The deformation and strength characteristics of rock materials are strongly influenced by the structure of their porosity. Some peculiarities of voids such as density, aspect ratio (ratio between maximum and minimum lengths), spatial distribution, together with the presence of fluids may affect the mechanical behaviour of rocks. In this paper some results from laboratory tests on a clastic rock (calcarenite) are illustrated and discussed. In particular, special attention is paid to the mechanical aspects which are governed by porosity. The direct observation of thin section at the optical microscope has evidenced that the rock is constituted by organic clasts and that porosity is high (around 30%). The geometrical aspect of porosity varies widely: some pores in the matrix appear to be almost equidimensional, while others in the clasts are much smaller and apparently constitute a closed porosity. Moreover, sample weights at maximum saturation show that a certain portion of porosity (0.3%) is not “hydraulically connected”. The experimental tests have included measurements of ultrasonic velocities in both dry and saturated conditions. In particular, drainage tests and imbibition tests have been carried out, together with measurements of the longitudinal velocity. These tests have outlined the complexity of the porosity structure and the composite distribution of water inside pores. In fact, longitudinal velocities in partially or fully saturated conditions are higher than in dry condition in drainage tests, while the contrary happens in imbibition tests. A comparison of these results with the longitudinal velocities measured in situ has shed some light on the interpretation of this parameter. To investigate the mechanical behaviour of the rock during loading, triaxial tests in both isotropic and deviatoric conditions have been carried out. The static measurements have been supplemented by dynamic measurements of the longitudinal and transversal velocities in the direction of the applied load. By this kind of measurements it has been possible to identify the elastic characteristic of the rock and to follow the evolution of plastic strains until failure.
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|Titolo:||The role of dynamic measurements in characterizing the mechanical behaviour of a high porosity rock|
|Data di pubblicazione:||1998|
|Appare nella tipologia:||04b Atto di convegno in volume|