International Pipeline Conference — Volume I
ASME 1998
IPC1998-2054
THE SIGNIFICANCE OF SOIL FREEZING FOR STRESS CORROSION CRACKING
Peter J. W illiam s
Thom as L. W hite
J. Kenneth Torrance
Geotechnical Science Laboratories, Carieton University
1125 Colonel By Drive
Ottawa ON K1S 5B6 Canada
Tel: (613)520 2852 Fax: (613)520 3640 E-mail: [email protected]
ABSTRACT
The microstructure of soils (the arrangement of pores and voids,
aggregation and surface characteristics of particles) is substantially
modified by freezing. Soils so modified differ, in a number of
important properties, from soils not previously frozen. Furthermore,
each time a soil is frozen there is a redistribution of particles,
moisture and solutes. Corrosion of buried pipes is known to be
related to the ground conditions. Accordingly the particular nature
of frozen ground needs consideration in this respect.
Studies of microstructure o f samples of freezing, frozen and
unfrozen soils, many obtained from a full-scale experimental study
of the effects o f freezing on a buried pipeline, have provided an
explanation for measured changes in bulk geotechnical properties of
the materials. The microstructure viewed by optical microscopy,
reveals the soil structure as having a complex and striking
dependence on freezing history. Scanning electron microscopy shows
further details in clay rich soils.
Freezing at tem peratures occurring in nature normally does not
convert all the soil w ater to ice. The effects of particle surface forces
is to reduce the freezing point of the w ater nearest a mineral
surface. The distribution of solutes is radically altered, with pockets
of high concentration interconnected by a liquid phase of varying
concentration.
A variety o f other effects, relating to chemical and mechanical
properties o f soils subjected to freezing, have been demonstrated or
can be postulated. Some o f these are important in corrosion
phenomena. The stresses that have been shown to occur in a pipe
as a result o f frost heave in the freezing soil, will also tend to
increase the possibility of stress corrosion cracking.
INTRODUCTION
Most people would agree that understanding stress corrosion
cracking (SCC) requires understanding the soil, or ground,
environment o f the corroding structure. There is a significant body
of knowledge on the importance of the soil conditions in those
regions where the problems of SCC are greatest. The cold regions
of the world are not those we think o f in this context. Yet in fact,
the cold regions, those where freezing of the ground is extensive,
have particular importance: in Russia the troubled pipeline network
of Siberia in the permafrost regions is of great political and
industrial significance. In North America we have the prestigious
example of the TransAlaska oil pipeline, and the smaller, Canadian
Norman Wells-Zama oil line (although no large gas pipelines at all)
passing through the permafrost. In southern Canada and the
northern US there are many pipelines where the adjacent soil may
be frozen for much of the winter.
Probably the world’s largest oil spill on land, the Russian, Komi
spill of 1994, has been ascribed to corrosion problems. Those of us
who study the effects of frost heave on pipelines have also
considered how this failure and others might relate to the cold
conditions. In fact, there may well be multiple causes. A pipe may
have failed due to corrosion which had been exacerbated by the
stresses associated with the frost heave displacements and also by
the particular conditions for corrosion that occur in freezing ground.
Even geotechnical engineers often do not realise just how
significantly different freezing soil is from ’normal’, that is unfrozen,
soil. Frozen soil is usually thought o f as cold and bard. Both
conditions sound relatively benign.
THE NATURE OF FREEZING GROUND
When water freezes in small spaces it behaves differently than
’normal’water. Notably the tem perature at which freezing occurs is
less than 0 ° (by up to a few degrees) and the pressure conditions
of the ice and the w ater are quite different from those of ’ordinary’
or bulk ice - that is, ice which is not confined in small spaces. As
much as half of the moisture content, in some clays, is unfrozen at 5 ° C. Sands and other coarse-grained soils have little unfrozen water
while medium textured soils lie between these extremes. In brief, at
the microscopic scale the behaviour is dictated by forces at the
molecular level, associated with the interfaces between the mineral
surfaces of the particles, the w ater and the ice (Williams and Smith
1991). The area of mineral particle surface in a gram of clay can be
as large as the area o f a football field (consider how the area of
particle surface increases as the particles are divided into smaller
and smaller particles). In turn these surface forces and the
distribution of the stresses at the microscopic level cause
displacements of the particles and of the w ater and ice (White and
Williams, 1994). Plate 1 shows the result of a single freezing (and
thawing) of a silt soil. The change in the soil’s microstructure from
the unfrozen state to the once-frozen state is very clear.
We all know how important the microstructure of steel is for its
properties and it should come as no surprise that the microstructure
of a soil has effects on the mechanical, hydraulic, chemical, and
geotechnical properties. How significant is freezing of the ground
likely to be in the question of stress corrosion cracking? Because
the properties of freezing soils are so different to those of unfrozen
soils, they must be considered in the analysis of SCC in the cold
regions.
