| Summary: | The engineering profession has been called upon by the
public in recent years to provide ever increasing degrees of
containment for low level waste contained in shallow subsurface
waste containment facilities. To provide this
protection, the use of engineered liners or barriers has
become a common design feature. In many cases, these
barriers are constructed out of natural clays or artificial
mixtures of clay minerals.
The use of soil barriers to contain waste products
consisting of strong electrolyte solutions, has met with
mixed success. Clays of lowest permeability are also those
whose behavior is the most influenced by the presence of
salt solutions. Failures of clay liners exposed to
electrolyte solutions have not been well documented. The
mechanism for failure seems to be the result of shrinkage of
the clay, which then leads to the development of a secondary
structure of cracks and fissures. Field cases of liner
failure have often been described as occurring as a result
of "osmotic desiccation".
The general objective of this study was to define, and
quantify, the mechanisms controlling the rate and magnitude
of volume change in clay soils exposed to strong electrolyte
solutions. A review of the literature presented two
possible mechanisms
termed osmotically
consolidation.
Osmotically induced consolidation occurs as a result of
for osmotic volume change.
induced consolidation and
These were
osmotic
rapid flow of water out of the sample in response to osmotic
gradients. Osmotic consolidation occurs as as result of a
reduction in the net electrostatic repulsive stresses
between clay particles. A general theoretical description
of osmotically induced consolidation and osmotic
consolidation was developed. A phenomenological approach was
adopted to describe fluid flow in response to osmotic
gradients. A Darcy type flow law was used to related
osmotic flows to osmotic gradients through a conductivity
term called the osmotic permeability. To describe osmotic
consolidation, the osmotic pressure of the pore fluid was
selected as a stress state variable. Volume changes were
linked to the osmotic pressure of the pore fluid through a
constitutive relationship. The soil property used to define
changes in soil volume due to osmotic pressure changes was
called the osmotic compressibility.
A numerical solution to the theoretical description of
osmotic flow and volume change was developed using finite
element techniques. This model was used to characterize the
processes of osmotic and osmotically induced consolidation.
A laboratory program was undertaken to monitor the osmotic
flow and volume change in two clays; Regina Clay and an
Ottawa SandI Na montmorillonite mixture. From the results
of these tests the dominant mechanism of volume change for
these clays was found to be osmotic consolldatipn. The test
procedures developed allowed the soil properties describing
osmotic flow and volume change to be evaluated.
A technique was developed by which the electrostatic
repulsive stresses within a clay could be measured
indirectly through laboratory testing. The results
indicated that the onset of fracturing may be predicted by
comparing the change in the net repulsive stress that occurs
as result of changing pore fluid concentrations, to the
confining stress within the soil.
|
|---|
