1,721,008 research outputs found
Point load testing for classification of porous effusive rocks
No full textThe choice of the K ratio between the uniaxial compression strength and the size-corrected point load strength for the estimate of strength from point load tests can have significant consequences on design issues. Several Authors indicated that K ratio departs significantly from the typical suggested values, especially in weak rocks. Experimental evidences from this study confirm that significant variations of K occur even for the same material. Therefore the recourse to literature data of the K ratio for rock materials similar to those tested is possible only if the dependence of K on physical/textural characters of the rock is accounted for. With reference to a scoriaceous lava with widespread and irregularly distributed macropores, the K coefficient was found to be dependent on porosity, similarly to other porous rocks, through an inverse (e.g. a negative power-law) function. The relationship was successively extended to the same type of rocks from worldwide, obtaining very low confidence regions for the function exponent. The present results indicate that lithology and texture strongly affect both shear and tensile modes of failure
Mechanical properties of a weak pyroclastic rock and their relationship with microstructure
No full textThe geotechnical behaviour of very weak pyroclastic rocks controls both the failure mechanisms at the margins of rock mesas, where historic hill towns are often sited, as well as the stability of old underground cavities, in urban areas of Central Italy. The study focuses on the mechanical behaviour of one of the pyroclastic materials forming the Orvieto mesa (pozzolana), not unlike other pozzolanas in Central and Southern Italy and other pyroclastites from volcanic districts worldwide. The mechanical properties under static conditions of this weakly cemented rock are reported. A petrographic and physical characterization of the material was preliminary conducted, followed by a wide range of mechanical tests: oedometer, uniaxial and isotropic compression tests and indirect tensile tests. The stress-strain and strength behaviours of the pozzolana are highlighted and compared to those of the lithic facies of the pyroclastic formation (tuff). The mechanical behaviour of the pozzolana is related to its physical and textural characters, with special reference to continuity of the groundmass and porosity. Finally the role of the material behaviour at the field scale is discussed
Relazione tra parametri di resistenza in condizioni di carico monoassiale di rocce effusive porose
No full textLa relazione tra porosità e resistenza in condizioni di carico monoassiale per una categoria di rocce effusive, caratterizzate da una matrice afanitica con diffusi macropori, presenta caratteri specifici. Nella nota sono presentati dati di resistenza da prove di compressione uniassiale, carico puntuale e trazione indiretta (Brasiliane) condotte dagli autori e desunte dalla letteratura per un ampio intervallo di porosità dei materiali. I risultati indicano che i rapporti tra la resistenza a compressione uniassiale, σf, e l’indice di resistenza a carichi puntuali IS50, oltre che tra σf e la resistenza a trazione indiretta σt, diminuiscono all’aumentare della porosità, discostandosi in maniera significativa da quelli suggeriti dalle raccomandazioni ISRM (1985). La relazione tra il rapporto σt / IS50 e la porosità risulta invece pressoché assente. Questi risultati suggeriscono che la macroporosità influenza in maniera differente i due modi di rottura, per taglio o trazione, tipici rispettivamente delle prove di compressione uniassiale e trazione indiretta/carico puntuale
Dynamic characterization of soil and soft rocks of the Central Archeological Area of Rome
No full textThe paper presents the results of in-situ and laboratory tests aimed at defining the
cyclic properties of soils and soft rocks of the Central Archeological Area of Rome in the
framework of the seismic microzonation study of the area. The small-strain shear modulus
G0 (or analogously shear wave velocity VS) and the curves expressing shear modulus G and
damping ratio D variation with shear strain amplitude were investigated. A large amount
of in-hole tests integrated with active surface wave tests were utilized to characterize the
small-strain stiffness of the lithotypes identified in the area. Small-strain stiffness values
determined by geophysical testswere further compared with thosemeasured in the laboratory
showing different behaviors of soils and soft rocks. The effects of sample disturbance and
degree of jointing, for soils and soft rocks, respectively, was invoked to explain the observed
differences. The shear modulus and damping ratio versus shear strain amplitude curves were
determined by means of resonant column and cyclic shear tests, both simple and torsional.
