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THM simulations for laboratory heating test and real-scale field test
Simulations THM d’essais de chauffage en laboratoire et en vraie grandeur in situ
Xiong Y.L., Zhang F., Nishimura T., Kurimoto Y.
Nagoya Institute of Technology, Nagoya, Japan
ABSTRACT: In this paper, a program of finite element method (FEM) named SOFT, considering thermo-hydro-mechanical (THM)
behavior of soft rock, has been developed to simulate the THM behavior of the soft rock in geological disposal based on a thermo-
elasto-viscoplastic model. A thermal heating isotropic test for soil under drained condition in different over-consolidated ratio (OCR),
is firstly simulated by the proposed numerical. Meanwhile, a real-scale field test is also simulated by the proposed method. The
material parameters of the rock involved in the constitutive model are determined based on element tests for the rock. It is found that
the proposed numerical method can well describe the THM behavior of the soft rocks, such as, the temperature change, the change of
EPWP and the heat-induced deformation.
RÉSUMÉ : Dans cette étude, nous avons développé un programme aux éléments finis appelé SOFT intégrant le comportement
thermo-hydro-mécanique de roches tendres basé sur un modèle thermo-élasto-viscoplastique de façon à simuler le comportement
THM de ces roches constituant un site géologique de stockage. Un essai d’échauffement sous chargement isotrope en condition
drainée avec différents rapports de surconsolidation a été tout d’abord simulé par le modèle numériqu
e développé. Dans le même
temps, un essai en vraie grandeur réalisé in situ a également été simulé. Les paramètres de la roche considérés dans le modèle ont été
obtenus à partir d’essais élémentaires réalisés sur ce matériau. Les résultats obtenus montrent
que le modèle est capable de bien
décrire le comportement THM des roches tendres, incluant le changement en température, le comportement mécanique et les
déformations induites par l’élévation de température.
KEYWORDS: THM behavior, thermo-elasto-viscoplastic model, FEM, soft rock, numerical analysis, geological disposal
1 INTRODUCTION
In considering the problem about deep geologic disposal for
high level radioactive waste, not only artificial barrier, but also
the thermo-hydro- mechanical (THM) behavior of natural
barrier, most of which is sedimentary rock or granite, is also a
very important factor to be studied. High radioactive substance
might permeate with water through barrier systems to
biosphere. The temperature emitting from nuclear waste
canisters also requests the study of temperature effect on soft
sedimentary rock. The water may induce swelling phenomenon
which can yield to a damage of the nuclear waste containers due
to the generated temperature. All the above phenomena need to
be well understood in order to guarantee the safety and the
efficiency of the waste sealing construction.
In this paper, a program of finite element method (FEM)
named SOFT, considering soil- water-heat coupling problem,
has been developed to simulate the above-mentioned THM
behavior of geological disposal based on a thermo-elasto -
viscoplastic model (Zhang and Zhang, 2009). In order to verify
the applicability of the program, a thermal heating isotropic
tests for soils under drained condition in different over-
consolidated ratio (OCR), are firstly simulated by the proposed
program. In the test, the thermal volume change is found to be
dependent on OCR, which can also be found in other literature.
In this paper, however, the element test is not regarded as an
elementary behavior, but a boundary value problem due to the
different thermal expansion of water and soil particles. The
simulated results show that the THM phenomenon observed in
the laboratory test can be explained well by the proposed
numerical method.
Meanwhile, a real-scale field test reported by Gens et.al
(2007) is also simulated by the proposed numerical method. The
material parameters of the rock involved in the constitutive
model are determined based on element tests for the rock.
2 SIMULATION OF ELEMENTARY HEATING TEST
Heat-induced volume strain of geomaterials has been
investigated extensively in literature. Drained triaxial (isotropic)
consolidation test is usually used to investigate this effect by
heating the test specimen, in which a specimen is firstly
consolidated to a given stress and then unloaded to reach a
specified OCR state. After then it is heated to a prescribed
temperature in a very slow rate in order to prevent generation of
excessive pore water pressure (EPWP).
A very interesting result shown in the Figure 1 (a) was
reported by Baldi et al. (1988), in which the heat-induced
volumetric strain measured by the quantity of water discharge
was dependent on OCR, that is, the specimen will change from
contraction to dilation as OCR increases. Afterwards, some
other researchers also reported the same test results, e.g., the
works by Cekerevac and Laloui (2004) and Cui et al (2009).
In the present paper, the heating test for soft rock (Baldi et
al.,1988) is simulated by the proposed THM-FEM program
SOFT to explain the test results. The FEM model used in the
THM analysis is shown in Figure 2. Though the test is regarded
as a so-called element test of free thermal expansion test, its
thermal-
mechanical behavior, in the author’s viewpoint, is a
boundary value problem. In the simulation, all the boundary
conditions are the same as those in the test. For instance, FEM
model is only fixed in vertical direction (z) on bottom and other
movements are totally free. The initial temperature is 22°C and
the specimen is heated gradually up to 90°C with a rate of about
1°C /h. The material parameters and the physical properties of
