1986; STERPI et al., 1995; CIVIDINI et al., 1992; CIVIDINI & GIODA, 1992). In rock engineering practice, this evaluation is customarily based on the results of the necessary in situ investigation and of proper laboratory, tests, such as one dimensional, unconfined compression tests and standard triaxial tests (e.g. ADACHI & TAKASE, 1981; KING,'I983; HAIMSON, 1978; JOHNSTON, 1992). Confining our attention to the laboratory investigation, it is known that the experimental procedure and the characteristics of the testing' device could influence the experimental results and, consequently, the values of the derived mechanical parameters (see e.g. YUMLU & OZBAY, 1995; TATSUOKA et al., 1986; 1994; CIVIDINI, 1997). This work intends to provide an insight into this problem; with particular reference to the behaviour of a soft volcanic rock (tuff) from central Italy, known as "tufo di Riano".
- Geology > Geological Subdiscipline > Volcanology (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
Multiple-step Loading Triaxial Compression Test (ML-TCT) method is a useful tool to evaluate strength parameters from a single specimen. However, because of accumulated damage in the specimen with repeated cycles of axial loading/unloading, the shear strength is prone to be underestimated. Therefore, a Multiple-step Loading Damage (MLD) model was proposed to simulate ML-TCTs with various stress paths. Two series of ML-TCTs were carried out on a sedimentary soft rock of mudstone. The first series was to determine the geotechnical parameters to describe the MLD model, and the second series was to verify the MLD model. The results demonstrated that the proposed MLD model was applicable for simulation of ML-TCTs on the mudstone and correction the results of carried out tests. This model allows prediction of peak deviator stress and the relevant excess pore water pressure due to previous plastic deformations in the specimen for various types of rock.
Abstract The paper presents a pseudo-dynamic constitutive relationship describing undrained stress-strain responses of soft clays subjected to cyclic loads based on the equivalent visco-elastic theory and the creep theory. The relationship includes three basic parameters: the equivalent cyclic shear modulus, the equivalent damping ratio and the cyclic accumulative strain increment. The nonlinearity of cyclic stress-strain responses of soft clays is described using variations of the cyclic shear modulus and the damping ratio with the octahedral shear strain. The cyclic accumulative strain is described using the equivalent creep. The cyclic accumulative strain increment is determined using the Mises creep potential function and the associated flowing rule. They can be determined by unconsolidated and undrained cyclic triaxial tests of soft clays. A pseudo dynamic visco-elastoplastic finite element method to analyze the deformation instability process of suction anchor foundations subjected to static and cyclic loads in soft clays was developed using the constitutive relationship. For the method, a cyclic loading time history is divided into a series of incremental cyclic loading processes which includes one load cycle at least. Detailed cyclic stress-strain responses of soil elements at any time are not tracked but they are determined by the equivalent visco-elastic calculations for each incremental loading process. The cyclic accumulative strain increment of soil elements is determined based on the static stress, the cyclic stress of soil elements and the incremental number of load cycles. The cyclic accumulative deformation of suction anchor foundations is calculated using the initial strain method. Model test results of suction anchors subjected to inclined static and cyclic loads at the optimal load point are predicted using the method. Comparisons of predicted and model test results show that predicted the deformation instability process of suction anchor foundations are basically in agreement with model test results.
- Research Report > New Finding (0.49)
- Research Report > Experimental Study (0.34)
ABSTRACT The monotonic and cyclic shear behaviour of a volcanic soil Shirasu as a reclamation material was investigated in the present study. Shirasu usually has a high coefficient of uniformity and contains typically 20–30% non-plastic fines formed from the crushed remains of larger particles. A series of monotonic and cyclic triaxial tests was carried out on samples of Shirasu to assess the influence of the fines on the monotonic shear and liquefaction susceptibility. Tests were performed on both the original particle size distribution and soil which had the fines washed out. The existence of the fines was shown to dictate the mechanical properties of Shirasu when used as a fill material. INTRODUCTION In 1995 during the Great Hanshin earthquake, reclaimed areas filled with masado underwent major liquefaction damage. Since this earthquake event, Masado, a decomposed granite has been recognised as a material susceptible to liquefaction and many tests have been performed to establish the conditions under which it would liquefy. In addition to the igneous decomposed granites, Japan also has large areas covered by crushable volcanic ash sediments. These are particularly predominant in the south of Kyushu, particularly in Kagoshima prefecture where they are known as Shirasu. These ash deposits are frequently exposed as soft rock escarpments which are mined for use in reclaimed land fill. It is frequently pumped from inland locations as a slurry to form a loose saturated fill. In the 1968 Ebino earthquake sand boiling due to liquefaction was observed on the plains and river terraces formed from Shirasu and subsidence of bridge piers was reported. In March and May 1997 structural damage due to liquefaction was observed in the Kagoshimaken Hokuseibu earthquake in areas of coastal land reclamation at Akune harbour and Izumi and inland at Iriki.
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Geological Subdiscipline (1.00)