The experimental results were compared with literature data on similar soils highlighting
some peculiar behaviors. In particular the role of fine matrix in sandy soils and organic
matter content of clays on the cyclic properties was stressed. The results showed that an
increase in fine matrix and organic content results in a stronger linearity and lower damping
ratio
Modelling the instability phenomena on the NW flank of Stromboli Volcano (Italy) due to lateral dyke intrusion
No full textThe aim of the paper is to provide quantitative elements on the mechanisms that controlled the slope instabilities occurred after two major recent eruptions at Stromboli volcano (i.e. 2002–2003 and 2007). After a brief description of the geological setting and of the largest well documented instability phenomena on volcano flanks, the main objective is pursued using three-dimensional stress-strain analyses for modelling the effects of the magma intrusion on the stability of the volcano flank. Modelling is based on the results of an extensive geotechnical characterization of the volcaniclastic and lava materials. The numerical analyses investigate the influence of different paths and geometry of magma intrusion as well as the spatial variation of mechanical properties. As a result, favourable conditions for specific failure modes are identified. The stress-strain analyses show that magma intrusion can cause both a local failure of a wedge immediately downslope from the dyke or deep-seated movement involving large part of the slope; these two mechanisms were consistent with the deformative patterns observed during the 2007 and 2002–2003 eruptions, respectively. The magma thrust induces shear strains up to levels associated to appreciable grain crushing even below the sea level, where flow liquefaction can be invoked to explain the occurrence of past submarine slides. The submarine landslide is analysed by a combined finite element – limit equilibrium approach that demonstrates that instability conditions can develop if the loading exerted by the upper portion of the slope is sufficiently fast to produce undrained conditions. The analyses also support the hypothesis that at the 2002 subaerial failure occurred, due to combined effects of a magma mild thrust and the removal of the toe support caused by the preceding submarine slide
Strength behaviour in monoaxial loading conditions in effusive rocks: the influence of porosity
No full textRelations between strength index properties from monoaxial tests (uniaxial compression, point load and indirect tensile tests) are extensively used in engineering practice and are object of suggestions by ISRM (1985). The results of an extensive test campaign, that investigated strength and static/dynamic deformability of effusive volcanic rocks under monoaxial loading conditions are presented. The tested lithotypes span a wide range of porosity (n = 1 - 50 %) and consist of an aphanitic groundmass with interspersed macropores and rare minute phenocrysts. The microstructure was observed through optical microscopy, scanning electron microscopy and 3D computed tomography images. The analysis of test
results and literature data on similar lithotypes indicate that porosity controls quite strictly both the strength parameters and the ratios between them. The inadequacy of the ratios between point load and uniaxial/tensile strengths proposed in the suggested methods is apparent for n>10%. Some clues on the failure process were highlighted through theoretical and microstructural models in which the strengths obtained from different monoaxial tests are related to porosit
Cyclic and dynamic behaviour of a soft pyroclastic rock
No full textThe mechanical behaviour under cyclic and dynamic conditions of a weakly cemented pozzolana, taken from the subsoil of a historical town lying in a seismically active area, was investigated through laboratory tests and in-situ measurements. The interest for these soft pyroclastic rocks, widespread in many volcanic regions worldwide, is noticeable because they are often found alternated to stronger volcanic rocks in thick sequences, thus giving a significant impedance contrast that could increase the ground-shaking hazard. A cross-hole (CH) test and a spectral analysis of superficial waves (SASW) test, carried out from the ground surface and in the floor of a cavity respectively, provided in situ vertical profiles of the shear waves velocity. Cyclic and dynamic properties were investigated in the laboratory through velocity measurements of ultrasonic pulses (UPV), cyclic simple shear (DSDSS), torsional shear (TS) and resonant column (RC) tests. Most testing procedures and devices, originally conceived for soils, posed some challenges. The collected data allowed the analysis of the influence that mean confining stress, strain amplitude and number of cycles have on both shear stiffness and material damping. The exhaustive geotechnical characterization at the laboratory scale has an augmented importance because in these deposits the small- and large-scale behaviour are comparable due to the large discontinuity spacing
Cyclic behavior of soft offshore clays at small to large strains
No full textTwo soft offshore clays, recovered from the Adriatic shelf and the Tyrrhenian shelf margin (Italy), were subjected to cyclic simple shear tests. The influence of the cyclic shear strain amplitude as well as of vertical stress, void ratio and stress history on stiffness and damping properties of the soils, in a wide strain range (i.e. from 10-4 % to more than 1%), is examined and discussed. The role of soil structure on the cyclic behavior is also explored. © 2016, IMEKO-International Measurement Federation Secretariat. All rights reserved
Simple shear testing of very soft offshore clay for wide strain range
No full textStiffness and damping properties of sensitive, very soft clay sediments of the Italian Adriatic continental shelf are determined by means of two series of cyclic simple shear tests (one with 12 stages and one with two stages). The apparatus used in this research is capable of investigating the stress–strain behaviour of the soil in a wide range of shear strains from about 0.0004% to 1%. Test results were expressed in terms of small-strain shear modulus (G0), normalized equivalent shear modulus (Geq/G0), and damping ratio (D) versus cyclic shear-strain amplitude (γc). These parameters were analyzed in the framework of existing literature by comparison with empirical correlations developed for onshore materials of different plasticity and, limited to G0, also for soft soils. The dependence of G0, Geq/G0–γc, and D–γc on factors such as void ratio, stress history, and loading cycles is analyzed and discussed
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