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New thermal triaxial apparatus for unsaturated soils using the osmotic
method
Article in Arabian Journal of Geosciences · June 2014
DOI: 10.1007/s12517-014-1471-2
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Arab J Geosci (2015) 8:3365–3380
DOI 10.1007/s12517-014-1471-2
ORIGINAL PAPER
New thermal triaxial apparatus for unsaturated soils using
the osmotic method
Moulay Smaïne Ghembaza & Said Taïbi &
Jean Marie Fleureau
Received: 24 February 2014 / Accepted: 14 May 2014 / Published online: 29 May 2014
# Saudi Society for Geosciences 2014
Abstract The study of the temperature effects on the behaviour of saturated and unsaturated soils on triaxial paths requires the development of a new triaxial experimental device
of 3.5 MPa of confining pressure and equipped with a heating
collar-controlled temperature. In addition to this case of the
unsaturated soils, a special pedestal is developed, so as to
impose suctions up to 9 MPa using an osmotic solution of
polyethylene glycol (PEG) 6000 through a permeable semimembrane. Measurements of the degree of saturation and the
variation of volume during tests induce various problems,
primarily related to the thermal dilation of the osmotic solution and the cell-water system. These problems have been
solved by carrying out a preliminary calibration. The applicability of this new apparatus concerns environmental
geotechnics and geological engineering such as the design
and behaviour of engineered barriers as well as the behaviour
of the receiving layers of storage sites like radioactive waste.
Keywords Thermohydromecanical behaviour . Saturated
soils . Unsaturated soils . Suction . Thermal triaxial cell . PEG
osmotic solutionm Isotropic compressibility . Deviatoric
triaxial paths
M. S. Ghembaza (*)
Laboratory of Civil Engineering and Environment, Djillali Liabès
University, 22000 Sidi Bel-Abbès, Algeria
e-mail: ghembaza_moulay@yahoo.fr
S. Taïbi
Laboratory LOMC, University of Le Havre and CNRS, Le Havre,
France
e-mail: Said.taibi@univ-lehavre.fr
J. M. Fleureau
Laboratory MSS-Mat, Ecole Centrale Paris and CNRS,
Châtenay-Malabry, France
e-mail: jean-marie.fleureau@ecp.fr
Introduction
To investigate the thermal effects on the soil behaviour, the
testing equipment needs to be capable of independent measurement and control of temperature. Moreover, special techniques need to be devised for the overall volume change
measurement, as the conventional methods of measuring the
sample volume change fail to yield accurate results due to
thermal expansion of both liquid and solid phases in the
sample. To examine thermo-hydro-mechanical processes in
unsaturated soils, the testing equipment must be able to control cell pressure, pore water pressure, pore air pressure and
temperature independently. Furthermore, it needs to be capable of measuring pore water pressure, pore air pressure, pore
water volume change, total volume change and temperature.
To study thermo-hydro-mechanical behaviour in saturated and
unsaturated soils, several researchers carried out controlled
temperature tests using shearing apparatus (triaxial cells, direct shear test) or oedometric and isotropic cells. One of the
major problems is the heating system of these devices. Full
scale experimental tests showed that the temperature inside
the engineered buffer was ranging from 20 to 100 °C; the
relative humidity varied between 30 and 100 %, which corresponds to suction range of 1–500 MPa; the mean net stress
could reach 10 MPa (Huertas et al. 2000; Chijimatsu et al.
2001 ). Certain authors carried out their tests on triaxial cells
immersed in a thermostat bath which can be filled with water
(Paaswell 1967; Noble and Demirel 1969; Habibagahi 1977;
Tang et al. 2007) or with oil (Boisson et al. 1993). Others used
cells in enclosures of air at constant temperature (Campanella
and Mitchell 1968). De Bruyn (1999) and Cekerevac and
Laloui (2004) used the triaxial cells whose heating system is
ensured by a copper spiral surrounding the sample where a
liquid is controlled by a thermostated bath circulates. Some
other authors used thermal triaxial cells whose heating system
is ensured by using electrical resistances placed outside the
3366
cell, such as Horseman et al. (1993), Baldi et al. (1988), Saix
and Jouanna (1990), Sultan (1997), Devillers (1998) and
Jamin (2003). On the other hand, Wiebe et al. (1998)
imposed their temperature using the resistance of thermal
devices placed inside the cell.
For the study of the behaviour in temperature of saturated
soils, the triaxial apparatus at high temperatures and pressures
developed by Baldi et al. (1988) for tests of compression
covers a range of temperatures from 20 to 200 °C with an
increment of 0.5 °C. The temperature is imposed by resistances of 1,700 W placed outside the cell. This apparatus is
equipped with thermocouples placed inside the cell to control
the imposed temperature, and thermocouples are used to
measure the simple temperature.
The work of Kuntiwattanakul et al. (1995) is concerned
consolidated with undrained triaxial tests on saturated soils.
This required the development of a temperature-controlled
triaxial apparatus which was composed of two cells, namely
inner and outer cells. The inner cell was filled with water
whose temperature was controlled by a variable power heater
submerged directly in the water. The amount of power supplied to the heater was automatically adjusted by a controller
unit in order to balance the amount of heat transferred from the
cell to the surrounding environment. The method of temperature control allows values to be imposed from the ambient
temperature to 90 °C with an accuracy of 0.1 °C. The chamber
between the inner and the outer cell was filled with air. This air
was used to transfer the pressure to the water contained in the
inner cell and also to create heat insulation.
A high temperature triaxial apparatus (HTTA) had been
previously designed and manufactured at the Sydney
University by Chiu (1996), to investigate the
thermomechanical behaviour of clay at higher temperature
>50 °C. The design of the HTTA was completely
reconsidered and modified by Ghahremannejad (2003).
