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To determine the impact of temperature on the hydro-mechanical properties of the bentonite
and the reversibility of the modifications observed.
Laboratory work has also been done to optimize both the design of the single hole granite probes
and the interpretation of the TDR measurements made in the FEBEX in-situ tests.
Tests were done on small cylindrical specimens to investigate the effect of the hydraulic gradient on
the permeability of bentonite. The results from the test with low gradients could indicate if there
exists a critical gradient for the bentonite. The critical gradient is the hydraulic gradient below
which flow occurs but it is not Darcian. The possible threshold hydraulic gradient depends on the
dry density and the injection pressures applied.
FEBEX bentonite compacted to dry density 1.65 g/cm 3 with hygroscopic water content was hydrated
with granitic water in 40-cm long cylindrical cells under isothermal conditions and under
thermal gradient for five years to investigate the effect of the thermal gradient on hydration. The
initial saturation of compacted bentonite takes place quicker under thermal gradient than at laboratory
temperature. Afterwards, the water intake is higher for the sample tested under room temperature.
In both tests, the 10 cm of bentonite closest to the hydration surface seem to have reached
steady-state conditions with respect to relative humidity (Figure 1).
Relative humidity (%)
100
90
80
70
60
50
40
30
0 10000 20000 30000 40000 50000
Time (hours)
RH1
RH2
RH3
Relative humidity (%)
196
100
90
80
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40
RH1
RH2
RH3
30
0 10000 20000 30000 40000 50000
Time (hours)
Figure 1: Evolution of relative humidity in the isothermal test (left) and the test performed under
thermal gradient (right) during infiltration (sensor 1 placed 30 cm from the bottom, sensor 2 at 20
cm and sensor 3 at 10 cm). The thick vertical lines indicate periods of failure
A heating/hydration test in a 60-cm long bentonite column was dismantled after 7.6 years operation.
The final average degree of water saturation was 92 percent, and an important gradient of water
content and dry density was originated along the column, what indicates that the state analysed is
still a transient one. Those gradients condition the hydro-mechanical properties of the bentonite,
leading to an inhomogeneous distribution of swelling capacity which nevertheless, after the long
treatment, still remains in values close to those expected for the untreated bentonite.
Systematic small scale tests to study the movement of moisture have been performed on MX-80
bentonite. In these tests a number of different thermal and thermal-hydraulic gradients have been
applied across the samples and transient measurements of moisture content, temperature, dry density
and ion concentrations have been made. Figure 2 illustrates chemical concentration variations
for a thermal gradient test. The movement of moisture due to drying processes and chemical ions
due to the so called heat pipe process can be clearly observed.