Copyright © 1998 by ASME
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the pipeline (Williams et al. 1993), there are a variety o f less direct
effects which may be even more important.
If the soil which has experienced frost heave thaws, its strength
falls to a value lower, probably much lower, than it had prior to
<*>)
Plate 1. (a) Micrograph of Caen silt, unfrozen (b) Similar
m aterial after one freeze-thaw cycle (both frames
are 13.5 mm wide in reality).
UNIQUE CHARACTERISTICS OF FREEZING SOILS
Mechanical properties: These are dominated by creep and by the
process of frost heave. The former is the tendency to deform over
long periods of time at rather small stresses, notwithstanding the
demonstrably high strength under short term loading. Frost heave
is a related phenomenon best known as causing irregular uplift of
the ground surface where winters are cold; frost heave may disrupt
driveways or highways, building foundations and underground
services (Ladanyi and Andersland 1995). It is less widely known that
it is due to the movement of water from unfrozen soil to the
freezing zone giving rise to layers of ice within the frozen soil (PI. 2).
Known only to specialists is that the water, in a still liquid state
within solid-frozen soil, moves slowly along microscopic pathways in
tiny pores and along particle surfaces to freeze at some lower
temperature where it gives rise to high stresses and small
deformations within the rock-like material.
A pipeline buried in frozen ground will often experience high
stresses on this account with a slowly developing deformation
(Williams 1989). In addition to the direct effects of frost heave on
Plate 2. Frozen silt soil (sample approx. 8 cm high) showing ice
layers.
freezing. A pipeline may subside -the essential problem of oil
pipelines in permafrost - or if a gas line, it may be displaced
upwards by buoyancy in the water saturated soil.
The weakened soil following thaw, on sloping ground will be
exposed to risk of erosion by surface meltwater such as to expose
the pipe. There is abundant evidence (PI. 3 ) o f all these effects in
the Russian pipeline system in the permafrost regions (Williams,
1992, Kharionovsky, 1992). The problems associated with thaw have
been tackled in the TransAlaska pipeline, although at great expense,
and in the Norman Wells - Zama oil line.
There remains the problem of SCC. Even when the stresses
originating through freezing and thawing are held within acceptable
operating limits, these stresses are a factor in the development of
SCC over the life-time of the pipeline. The thermodynamic
processes associated with freezing of soils must be considered,
because of the stresses they produce in a pipe.
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Plate 4 (cont’d). (b) sim ilar m aterial contaminated with
10 parts per million aviation diesel and exposed to
two further freeze-thaw cycles.
P late 3. Pipeline
deformed by frost heave and exposed
by erosion of thawed frost heaved materials. Siberia.
When w ater freezes pure ice is formed and dissolved materials
are excluded. Solutes become concentrated in the water that
remains (in certain circumstances, for example, sea ice, the
crystalline ice may come to enclose a small body o f highly
concentrated solution or even o f precipitate - but the dissolved
material does not become integral with the ice). In frozen soils the
solutes will be concentrated in unfrozen w ater located on the surface
of particles and in small pores, and there may be pockets o f highly
concentrated solution.
Physico-chemical properties: The effects of freezing are so
fundamental, as is evidenced by the modification of microstructure,
that it is not surprising they modify the chemistry of the soil as well
as its physics. M ore surprising is the degree to which a small
admixture, literally parts p er million of certain chemicals, can cause
substantial modifications o f the microstructure (Pis. 4 and 5) of soils
exposed to freezing (W hite and Coutard 1998) . This is further
evidence of the close and sensitive relations of soil chemistry to soil
microstructure in freezing conditions.
Plate 5. Electron micrographs of sam e m aterial as in Plate 4 (b).
An aggregate structure has developed with the individual a l t
grains closely packed into th e aggregates, with large
c h a n n e ls between intersections o f aggregates, (a) image
length approx 0 3 mm
Plate. 4 (a) Optical micrograph of Caen silt after
exposure to two cycles of freeze-thaw.
Thermal properties: The fact of freezing is always important for
thermal properties. The conversion of w ater to ice requires the
extraction of latent heat of fusion, 334 J g m 1. By contrast, to lower
the tem perature of w ater by one degree C requires the removal of
only 4.12 J g m 1. This means that tem perature changes will occur
much more slowly in freezing ground. Ground freezing is a climatic
effect and the thermal properties are important over the whole
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facility at the Centre de Geomorphologie, CNRS, Caen, France,
(extensively reported, see Geotechnical Science Laboratories, 1997).
The experiment showed how a pipe could be deformed as a
consequence of differential frost heave arising from a transition from
silty to sandy soil (Figure 1 and Fremond and Mikkoia 1993,
Williams et al. 1992).