Sultan (1997) developed a triaxial cell, especially adapted to
expansive soils under thermal and mechanical loading. This
cell was used to carry out test on saturated samples. The
heating system on the outside of the cell is ensured by electrical resistances controlled by a regulator related to thermocouples inside the cell. The range of temperature varies from
20 to 100 °C (Sultan et al. 2002).
De Bruyn (1999) took again a thermal triaxial cell initially
developed by Virdi and Keedwell (1989). They adopted it for
the conducting tests at high temperatures going from ambient
temperature to 80 °C on saturated samples. Heating occurs in
the water-filled cell body through a copper spiral. The water is
heated in a bath out of the cell. The temperature measurement
is made using a sensor fixed in the room of the cell and close
to the sample. The axial loading is limited to 100 kPa which
represents the maximum capacity of the cell. The assembly is
sealed by a system of double membranes suggested by
Casagrande and Wilson (1960) and which constitutes a good
Arab J Geosci (2015) 8:3365–3380
improvement but without absolute guarantee, as mentioned by
Derenne (1996). The rate of heating is 0.5 °C/min.
Another triaxial apparatus at high temperature control
using GDS pressure-volume controllers was developed by
Cekerevac and Laloui (2004). It allows carrying out thermal
and mechanical tests at controlled temperatures. The
apparatus is composed of a cell and a heating system similar
to that of De Bruyn (1999). The temperature applied varying
from 5 to 95 °C is controlled by a regulator with a sensitivity
of up to ±0.25 °C. The temperature of soil specimen in triaxial
cell is often controlled by a heating coil that covers the outer
wall of the cell (Delage et al. 2000) or by a heater that is
immersed in the confining liquid of the cell (Cekerevac et al.
2003).
In addition, to study the effect of the initial state, Rahbaoui
(1996) used an oedometric apparatus at high temperature. This
apparatus is placed in drying ovens in order to impose temperatures of 20 to 80 °C. The rate of thermal loading ensured
by the potentiometer of the drying oven is of 1 °C/2.5 h. The
vertical stress applied can reach 30 MPa. In these works,
oedometer cell is often immersed in a thermostat bath
(Towhata et al. 1993) for controlling the soil temperature.
We realize through this review that the study of thermohydro-mechanical (THM) behaviour of the unsaturated soils is
rarely treated in the literature. This is due to the complexity of
control of several parameters at the same time such as suction,
stress, variation of volume and temperature. For the study of
the behaviour in temperature of the unsaturated soils, Saix and
Jouanna (1990) developed a triaxial cell in order to carry out
thermal consolidation tests on unsaturated specimens. The
range of temperatures used in their tests varies from the
ambient to 70 °C. A device, made up of a pressure gauge with
mercury, allows the simultaneous measurement and the regulation of capillary suction going from 0 to 70 kPa with a
precision of about 0.1 kPa. A regulation and measuring equipment allows determining the variations of water volume with
an accuracy of 0.05 cm3. The measured range of water volume
variations is imposed by the measured range of suction. This
apparatus was used to study the thermomechanical behaviour
of unsaturated silty sand (Saix 1991; Saix et al. 2000; Jamin
2003).
Devillers (1998) took again the same cell in order to
establish constitutive law-coupling effects of temperature,
mechanical stress and suction on an oedometric path. In
these tests, Devillers (1998) imposes the mechanical stress
by the pressurization of oil filling the chamber of the cell, and
thus the force is applied by a lever-arm system and marked
masses. The measurement of water content variation consists
in measuring the variations of water volumes entering or
leaving through the porous ceramics placed at the base of
the sample.
Wiebe et al. (1998) carried out triaxial compression tests on
unsaturated samples using a thermal cell, confining pressures
Arab J Geosci (2015) 8:3365–3380
ranged from 0.2 to 3 MPa. Water was used as cell fluid for
tests at room temperatures (26 °C) and silicone oil for elevated
temperatures from 65 to 100 °C. The measure of temperatures
is made by using two resistance thermal devices fastened at
the mid-height of the specimen. Two membranes were used,
either latex rubber for low-temperature tests or one butyl and
one silicone rubber membrane for high-temperature tests.
Suction is imposed using overpressured air technique.
Romero (1999) contributed to the development of a new
oedometric cell at high temperature modified in order to study
isotropic swelling with controlled suction. The temperature
applied using electrical resistance is controlled by a temperature regulator. Furthermore, he developed a new thermal triaxial cell with controlled suction on isotropic paths. The range
of temperatures applied varies from 22 to 80 °C, and imposed
suction is limited to 1.9 MPa. The axis translation technique
was used to control the soil suction in oedometer (Romero
et al. 1995) and in triaxial apparatus (Romero et al. 1997)
under temperatures varying from 22 to 80 °C. Recently, a
series of controlled laboratory tests were carried out by
Uchaipichat (2005) on a compacted silt sample using a triaxial
of Bishop-Wesley cell which was modified for testing unsaturated soils at elevated temperatures. Image processing technique was used for measuring the volume change of the
samples subjected to mechanical, thermal and hydric loading.
To allow independent measurement and control of pore water
and pore air pressures, a high air entry disc was placed
between the base of the sample and the pedestal. The high
air entry discs used had a rated air entry value of 500 and
1,500 kPa.
The work of Devillers (1998) and Jamin (2003) forwarded
some disadvantages. On the one hand, the use of oil as
confining liquid requires a considerable time of filling and
draining while assembling and disassembling the apparatus.
On the other hand, this device presents difficulties for
measuring radial strains of the sample. Jamin (2003) proposed
the following improvements: (1) The oil was replaced by a
compressed air circuit with controlled pressure. This new
device led to the use of neoprene membranes to coat the
sample instead of the latex ones which appeared permeable
to air. (2) A system of interdependent loading plate and piston,
allowing to follow the vertical deformation of the sample for isotropic mechanical loadings. (3) The suction,
initially imposed by tensiometric plates (Devillers 1998),
is imposed by the method of overpressured air. (4)
Improvement of the volume-measuring equipment by
measuring a displacement based on the variation of an
electromagnetic field.
