106 research outputs found
Numerical upscaling of the permeability of a randomly cracked porous medium
The equivalent permeability of a randomly cracked porous material is studied
using a finite element program in which a four-nodes zero-thickness element is
implemented for modelling the cracks. The numerical simulations are performed
for geometries with different cracks densities and for different values of
matrix permeability and cracks conductivity, but the cracks length are taken
equal to one. The method used for determination of the equivalent permeability
resulted in a perfectly symmetric equivalent permeability tensor for each case.
Based on the obtained results a simple relation is presented for the equivalent
permeability of a randomly cracked porous material as a function of the matrix
permeability and the cracks density and conductivity. This relation is then
generalized for the cracks of any length using a linear transformation
Temperature induced pore fluid pressurization in geomaterials
The theoretical basis of the thermal response of the fluid-saturated porous
materials in undrained condition is presented. It has been demonstrated that
the thermal pressurization phenomenon is controlled by the discrepancy between
the thermal expansion of the pore fluid and of the solid phase, the
stress-dependency of the compressibility and the non-elastic volume changes of
the porous material. For evaluation of the undrained thermo-poro-elastic
properties of saturated porous materials in conventional triaxial cells, it is
important to take into account the effect of the dead volume of the drainage
system. A simple correction method is presented to correct the measured pore
pressure change and also the measured volumetric strain during an undrained
heating test. It is shown that the porosity of the tested material, its drained
compressibility and the ratio of the volume of the drainage system to the one
of the tested sample, are the key parameters which influence the most the error
induced on the measurements by the drainage system. An example of the
experimental evaluation of undrained thermoelastic parameters is presented for
an undrained heating test performed on a fluid-saturated granular rock
Experimental artefacts in undrained triaxial testing
For evaluation of the undrained thermo-poro-elastic properties of saturated
porous materials in conventional triaxial cells, it is important to take into
account the effect of the dead volume of the drainage system. The
compressibility and the thermal expansion of the drainage system along with the
dead volume of the fluid filling this system, influence the measured pore
pressure and volumetric strain during undrained thermal or mechanical loading
in a triaxial cell. A correction method is presented in this paper to correct
these effects during an undrained isotropic compression test or an undrained
heating test. A parametric study has demonstrated that the porosity and the
drained compressibility of the tested material and the ratio of the vol-ume of
the drainage system to the one of the tested sample are the key parameters
which influence the most the error induced on the measurements by the drainage
system
Association of macroscopic laboratory testing and micromechanics modelling for the evaluation of the poroelastic parameters of a hardened cement paste
The results of a macro-scale experimental study performed on a hardened class
G cement paste [Ghabezloo et al. (2008) Cem. Con. Res. (38) 1424-1437] are used
in association with the micromechanics modelling and homogenization technique
for evaluation of the complete set of poroelastic parameters of the material.
The experimental study consisted in drained, undrained and unjacketed isotropic
compression tests. Analysis of the experimental results revealed that the
active porosity of the studied cement paste is smaller than its total porosity.
A multi-scale homogenization model, calibrated on the experimental results, is
used to extrapolate the poroelastic parameters to cement pastes prepared with
different water-to-cement ratio. The notion of cement paste active porosity is
discussed and the poroelastic parameters of hardened cement paste for an ideal,
perfectly drained condition are evaluated using the homogenization model
The effect of undrained heating on a fluid-saturated hardened cement paste
The effect of undrained heating on volume change and induced pore pressure
increase is an important point to properly understand the behaviour and
evaluate the integrity of an oil well cement sheath submitted to rapid
temperature changes. This thermal pressurization of the pore fluid is due to
the discrepancy between the thermal expansion coefficients of the pore fluid
and of the solid matrix. The equations governing the undrained
thermo-hydro-mechanical response of a porous material are presented and the
effect of undrained heating is studied experimentally for a saturated hardened
cement paste. The measured value of the thermal pressurization coefficient is
equal to 0.6MPa/°C. The drained and undrained thermal expansion
coefficients of the hardened cement paste are also measured in the heating
tests. The anomalous thermal behaviour of cement pore fluid is back analysed
from the results of the undrained heating test.Comment: Cement and Concrete Research (2008) In pres
THERMAL PRESSURIZATION AND ANOMALOUS THERMAL EXPANSION OF THE PORE FLUID OF A HARDENED CEMENT PASTE
International audienceTemperature increase in a fluid-saturated porous material in undrained condition leads to pore pressure increase. This phenomenon of thermal pressurization is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is found equal to 0.6MPa/°C. The experimental observation that this coefficient does not change with temperature between 20°C and 55°C is attributed to the anomalous thermal behaviour of cement paste pore fluid. It is shown that the thermal expansion of the cement paste pore fluid is higher than the one of pure bulk water and is much less sensitive to temperature changes. This anomalous thermal behaviour is due to the confinement of the pore fluid in the very small pores of the microstructure of the cement paste, and also to the presence of dissolved ions in the pore fluid
Numerical modelling of the effect of weathering on the progressive failure of underground limestone mines
International audienceThe observations show that the collapse of underground limestone mines results from a progressive failure due to gradual weathering of the rockmass. The following stages can be considered for the limestone weathering and degradation process in underground mines: condensation of the water on the roof of the gallery, infiltration of water in the porous rock, migration of the air CO2 molecules in the rock pore water by convection and molecular diffusion, dissolution of limestone by CO2 rich water and consequently, reduction of the strength properties of rock. Considering this process, a set of equations governing different hydrochemo-mechanical aspects of the weathering phenomenon and progressive failure occurring in these mines is presented. Then the feasibility of numerical modelling of this process is studied and a simple example of application is presented
Pressurisation thermique d'une roche saturée
L'augmentation de la température dans les milieux poreux saturés en conditions non-drainées conduit à un phénomène de pressurisation thermique du fluide interstitiel due à la différence des coefficients d'expansion thermique du fluide et de la matrice solide. Cette augmentation de la pression interstitielle induit une diminution de la contrainte moyenne effective et peut conduire à une rupture en cisaillement ou à une fracturation hydraulique. Ce phénomène de pressurisation thermique est étudié ici expérimentalement pour une roche granulaire saturée à partir d'essais de chauffage non-drainés sous un état de contrainte isotrope maintenu constant. Les résultats de l'essai de chauffage non-drainé sont modélisés à l'aide d'un modèle de comportement thermo-poro-élastique non-linéaire de la roche granulaire
Micromechanics analysis of thermal expansion and thermal pressurization of a hardened cement paste
The results of a macro-scale experimental study of the effect of heating on a
fluid-saturated hardened cement paste are analysed using a multi-scale
homogenization model. The analysis of the experimental results revealed that
the thermal expansion coefficient of the cement paste pore fluid is anomalously
higher than the one of pure bulk water. The micromechanics model is calibrated
using the results of drained and undrained heating tests and permits the
extrapolation of the experimentally evaluated thermal expansion and thermal
pressurization parameters to cement pastes with different water-to-cement
ratios. It permits also to calculate the pore volume thermal expansion
coefficient f a which is difficult to evaluate experimentally. The anomalous
pore fluid thermal expansion is also analysed using the micromechanics model
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