RD&D-Programme 2004 - SKB

Investigations of the hydromechanical properties of the bentonite, above all the influence of
swelling pressure evolution, mechanical constraint and external pressure on the pore pressure
potential at incomplete water saturation, are under way. The results will be used to determine
values for material parameters in Code Bright. With a better model for how the porosity
distribution changes during swelling and compression, and for how the redistribution affects
e.g. the pore pressure potential, it will be possible to model processes around gaps, for example
at the transition between rock and buffer, in a physically realistic manner.
A number of model studies have also been done with Abaqus, see section 17.2.4.
Field studies
Experimental studies of the coupled THM processes under unsaturated conditions have been
conducted both in the field and in the laboratory. A number of field tests are under way in the
Äspö HRL where these processes are being measured under realistic conditions. The Prototype
Repository, Canister Retrieval Test and TBT test are the foremost examples. These tests cover
both wet and dry conditions and will be conducted for various lengths of time. This means that
the measurement results from installed sensors, as well as measurements on samples taken from
future excavations, will be able to be used for many different wetting cases in the evaluation of
the coupled THM processes by comparisons with model calculations.
Laboratory experiments
Fundamental understanding of and relationships between the negative pore water pressure, the
swelling pressure during wetting and the degree of water saturation are being investigated in
an ongoing doctoral project whose results will be reported this year. In this project, laboratory
technique for measurement of the swelling pressure at different relative humidities has been
developed. The methods have been used to determine the influence of pressure (swelling pressure
and total pressure) on the negative pore water pressure. These studies consist of e.g. enclosing
a sample of unsaturated bentonite in an oedometer and controlling the relative humidity
in the sample either by circulating air with a given relative humidity through filters that are in
contact with the sample, or by surrounding the sample with an atmosphere with a given relative
humidity. By changing the relative humidity gradually and continuously measuring the relative
humidity in the middle of the sample, measuring the sample’s swelling pressure and determining
the sample’s water ratio, an understanding has been gained of the evolution of the swelling
pressure and its influence on the negative pore water pressure, as well as important relationships
for the material models.
The wetting process with associated swelling and homogenization has been studied in the
laboratory for the KBS-3H concept. A 1:10 scale model of a horizontal deposition hole has been
built in the laboratory. A deposition tunnel with two parcels containing canister, bentonite block
and perforated steel container as well as spacer plugs was modelled in the experiment. The rock
was simulated by a steel tube partially lined with filters for wetting of the bentonite and for the
application of hydraulic gradients between the filters so that the resulting flow can be studied.
Figure 17-3 shows a cutaway diagram of the experimental set-up.
The model was instrumented with total pressure sensors, pore pressure sensors and RH (relative
humidity) sensors, which were attached to the outer steel tube and to one of the canisters. The
canisters were not heated.
The experiment was terminated one year after start to permit evaluation of wetting, homogenization
and interaction between buffer and perforated container. The results show that wetting
and pressure build-up had proceeded largely as expected but more slowly towards the end. To
study the hydraulic function of spacer blocks and the rest of the buffer, a hydraulic gradient was
applied between the filters in different increments. The hydraulic conductivity of the bentonite
behind the perforation has been evaluated at about 10 –12 m/s, which is slightly higher than
expected from the swelling pressures and the water ratio measured in the model test and from
RD&D-Programme 2004 213