In summary, we can see that (1) In saturated field, researchers have developed a number of devices that has the
advantage of varying the temperature with different means (a
thermostat bath, a heating coil that covers the outer wall of the
cell and heater that is immersed in the confining liquid of the
3367
cell). (2) In unsaturated field, three techniques are often used
to control the suction: (1) axis translation is used to generate
suction from 10 to 1,500 kPa; in some special cases, the
suction value can reach 14 MPa; (2) osmotic suction is comprised between 0 and 8.5 MPa and may reach 12 MPa; (3)
vapour equilibrium can cover the suction range from 4 to
350 MPa. The system of controlling or imposing of temperature is applied in the same manner as the saturated field.
The aim of this work is to present a new triaxial apparatus
dedicated to the study of THM behaviour of unsaturated
normally consolidated, overconsolidated, compacted soils
and also soft rocks, where suction is controlled by using the
osmotic technique with the range from 1 to 8.5 MPa and
temperature ranging from 20 to 80 °C.
Triaxial apparatus of the THM tests on saturated
and unsaturated soils
Saturated soils
The study of the THM behaviour of soils on triaxial paths
according to the temperature requires the development of a
particular cell starting from a Wykeham Farrance standard cell
of 3.5 MPa of confining pressure, equipped with a pedestal of
35 or 50 mm in diameter. The heating system is composed of
an insulated heating collar, in which electric resistances are
submerged, having an inner diameter of 146 mm and a height
of 230 mm and developing a power of 1,800 W. This collar is
wrapped around a hollow aluminium roll. A given temperature is imposed on the sample and is controlled by an electronic temperature regulator along with a Pt100 probe. This
makes it possible to obtain a given temperature from the
ambient to 80 °C, with a 3 °C precision. The time required
to carry out an increase in a few degrees depends on the target
temperature and the thermal consolidation time of the sample
for each thermal step. Thermostats like the safety system were
fixed in various places of the cell. A thermocouple is used to
measure the imposed temperature. The choice of an aluminium cell constitutes the best compromise to carry out temperature tests. It is sufficiently light to be handled often and
sufficiently resistant to produce repetitive tests. Figure 1 presents the general diagram of the experimental device.
Unsaturated soils
The triaxial device used for unsaturated tests remains the same
one as that of the saturated tests at an imposed temperature. In
addition, a special pedestal at the bottom of the sample is
developed to make it possible to impose negative pore water
pressure using the osmotic technique.
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Arab J Geosci (2015) 8:3365–3380
Fig. 1 Thermal triaxial cell for
saturated soils. 1 porous stone. 2
neoprene membrane. 3 the
sample. 4 collar heating. 5 force
sensor. 6 displacement sensor. 7
purge and thermostat. 8 pressure
transducer. 9 GDS, controller of
uw. 10 electrical connector. 11
GDS, controller of σ3. 12
temperature regulator. 13 Pt100
temperature Probe. 14
thermocouple. 16 mechanical
loading frame
Osmotic technique The first adaptation of osmotic technique
to geotechnical testing was by Kassif and Ben Shalom (1971)
in an oedometer to study expansive soils. Subsequent work
has been done on a hollow cylinder triaxial apparatus by
Komornik et al. (1980) and on a standard triaxial apparatus
by Delage et al. (1987). Some improvements on the
oedometer have been made by Delage et al. (1992), involving
the use of a closed circuit and a pump to ensure the circulation
of the solution and to allow a more precise control of the
volume of the water exchanged.
Drying of the sample is caused by the process of osmosis,
whereby the sample is placed on a cellulotic, semipermeable membrane which is permeable to water, and a
solution of polyethylene glycol (PEG) is fed beneath the
membrane. The large-sized PEG molecules cannot penetrate the membrane, resulting in an osmotic suction applied
to the sample through the membrane. Various sizes of PEG
molecules exist; they are defined by their molecular
weight. The most commonly used PEG solutions have
molecular weights of 6,000 and 20,000 g. In order to avoid
any penetration of PEG molecules through the membrane,
a PEG solution should be used with a semi-permeable
membrane corresponding to its size. Semi-permeable
membranes are defined by their molecular weight cut-off
(MWCO); a MWCO 14000 membrane is used with PEG
20000 and a MWCO 4000 membrane with PEG 6000
(Delage et al. 1998).
The suction value depends on the concentration of the
solution, the higher the concentration, the higher the suction.
The relationship between osmotic pressure and PEG concentration is well known for two molecular weights (PEG 6000
and 20000), since consistent results have been obtained by
different authors, as shown by Williams and Shaykewich
(1969). The range of possible suctions is confined to
1,500 kPa, for concentrations increasing from 0 to 28 g of
PEG per 100 g of solution, for both PEG 6000 and 20000. A
suitable method for measuring the PEG concentration of the
solution is the measurement of its refractive index (Suraj de
Silva 1987).
Figure 2 was plotted using the experimental results collected by Williams and Shaykewich in a diagram giving the
square root of the suction as a function of the concentration
in grams of PEG per gram of water for PEG 6000 and 20000.
All points fit one line, and the following parabolic relation is
obtained (Delage et al. 1998):
s ¼ 11c2
ð1Þ
where s is the suction expressed in MPa and c is the concentration (grams of PEG per gram water). It is important to note
that the calibration does not depend on the nature of the PEG.
The extension of the osmotic technique to higher suctions was
examined by Delage et al. (1998). They supplemented the
calibration of the solution for very high suctions (8.5–9 MPa)
Arab J Geosci (2015) 8:3365–3380
Fig. 2 Calibration curve of the negative pore pressure versus the concentration of PEG (Delage et al. 1998; Cuisinier 2002)
by using a concentration of 0.79–0.885 g of PEG 6000 per 1 g
of water. Calibration results are plotted in Fig. 2.
