3403
Determination of the thermal parameters of a clay from heating cell tests
Détermination des paramètres thermiques d'une argile à partir d’essais dans une cellule de
chauffage
Romero E., Lima A., Gens A., Vaunat J.
Department of Geotechnical Engineering and Geosciences, Universitat Politècnica de Catalunya, Barcelona, Spain
Li X.L.
EURIDICE / SCK.CEN, Mol, Belgium
ABSTRACT: Boom Clay is being studied in Belgium in connection with the design of a repository for radioactive waste. Within this
context, thermal impact may play an important role on the behaviour of this low-permeability clay. To evaluate this impact, heating
pulse tests on intact borehole samples were carried out using an axi-symmetric and constant volume heating cell with controlled
hydraulic boundary conditions. Attention is focused on the time evolution of temperature and pore water pressure changes along
heating and cooling paths –i.e., pore pressure build-up during quasi-undrained heating and later dissipation to the applied hydraulic
boundary conditions–. A coupled thermo-hydro-mechanical finite element program was used in a first stage to determine thermal
parameters by back-analysis and then to simulate the experimental results.
RÉSUMÉ: L’argile de Boom est un matériau actuellement étudié en Belgique dans le cadre de la conception d’un centre de stockage
de déchets radioactifs. Dans ce contexte, l’impact thermique est susceptible de jouer un rôle important dans la réponse de la formation
argileuse, de faible perméabilité. Afin d’évaluer cet impact, des essais de chauffage par impulsion ont été réalisé sur des échantillons
intacts, dans une cellule axisymétrique à volume constant qui permet de contrôler les conditions hydrauliques à sa frontière. Les
mesures obtenues en termes d’évolution de température et de pression d’eau lors de cheminements de chauffage et de refroidissement
indiquent le développement initial de pression de pores en conditions quasi-non drainées (cas du chauffage), suivi d’une dissipation
postérieure vers un régime stationnaire équilibré avec les conditions hydrauliques appliquées. Un programme Éléments Finis couplés
thermo-hydro-mécanique a été utilisé pour rétro-analyser, dans un premier temps, les paramètres de conductivité thermique et pour
simuler, dans un deuxième temps, les résultats expérimentaux.
KEYWORDS: heating cell, clay, thermal conductivity, back-analysis, experimental results, numerical simulation
1 INTRODUCTION
Thermal impact may play an important role on the behaviour of
low-permeability saturated clayey host formations in connection
with the design of a repository for ‘High-Level Radioactive
Waste’. Boom Clay is currently the subject of extensive
research on hydrothermal and mechanical phenomena that may
possibly affect its performance as potential geological host
formation.
There are a number of laboratory results concerning the
saturated hydro-mechanical behaviour of Boom Clay under a
constant temperature field and studies on this area are described
-to cite but a few of them- in De Bruyn (1999), Le (2008) and
Lima (2011). Nevertheless, there is less information on clay
hydro-mechanical response on heating and cooling paths under
controlled small-scale laboratory condition. To this end, the
paper presents results from a comprehensive testing program
performed on Boom Clay to determine thermal and hydraulic
parameters using an axi-symmetric heating cell with
measurement of temperature and pore pressure. Pore-pressure
built-up and dissipation on fast heating pulse tests have been
analysed using experimental results assisted by numerical
simulations carried out with a coupled thermo-hydro-
mechanical finite element code.
2 EXPERIMENTAL SETUP AND TESTED MATERIAL
Laboratory tests have been performed on Boom Clay (Mol,
Belgium). Table 1 summarises the main properties of this
Tertiary clay (20%-30% kaolinite, 20%-30% illite and 10%-
20% smectite), which is slightly overconsolidated (Horseman
et
al
. 1987, Coll 2005, Lima 2011).
Figure 1 shows a scheme of a constant volume and axi-
symmetric heating cell (Muñoz
et al
. 2009, Lima
et al
. 2010,
Lima 2011), which has been used to perform heating pulse tests
with controlled power supply and controlled hydraulic boundary
conditions. Soil sample size is 75 mm in diameter and 100 mm
high. A controlled-power heater (
H
in the figure) is installed
along the axis of the sample in the lower part of the cell.
Different transducers monitor the sample response, as shown in
the figure: two miniature pore water pressure transducers (
Pw
1
and
Pw
2
), and three thermocouples (
T
1
,
T
2
and
T
3
). The cell is
equipped with top and bottom valves to apply controlled
hydraulic boundary conditions (
u
u
and
u
b
). The heater with
controlled power supply remains switched on for 24 hours
during the heating stage and later it is switched off to perform
the cooling phase.
Table 1. Main properties of Boom Clay.
Property
Value
Density,
2.05 Mg/m
3
Dry density,
d
1.65 to 1.67 Mg/m
3
Gravimetric water content,
w
25 %
Density of soil solids,
s
2.67 Mg/m
3
Void ratio,
e
0.60 to 0.62
Degree of saturation,
S
r
100 %
Liquid limit,
w
L
56
Plastic limit,
w
P
29
Vertical water permeability at 20ºC,
k
wv
2.3
10
-12
m/s
Vertical water permeability at 80ºC,
k
wv
6.5
10
-12
m/s
Horiz. water permeability at 20ºC,
k
wv
4.5
10
-12
m/s
Small-strain shear modulus,
G
0
347 MPa
Poisson’s ratio,
0.20