The results of this study continue to be analysed and with the
slow-down in plans for extraction o f oil and gas in the Canadian
Arctic, the design response for pipelines in permafrost is still
unclear. It was established that slow heaving and accompanying
deformation occurs in frozen ground when the soil is a degree or so
below freezing point and is therefore frozen solid (Smith and
Patterson, 1989). Furthermore the associated heaving stress is
transmitted to the pipe and the effect may increase with time even
after there is a quasi-steady tem perature state.
Plate 5 (coat’d) (b) further enlargement, o f angle
aggregate seen in left centre o f (a>
Chilled Air
Ground Surface -Q.7S*C
E
range of studies associated with cold terrain. Quite generally,
therefore, therm al properties will have to be considered in SCC
although the significance is not as immediately relevant as the direct
frost-heave induced stresses.
Biological properties: It is wrong to think of frozen ground as a
barren material with respect to biological activity. Apart from the
living organisms that survive in ice (such as the algae which may give
a pinkish colour to glacier ice, or the bacteria leading to
deterioration of frozen foods with tim e) the often large amounts of
still mobile w ater provide an environment for a range of
microbiological organisms. In so far as organisms such as bacteria
are believed to play an im portant role in SCC, it should be noted
that this should also be true in freezing ground -although the
organisms will probably be fewer and their activities less than in
unfrozen materials.
SIGNIFICANCE OF THE PROPERTIES OF FREEZING SOIL
FOR THE PROCESSES OF STRESS CORROSION CRACKING
In speaking generally of the importance o f soil freezing in
pipelines it is im portant to rem em ber that gas pipelines are often
colder than the surrounding ground and therefore can cause
freezing. Oil pipelines in the cold regions are frequently warmer
than the natural soil and thus are particularly associated with
problems of thawing. Stress corrosion cracking needs to be
considered from both points of view. Indeed, many pipelines will be
exposed to conditions where tem perature gradients are, at different
times, towards and away from the pipe. Certainly the direction of
the temperature gradient is very important when considering in
detail what occurs in the soil at freezing temperatures.
Frost heave: Two main effects of frost heave must be considered.
Firstly, frost heave itself may be sufficient to render a pipeline
inoperable (Williams 1989). If the pipeline is already weakened by
SCC then com pared to otherwise similar ground freezing conditions,
the frost heave is likely to be a still larger threat to the pipeline.
Secondly, the SCC process itself is accelerated by the stresses
produced in the pipe by frost heave.
Concern over the uncertain magnitude of the mechanical effects
of frost heave led to a major experimental study of an 11" diameter
gas pipe placed in freezing ground in a controlled environment
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Figure 1. (a) arrangement o f pipeline in experimental
facility (b) deformation of pipe caused by freezing
o f soil (note vertical scale is mm, horizontal is m).
In the experiment the pipe was held at a lower (freezing)
tem perature than the soil. It was observed that a cavity developed
below the pipe and running throughout its length in the silt section.
During the period of freezing, occasional observations revealed
substantial accumulations of ice in the cavity. The cavity could be
clearly seen on excavation following thaw (PI. 6).
The cavity appears to be analogous to that known to form
naturally under boulders as these are lifted relative to the soil
matrix, to ultimately appear as a 'growing stone’ at the ground
surface (Washburn 1979, Williams and Smith 1991). Similar effects
result in fence posts, utility poles and similar being progressively
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Plate 6. Cavity below pipe, following freeze-thaw cycle, revealed
by excavation (the cavity is seen as the dark opening
immediately below the pipe).
displaced upwards sometim es to the extent of their falling over. The
mechanism, or more probably, mechanisms, have been widely
discussed in the geological literature. Broadly speaking, the object
(in this case the pip e) is either heaved up by a growing ice
accumulation below the pipe, or the pipe is pulled up by expanding
freezing soil adhering to it laterally. In either case, at thaw the
extraneous object is unlikely to settle back at its original level
because o f small stones, gravel etc that tend to fall into the space
vacated by the ice. T he significance o f the cavity and the attendant
soil shear is that they show the forces which freezing can exert and
which may disrupt a protective coating.
The formation o f discrete ice masses in freezing soils is an effect
of the fine-porous nature of the soil and water migration in the
direction of the tem perature gradient. A space between the pipe
surface and disturbed coating may offer the thermodynamic
conditions for ice accumulation, close or attaching to the steel
surface. This may be an analogous situation to the disbondment of
coatings in the presence o f montmorillonite (shrinking and swelling)
clays (Wilmott, pers. comm.). Particularly significant would be the
presence of small soil masses trapped between the pipe and the
coating, as may accidentally occur. If w ater can reach the entrapped
soil through a punctured coating, further ice accumulation there is
likely with further disruption o f the coating. Solutes might also be
concentrated close to the pipe surface.