Special osmotic pedestal A chamber, 35 mm in diameter and
2.5 mm in depth, is machined in a duralium pedestal of 50 mm
in diameter (Fig. 3). The PEG solution circulates uninterruptedly inside this chamber via input and output pipes. A brass
plate, 2.5 mm in thickness and 35 mm in diameter, perforated
by holes of 1.5 mm in diameter, caps this pedestal. This plate
Fig. 3 Pedestal for osmotic solution
3369
serves as a basis for the sample. Between the perforated plate
and the sample, a disc of dialysis membrane is placed over the
perforated plate. This perforated plate type is largely validated and used on oedometric cells for 15 years (Fleureau
et al. 1992; Fleureau and Taibi 1993). To avoid the damage of the dialysis membrane, perforated plate has round
circumference instead of sharp angles. The perforated plate
and membrane are maintained in place using a nut which
is screwed in a threading in the pedestal. This nut is
equipped with a toric seal ensuring perfect sealing between
the chamber containing the PEG solution and the remainder of the cell. The choice of the dialysis membrane and
PEG solution couple is significant for a good test schedule. The membrane used is SPECTRA/POR n°3,
manufactured in triacetate cellulosis, with a MWCO equal
to 3500, and an average thickness of 47 μm. The dimension of their pores is close to 40A⋅ , generating a very low
permeability, about 10−12 m/s (Yahia-Aïssa 1999). In order
to prolong the lifespan of the dialysis membrane and to avoid
its degradation by bacteria naturally present in the soil, a small
quantity of benzoic acid, acting like an anti-bacterial, is added
to the PEG solution. In addition, the imposition of a negative
pore water pressure in the soil sample using the osmotic
technique is based on the assumption that the air pressure
within the sample is equal to the atmospheric pressure. This
condition is ensured by putting the draining circuit at the top
of the sample to the atmospheric pressure. To avoid evaporation of water related to this boundary condition, an air semipermeable membrane made in Teflon comes to cap the sample
and makes it possible to maintain a homogeneous humidity in
this one.
To maintain a constant concentration of the osmotic solution in the circuit, a circulation between the chamber of the
pedestal and an external tank with a capacity of 1 l is carried
out by means of a peristaltic pump of low flow. The peristaltic
pump has the advantage of not causing turbulences in the
circuit and of not having direct contact with the blades of the
pump and the PEG solution. However, the cyclic crushing of a
flexible pipe may deteriorate the properties of this pipe and
thus decrease its lifespan. To avoid these hazards, neoprene
flexible pipes were selected and the speed of circulation of the
solution is reduced to the minimum to limit wear. The volume
of water exchanged between the soil sample and the osmotic
solution is measured by using a capillary tube of 2.9 mm in
interior diameter, equipped with a pressure transducer of
20 mbar (Fig. 4). A detailed attention is given to certain
accessories of the cell such as the rings ensuring the sealing
of the cell. Indeed, it is noticed that these joints become
deformed in an irreversible way after 2 or 3 cycles of temperature. Nitrile (NBR) joints are chosen. In addition, air traps
and purgings were made to maintain the various circuits
perfectly saturated in order to allow measurements of volume
variation.
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Arab J Geosci (2015) 8:3365–3380
Measurement and control devices
Heating rate
To impose or control pressures and volumes, three GDS
pressure-volume controllers were used. GDScontroller is a
hydraulic jack, controlled by a microprocessor and which
makes it possible to control and measure water pressure
with an accuracy of 1 kPa and its volume with an accuracy of 1 mm3. The whole of GDS controllers is connected to a power station via an RS232 interface. The axial
loading is carried out using a 25-kN loading frame, making it possible to impose constant axial strain rates going
from 1.2×10−5 min to 0.0163 min−1. Displacements are
measured using a 50-mm displacement sensor. The axial
force is measured using a 2.5-t force sensor, with a sensitivity of 0.2 %. Pore water pressure is measured using a
2 MPa absolute pressure transducer. Thermocouples measuring the temperature of the fluid are of type K and
Pt100 probe. This instrumentation is controlled via an
HP station.
The thermal consolidation consists in imposing stages of temperature according to time. The heating rate is controlled by the
need of a good homogenisation of the temperature in the
sample and by the dissipation of the pore water pressure
generated by the variation in temperature applied. For that, this
rate must be sufficiently slow. As an indication, Fig. 5 shows
the change of the temperature according to time for each stage
of imposed temperature gradient. It can be seen that the temperature is stabilized after 1 h. This thermal stabilization is
preceded by a low peak of ΔT=3 °C which is created by the
triaxial cell inertia. Indeed, Fig. 5b, c respectively show thermal consolidation curves at constant initial effective stress and
variation of the pore water pressure curves versus time, for two
steps of temperature (40 and 80 °C). It is noted that the time of
primary consolidation of material is largely reached after 1 h.
In addition, interstitial overpressures are dissipated quasi instantaneously and cause neither variation of effective stress nor
Fig. 4 Unsaturated triaxial cell with controlled temperature and imposed
negative pore pressure by the osmotic technique. 1 osmotic solution
chamber. 2 dialysis membrane. 3 neoprene membrane. 4 the sample. 5
collar heating. 6 force sensor. 7 displacement sensor. 8 purge and
thermostat. 9 PEG solution tank. 10 thermostated bath at 22 °C. 11 pump.
12 pressure transducer. 13 capillary tube. 14 GDS controller. 15 electrical
connector. 16 Pt100 temperature probe. 17 temperature regulator. 18 air
trap. 19 osmotic pedestal. 20 thermocouple. 21 mechanical loading frame
Temperature (°C)
Arab J Geosci (2015) 8:3365–3380
90
80
70
60
50
40
30
20
10
3371
Table 1 PEG solutions tested to calibration with temperature
T(°C)
PEG 6000
T = 22 - 40°C
T = 40 - 80°C
(a)
0.7
1
10
Time(min)
(b)
Void ratio
0.66
0.64
T = 22 - 40 °C
T = 40 - 80°C
t (min)
0.62
0.1
800
1
10
100
uw (kPa)
760
0.37
1.5
0.27
0.8
0.67
0.85
5
8
0.37
1.5
100
e
0.68
Water pore pressure (kPa)
Concentration g/g Succion (MPa) Concentration g/g Succion (MPa)
of water
of water
t (min)
0.1
PEG 20000
(c)
720
680
6000 and 20000, summarized in Table 1, were prepared with
various concentrations. Using a thermostated refractometer as
shown in Fig. 6, the Brix index were measured at imposed
temperatures varying from 30 to 70 °C. Results are plotted in
Fig. 7. It is noticed that for a given concentration, the value of
the Brix index decreases when the temperature increases. To
check the reversibility of the Brix index with the temperature,
these osmotic solutions have been exposed to a 30-80-30 °C
temperature cycle. It is noted that after this cycle, the values of
the Brix index are of the same order of magnitude as the initial
value at 30 °C. Consequently, if it is supposed that the imposed suction by a solution of PEG depends only on its mass
concentration, it can be admitted that for a given concentration, suction remains constant independently of the temperature. Currently, calibration tests using simultaneously osmotic
solutions and salt solutions are in hand to validate this
assumption.
Brix
T = 22 - 40 °C
T = 40 - 80 °C
640
As an indication, the function c ¼ 1−90Brix proposed by Cui
90
t (min)
600
0.1
1
10
Time (min)
100
Fig. 5 Thermal consolidation. a Heating rate, b void ratio versus time
and c water pore pressure versus time
thermal overconsolidation. It can be concluded that the low
peak of 3 °C causes no thermal overconsolidation in our case.
To ensure the homogeneity of the temperature in the sample, the settling time is increased to up to 4 h. To avoid the
remoulding of the sample by the presence of temperature
sensor in its centre, the temperature measurement is taken
around and not inside the sample. The settling time 4 h that
we chose for each thermal step is enough to ensure the
temperature homogeneity within the sample. Comparatively,
Towhata et al. (1993) measured the temperature inside and
outside the sample but that can induce a significant
remoulding of the latter.
(1993), quoted by Cuisinier (2002), was plotted as shown in
Fig. 7. One notices a good agreement at the temperatures near
30 °C. On the other hand, this function must be parameterized
in temperatures beyond 30 °C.
Thermal expansion of the osmotic solution The study of the
volume variation of a PEG solution under the effect of
Effect of temperature on the expansion of osmotic solution
Calibration of the PEG solution The control of the temperature effect on the Brix index and osmotic solutions of PEG
Fig. 6 Thermostated refractometer
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Arab J Geosci (2015) 8:3365–3380
Brix
50
c=(Brix/90)/ (1-(Brix/90))
(Cui , 1993)
40
Degree of Brix (%)
to an imposed suction of 8.5 MPa. This variation can be
described using a linear relation which is written as:
ΔV osm;T
¼ 0:08ðT −T o Þ
ΔV osm;T o
30
PEG 20000
30°C
50°C
60°C
70°C
Cycle 30-80-30 °C
20
PEG 6000
30°C
50°C
60°C
70°C
Cycle 30-80-30°C
10
0
0
0.2
0.4
0.6
0.8
Concentration (g PEG/g of water)
1
c
Fig. 7 Thermal Brix index calibration versus PEG concentration according with temperature
variations in temperature consists in calibrating prepared osmotic solutions with various concentrations. The principle
consists in plunging a tank containing the PEG solution and
equipped with a capillary graduated tube in a thermostated
bath as shown in (Fig. 8). Cycles of temperature stages of 1 °C
varying from 22 to 34 °C are applied. Once the target temperatures are stabilized, the volume variations of the osmotic
solution are raised on the capillary tube. As an example,
Fig. 9 represents the variation of volume according to the
temperature of a solution of PEG 6000 prepared with a concentration of 0.85 g of PEG per gram of water corresponding
ð2Þ
with (ΔVosm, T) as the variation of volume of the PEG 6000
osmotic solution and with c as 0.85 at a given temperature, T
as the temperature in degrees Celsius and (Vosm, T) as the
initial volume of the osmotic solution at T=To (mm3).
It is noted that the osmotic solution is very sensitive to the
temperature and that the variation of volume is proportional
according to the cycles of temperature.
Measurement of water content
The measurement of the variation of the water content in the
soil during the test is deduced from the quantity of water
exchanged between the sample and the osmotic solution.
This measurement is taken using a volumeter made up of a
capillary tube connected to the tank containing the osmotic
solution and equipped with a pressure transducer as shown in
Fig. 4. Figure 10 shows a calibration curve of the volumeter
representing the pressure reading according to the level of the
osmotic solution in the capillary tube. One notices the linear
and reversible response of this calibration.
At ambient temperature, the expression relating the volume
in the capillary tube to the pressure is given by:
ΔV ¼ 3805:Δp
ð3Þ
as shown in Fig. 10, where Δp is the variation of the pressure
indicated by the transducer in kilopascal, ΔV is the variation
in volume of osmotic solution measured in the capillary tube
(mm3).
Fig. 8 Calibration of osmotic
solution dilation versus
temperature. 1 water supply.
2 float. 3 thermostated bath. 4
polystyrene lid. 5 toric seal. 6
immersion heater. 7 osmotic
solution. 8 capillary tube
1. Water supply. 2. Float. 3. Thermostated Bath. 4. Polystyrene lid. 5. Toric seal. 6. Immersion
heater. 7. Osmotic solution. 8. Capillary tube
Arab J Geosci (2015) 8:3365–3380
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ΔVwc is the variation of volume of osmotic solution in the
capillary tube corrected in temperature (mm3). ΔPw is the
variation of the water weight (g). Ps the dry weight of sample
(g). γw is the density of water in grams per cubic millimetre.