PHYSICO-CHEMICAL EFFECTS OF FREEZING IN RELATION
TO CORROSION AND STRESS CORROSION CRACKING
The water in soils generally contains dissolved salts in low
concentrations (certain soils, in salt marshes, solonetz soils and
others, of course contain high concentrations and require special
attention). As the soil w ater freezes the salts excluded from the ice
become concentrated in the remaining water. The films are
important in frost heave. They must also be continuous paths of
relatively high electrical conductivity through the frozen soils. The
effects will be greater the finer-grained the soil, because of the
larger amounts of mineral surface and therefore of film water in
such soils.
Because o f the water that remains unfrozen, dissolved materials
are mobile. The permeability of frozen soils to water, although
sounding a contradiction in terms, is in fact regarded by many as the
most important topic for research in to the geotechnical behaviour
of freezing ground. In fine-grained soils movements of water,
towards lower tem perature, are responsible for the continuing frost
heave of the frozen material over time. As the w ater moves it
carries ions with it - advective transport - in the directions of the
temperature gradient. Cbuvilin et al., (1996) describe the movement
of ions in the water films around soil particles. Nye (in Dash et al.
1998) discusses the unfrozen w ater al crystal interfaces in ice, which
represents another possible path for ion movement, through ice
occupying pores or ice layers in the soil.
Chemical diffusion o f ions occurs in the unfrozen water in the
frozen soil but this is generally in the opposite direction because
solutes move from high to low concentrations (and the higher
concentrations occur at the lower tem peratures). In fine-grained
soils the advective transport towards and into the freezing layer
exceeds chemical diffusion in the opposite direction, when frost
heave is actively occurring (Chuvilin 1998). A complication is the
adsorption of ions on to mineral particle surfaces. Ion exchange is
significant, with certain ion species being held more strongly than
others by the particle surfaces.
Consequently the relative
importance of movements o f different ions by the advection process
or by diffusion becomes a complex question.
There may be small, near-microscopic, pockets of w ater in the
freezing soil having high concentrations of dissolved materials.
Layers of ground of considerable thickness, known as cryopegs, can
also remain unfrozen because o f their high concentration of
dissolved materials. Cryopegs are particularly likely in coarse­
grained soils where quantities o f unfrozen w ater are small and the
freezing process is different. Coarse-grained soils without any fine
material do not accumulate excess ice on freezing and discrete layers
of ice are not formed. In situations of constraint or loading on the
frozen layer, when the pore w ater turns into ice the increase in
volume is accommodated by w ater being pushed ahead of the
freezing layer into the unfrozen material. Dissolved materials
excluded from the ice are concentrated in this water. The result is
a layer of unfrozen soil, at the interface between the frozen and
unfrozen ground, at a tem perature just below 0 ° C .
This is the origin of the cryopegs, which are widely reported and
extensively discussed in the Russian literature. They occur below
and within the permafrost. They are not commonly reported in
Canada. It may be that the different climatic history of the
permafrost in Russia explains the extensive, very large cryopegs
there, but it seems probable that smaller cryopegs (maybe localised,
metre-thick layers) will occur in the near-surface permafrost regions
of North America, especially where the deposits are coarse-grained.
These diverse aspects of the physical chemistry and
thermodynamics of freezing soils are the subject of current research
prompted by the global significance o f freezing processes (Dash et
al. 1998) -climate change, agronomy in cold regions, and pollution
in freezing ground (White and Williams 1998, Chuvilin 1998, and
Virtual Conference, 1997).
The fundamental importance in geotechnical engineering of the
freezing of soils has been realised in recent decades. Nevertheless
the potential importance o f freezing o f soil in stress-corrosion
cracking appears to have been largely overlooked.
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CONCLUSIONS
The characteristics o f frozen ground differ radically from those
of unfrozen ground in ways that are significant for SCC. In
particular:
1. Continuing accumulations of ice within freezing ground give
rise to pervasive and often large stresses and deformations in buried
pipelines
2. The ice accumulations can be disruptive of protective coatings.
3. The freezing porous soil medium has a high concentration of
ions in films on particle surfaces.
4. In spite of its apparently relatively inert and solid nature,
frozen ground is perm eable to w ater and ions can be transported by
advective transport. As a result of the thermodynamic conditions,
extremely high hydraulic gradients develop in association with
temperature gradients in freezing groundJons also move under
concentration gradients and gradients of electrokinetic potential.
5. Metre-thick layers of soil remain unfrozen at temperatures
below 0 0 C because o f high concentrations of solutes excluded from
ice.
6. In cold regions, research into the pipeline environment with
respect to stress corrosion cracking must consider the role of the
special characteristics o f freezing ground.
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