The water content calculated at the temperature T and at the
moment t is given by the expression:
wðt; T Þ ¼ wðt o ; T Þ−Δwðt; T Þ
ð6Þ
where w(to,T) is the initial water content at the temperature T.
In addition, to limit the dilation of the osmotic solution with
the temperature and to avoid the corrections related to this
dilation such as described in paragraph 3.2.2, the tank is
plunged in a thermostated bath maintained at ambient temperature of 22±1 °C and the length of the circuit of circulation of
the osmotic solution is reduced to the minimum (Fig. 4).
Fig. 9 Thermal dilation of PEG 6000 osmotic solution at a suction of
8.5 MPa
The variation of the water content of the sample is given by
the following expression:
Δwðt; T Þ ¼
ΔPw ΔV wc
¼
⋅γ w
Ps
Ps
ð4Þ
with
Variation in volume of osmotic solution in capillary tube (mm3)
ΔV wc ¼ ΔV − ΔV osm;T
ð5Þ
8000
ΔV
T= 22°C
Experimental data
dV (mm3) = 3805 *dP (kPa)
6000
4000
Measurement of the variation of volume of the sample
The most delicate measurement to be made in the case of tests
on unsaturated samples is that of their variation of volume. In
the case of the saturated samples, the variation of the volume
is deduced starting from the quantity of water exchanged by
the sample with the tank of the interstitial pressure GDS
controller. In the case of the unsaturated samples, the interstitial pores are filled with water and air (compressible) and
exchanges of water between the sample and the environment
are not enough to evaluate the variation of volume of the
sample. This variation of volume can be measured by measurements of external dimensions of the sample. These measures can be carried out either by the use of proximity sensors
(local measurements of strain) or by the measurement of the
variation of volume of the confining water surrounding the
sample. It is while being based on the latter case that a
calibration curve of the aluminium cell according to the temperature and the pressure is established. The principle consists
in putting a metal cylindrical sample inside the cell and
imposing three different confining pressures (600, 900 and
1,250 kPa), respectively, and maintaining the temperature
constant. Each pressure is imposed during approximately
48 h. Figure 11 represents the calibration curve of the cell
with the temperature at various pressures and can be represented using the following polynomial:
ΔV cell∕ Tcte ¼ AðT Þ:Δσ23 þ BðT Þ:Δσ3
2000
ΔP
0
0
0.4
0.8
1.2
Pressure (kPa)
Fig. 10 Calibration of capillary tube
1.6
2
ð7Þ
where the values of coefficients A(T) and B(T) are summarized
in Table 2, for two values of temperature (22 and 80 °C) which
are related to the unsaturated triaxial tests. ΔVcell/Tcte is the
function of calibration of the variation of volume of the cell
confining water at a given temperature T.
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Arab J Geosci (2015) 8:3365–3380
ΔV sample ¼ ΔV ccw −½ΔV cell =Tcte ð8Þ
where ΔVsample is the variation of volume of the sample at a
given temperature T. ΔVccw is the variation of volume of the
cell confining water.
Effect of the temperature on the pore water pressure
A correct measurement of the pore water pressure is difficult
at high temperature. Indeed, the variation of the viscosity of
pore water and its dilation is very sensitive to the temperature
when it exceeds 40 °C. Beyne and De Visschere (1995) noted
the great sensitivity of the pore pressure, and a wet rag
deposited on the pore water pressure transmitter caused the
measurement to fall in a notable manner. In the case of the
Table 2 Coefficients A
and B at two temperatures 22 and 80 °C
T (°C)
A (T)
B (T)
920
uw (kPa)
0.001
0.002
0.08
0.28
100
880
80
Temperature
840
60
σ = 800 kPa
3
800
40
760
t (min)
720
20
0
22
80
T (°C)
Water pore pressure
Temperature (°C)
The variation of volume of the cell depends on a certain
number of parameters of which, the implementation during
the assembly of the soil sample in the cell and the presence of
various compressible elements (seal joints, pipes, liquid in
temperature and air bubbles). The expression making it possible to measure the variations of volumes of the sample by
this method can be written in the following:
Water pressure in pore pressure circuit (kPa)
Fig. 11 Calibration of triaxial cell water volume versus temperature
undrained tests, the volume of water contained in the sample
and the pedestal of the cell tends to dilate and is put consequently at pressure at the time of heating or the shearing phase.
As an indication, the dilation of the cell water, calculated by
taking into account the thermal dilation coefficient αW equal
to 3.10−4 C, is equal to 200 mm3 when the temperature
increases from 22 to 40 °C. This dilated volume reaches
600 mm3 when the temperature increases from 22 to 80 °C.
It is observed that the prevention of this dilation when the
drainage is closed and generates interstitial overpressures,
which are sometimes higher than the confining pressure
(Tasiaux 1996), This disturbs the behaviour of the sample
and consequently all measurements of variation of volume
which result from this.
In order to highlight this phenomenon, a test is carried out
by using a metal sample. Water being at the base of the cell
cannot escape, and a significant increase in the water pressure
in the measuring circuit of the pore water pressure occurs at
the temperature of 80 °C. Figure 12 shows the evolution of the
pore water pressure and the temperature at a confining stress
σ3 =800 kPa. It shows that this increase continues even beyond the confining pressure and takes the sample off the
pedestal, and then the pressure is stabilized gradually when
the temperature becomes constant.
In the case of this THM cell, the drained tests on unsaturated soil samples are carried out with an imposed pore water
pressure using the osmotic solution. Interstitial overpressures
due to the temperature are dissipated quasi instantaneously as
shown in Fig. 5c. The corrections of volume of drained water
are made according to the temperature.
100
200
300
400
500
Time (min)
Fig. 12 Variation of water pressure in pedestal circuit versus time for an
increase of temperature (22–80 °C)
Arab J Geosci (2015) 8:3365–3380
3375
Table 3 Identification of sandy clay
wL (%)
Ip (%)
%<80 μm
%<2 μm
d60/d10
38
19
95
53
21
Preliminary tests on soil samples
Material
A laboratory material which has many results on saturated
triaxial paths at ambient temperature (Sayad-Gaidi 2003) is
selected for preliminary tests using this new triaxial cell. The
material consists of 90 % of kaolinite and 10 % of mixture of
sand and silica. The various physical characteristics of material are given in Table 3. The initial state of material is a
normally consolidated state, prepared starting from a slurry
(w = 1,5 w L ) and consolidated in a consolidometer on
oedometric path to an axial effective stress of 100 kPa. After
consolidation, a sample is cored and placed in the triaxial cell.
Then, a confining stress of 50 kPa is applied on the sample in
order to maintain it and also to push the butyl membrane
against the sample.
Figure 13 shows the soil-water characteristic curve
(SWCC) carried out on the normally consolidated material,
prepared starting from a slurry (w=1.5 wL), then dried up to
200 MPa of suction. The dried slurry was then wetted until
complete saturation.
Saturation of the sample
The saturation of the sample is carried out by using as a
reference the measure of the B Skempton coefficient, defined
as the ratio of the increase of the pore water pressure Δuw to
w
the increase of the isotropic confining stress Δσ3 B ¼ Δu
Δσ3 .
Fig. 13 Soil-water characteristic curve (SWCC) of the material
The follow-up of the evolution of the B parameter according
to the total confining stress depends primarily on the characteristics on compressibility coefficients βS and βW, respectively of the solid skeleton and pore water, on the porosity n of the
material and on its degree of saturation Sr. The saturation
phase consists in increasing the confining stress σ3 and measuring the increase in the pore water pressure Δuw, with
undrained conditions and isotropic path. For this material,
backpressure uw =700 kPa is needed to reach a value of B≈
0.9. This value of uw will be applied to all the following tests.
Compressibility paths
The study of the compressibility of material proceeds in three
phases:
–
–
–
The first phase is a thermal consolidation; it consists in
applying a temperature to the sample with the opened
drainage, by maintaining the mechanical effective stress
constant and equal to 100 kPa. At the time of this phase,
only the temperature varies.
The second phase is a mechanical consolidation; it consists in applying mechanical stress by stages, at constant
temperatures of 60 or 80 °C. Figure 14 shows the thermal
and mechanical follow-up paths.
The third phase is the hydrous consolidation (unsaturated
phase). When the isotropic effective stress is reached at
the end of the second phase at a given imposed temperature, one carries out the cancellation of the positive pore
water pressure of 700 kPa using a negative ramp of 1 kPa/
mn, while preserving the constant confining effective
stress. This phase consists in desaturation of the sample
Fig. 14 Thermal and mechanical consolidation paths
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Arab J Geosci (2015) 8:3365–3380
by applying negative pore water pressures using the osmotic method. The solution used makes it possible to
sweep negative pressures from 3 to 8.5 MPa. Three
imposed negative pressures are applied (3, 6 and
8.5 MPa) with a total confining stress of 1,250 kPa as
shown in (Fig. 15). During the equilibrium of the sample
with the negative pore water pressure in the triaxial cell,
the variations of volume and the water content of the
sample are given using the methods described in the
preceding paragraphs.
Preliminary results
Thermal saturated isotopic paths
Figure 16 represents the variation of the final void ratio at
equilibrium after the heating phase at constant isotropic stress
according to the logarithm of the temperature. This variation
could be described by a linear semi-logarithmic law
(Belanteur et al. 1997; Rahbaoui 1996). Figure 17 presents
the isotropic compressibility curves for three values of imposed temperature in the plan [log p′, Δe]. All the curves
follow initially an overconsolidated path until a stress about
200 kPa before joining the normally consolidated path. This
overconsolidation is due to the loading history of the samples.
It is a slurry prepared initially with w=1.5 wL and consolidated in a consolidometer at σ′v =200 kPa. Then, samples were
taken to carry out triaxial tests. It is noticed in addition that the
thermal consolidation does not completely erase this
Fig. 16 Variation in void ratio versus temperature after thermal consolidation at constant isotropic effective stress of 100 kPa
mechanical overconsolidation. It is noted that, in the normally
consolidated field, the paths are parallel whatever the imposed
temperature. The value of slope Cc of the normally consolidated paths is not affected by the imposed temperature. One
notices in the case of this material that the variation of the void
ratio Δe is the same for both temperatures (60 and 80 °C).
This observation confirms the one of several researchers
(Campanella and Mitchell 1968; Fleureau 1972, 1980;
Despax 1976; Eriksson 1989; Norotte 1991; Rahbaoui 1996;
Belanteur et al. 1997; Tanaka et al. 1997 and Sultan 1997).
0.00
Δe
Variation of void ratio
-0.04
-0.08
Imposed temperature
T = 22°C
T = 60°C
T = 80°C
-0.12
p' (kPa)
-0.16
100
200
400
600 800
1000
2000
Isotropic effective stress (kPa)
Fig. 15 Hydrous consolidation paths
Fig. 17 Variation of void ratio versus effective isotropic stress for three
imposed temperatures (22, 60 and 80 °C)
Arab J Geosci (2015) 8:3365–3380
3377
Hydrous consolidation at imposed temperature and stress
w (%)
Sandy clay
Drying path
σ3= 1250 kPa
24
0.1
Δe
0.05
Test 1
Variation of void ratio
0.0
Test 2
-0.05
-0.1
-0.15
-0.2
-0.25
-0.3
-0.35
t (h)
-0.4
0
100
200
300
400
500
Time (hours)
Fig. 18 Volume change on drying path at T=80 °C and σ3 =1,250 kPa
carried out by the measurement of the variation of confining water
volume surrounding the sample
s=3,0 MPa,
s=8,5 MPa,
s=3,0 MPa,
s=8,5 MPa,
Water Content (%)
During the desaturation phase, the variation of volume
and water content were measured using the techniques
described in the preceding sections. With regard to the
measurement of the variation of the sample volume,
Fig. 18 shows an example of measurement taken on two
samples during the desaturation phase under a confining
stress of σ3 =1,250 kPa and a temperature of 80 °C. It is
noticed that the order of magnitude of the variation in
void ratio is 0.2 for the first sample and 0.3 for the
second. These values are too high. Indeed, the sample is
strongly consolidated (σ3 =1,250 kPa), and its void ratio is
near to the shrinkage limit. Consequently, this measurement method does not lead to exactly quantify and appreciate the volume variation for a given suction. It is recommended to choose another technique based on the
integration of immerged thermal sensors around the sample. Works are actually started in the laboratory to prospect this novel idea. With regard to the measurement of
the variation of the water content, Fig. 19 shows the
evolution of the water content according to time for two values
of suctions and two imposed temperatures. One notices
that for T=22 °C, the variations of water content are
respectively about 2 % for s=3 MPa and of 3 % for
s=8.5 MPa. In addition, for T=80 °C, these variations
of water content are respectively about 3.5 and 7 % for
s=3 and 8.5 MPa. These values are in a good agreement with those measured on the same sample on
drying and wetting paths (Ghembaza 2004).
T=22°C
T=22°C
T=80°C
T=80°C
20
16
12
t (h)
8
0
200
400
600
800
Time (hours)
Fig. 19 Variation of water content versus time during drying paths,
corrected according with temperature
Deviatoric paths
After the hydrous consolidation (desaturation), several
deviatoric loading drained tests were carried out on unsaturated samples at both constant suction and temperature. The
deviatoric loading is carried out at a very low speed of
1 μm/min. The axial deformation is continued until approximately 30 %. The determination of the volume and water
content changes of the samples require corrections due to the
thermal dilation of the PEG solution. The tests carried out
using this triaxial cell made it possible to study the effect of
temperature on the failure criteria of unsaturated soils, e.g. a
clayey rock from a radioactive wastes storage experimental
site. At the end of the shearing triaxial test, various parameters
are measured. Figure 20 shows an example of the results of
unsaturated triaxial compression tests on a normally consolidated sandy clay, for two imposed suctions (0 and 8.5 MPa)
and two temperatures (22 and 80 °C) in [ε1,q] plan. It is
noticed that the maximum strengths decrease when the temperature increases (Ghembaza 2004). Near the origin of the
[ε1,q] coordinate system, one observes an increase in axial
strain with an almost null deviatoric stress. This is due to a bad
contact between the piston and the top of the sample at the
beginning of the loading which is provoked by the presence of
metal collar heating and which does not allow to visualize
well the good contact between the piston and the sample
during the experimental setup (Biarez and Taibi 1997).
Figure 21 shows the same result in [p′,q] plan for different
imposed suctions and two temperatures (22 and 80 °C). It is
noticed that the envelope of maximum strengths decreases
when the temperature increases.
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Arab J Geosci (2015) 8:3365–3380
Deviatoric stress (kPa)
1600
q (kPa)
1200
Conclusions
800
T = 22°C
s=0
s = 8.5 MPa
400
T = 80°C
s=0
s = 8.5 MPa
ε1%
0
0
10
20
30
Axial strain (%)
Fig. 20 Deviatoric stress versus axial strain for two imposed temperatures (22 and 80 °C) and two imposed suctions (3 and 8.5 MPa)
In general, one observes a good adaptation of the thermal
triaxial apparatus in particular for the unsaturated paths.
However, it appears that the measurement of the variation of
the water content still remains uncontrollable. This difficulty
could be due to the thermal dilation of the osmotic solution
and the cell itself in spite of their calibration according to the
temperature. The appearance of air microbubbles in the circuit
following the circulation of the osmotic solution using the
peristaltic pump could explain the disturbance in the measurement of the quantity of exchanged water. To cure it, the circuit
is equipped with an air trap.
1400
q (kPa)
M = 1,Saturated Criteria Failure at T = 22°C
T = 22°C
T = 80°C
1000
T = 22°C
The aim of this paper is to present new experimental technique
adapted to the study of the effect of temperature on the
behaviour of saturated and unsaturated soils. The techniques
usually used at ambient temperature are adapted and improved
to be able to study the THM behaviour on triaxial paths of
unsaturated soils, under controlled negative pore water pressure and temperature conditions.
On unsaturated triaxial paths, the osmotic technique using
solutions of PEG 6000 is adapted in a thermal triaxial cell. The
tests with imposed negative pore pressure are drained tests.
The determination of the volume and water content changes of
the samples require corrections due to the thermal dilation of
the osmotic solution. The principal experimental difficulties
and limits which are in the measurement of the volume variations of the unsaturated soils in the triaxial compression cell
were shown.
The results of laboratory study using a temperaturecontrolled triaxial cell have been presented. Thermal effects
on the mechanical behaviour of unsaturated sandy clay were
analyzed by comparing tests at various temperatures and
suctions. In the case of unsaturated soils, the maximum
strength decreases when the temperature increases, in spite
of the hardening produced by the negative pressure. This
strength increases with the suction, at a given temperature.
New techniques are under development such as the measurement of volume variation using proximity sensors, immersed in water at high temperatures and also the measurement of the degree of saturation during the test, using for
example a technique based on the propagation of ultrasounds
within the soil sample. The applicability of this new apparatus
concerns environmental geotechnics and geological engineering such as the design and the behaviour of the engineered
barriers as well as the behaviour of the receiving layers of
storage sites like radioactive waste.
s = 3 MPa
s = 8.5 MPa
T = 80°C
600
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