Literature report
M.Sc. Thesis
September 2011
ARTEK – DTU Byg
Daniel Lyngholm Jakobsen
s032180
A preliminary literature survey about
Frozen ground engineering
2
PREFACE
This preliminary literature survey on Frozen ground engineering is the basic for a 30 ECTS point MSc thesis.
The literature survey is elaborated in corporation with GEO and Arctic Technology Centre at Technical
University of Denmark.
I wish to thank thesis supervisor Thomas Ingeman-Nielsen and Helle Foged Christensen for inspiration and
guidance during the process.
______________________________________________________
September 2011, Daniel Lyngholm Jakobsen
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TABLE OF CONTENTS
PREFACE ..................................................................................................................................................................... 3
INTRODUCTION .......................................................................................................................................................... 5
1
THE INITIAL LITERATURE SURVEY ....................................................................................................................... 6
2
GEOLOGICAL DESCRIPTION ................................................................................................................................ 9
2.1
2.2
THE GEOLOGY OF GREENLAND .................................................................................................................................. 9
GEOLOGY IN AND AROUND ILULISSAT ....................................................................................................................... 10
3
PERMAFROST ....................................................................................................................................................12
4
PRACTICAL ISSUES RELATED TO FROZEN GROUND ENGINEERING .....................................................................14
5
SOIL FROST ACTION ...........................................................................................................................................17
5.1
5.2
6
EFFECTS OF FROST HEAVE ....................................................................................................................................... 17
THAW SETTLEMENTS AND CONSOLIDATION ................................................................................................................ 18
MECHANICAL PROPERTIES OF FROZEN SOIL ......................................................................................................20
6.1
6.2
6.3
STRENGTH AND DEFORMATION ............................................................................................................................... 20
CREEP ................................................................................................................................................................ 20
EFFECTS OF UNFROZEN WATER AND SALINITY ............................................................................................................. 21
7
PREVIOUS TEST PERFORMED ON FROZEN GROUND ..........................................................................................23
8
CONCLUSION .....................................................................................................................................................25
9
PROBLEM STATEMENT ......................................................................................................................................26
10
REFERENCES ......................................................................................................................................................27
4
INTRODUCTION
The primary purpose of this literature survey is to define a problem statement to be addressed in the
subsequent MSc thesis. Secondly, it allows the author to get acquainted with challenges connected to the
research field of frozen ground engineering. Through the elaboration of this report, previously conducted
studies including laboratory test and articles have been studied as well as field of personal interest. A
comprehensive and detailed understanding of core areas within frozen ground engineering will provide a
good starting point for preparation of the forthcoming thesis. The intention of this literature survey has
also been to clarify areas where performance of additional investigations could be appropriate.
This report documents the processes of gathering knowledge chronologically, from the initial beginning
with a broad focus to the final problem statement addressed to a specific subject.
Which parameters influence the geotechnical properties of frozen and thawed fine grained soil? In what
way does temperature and stress affect the mechanical behaviour of frozen soil?
5
1 THE INITIAL LITERATURE SURVEY
The literature study has been initiated with an overall focus on the concept of frozen ground engineering.
The purpose of focusing broadly at the beginning of the MSc thesis is to provide a broad focus of the
concept within frozen ground engineering. Many different issues are related to the subject, and an overall
understanding of the concept is therefore essential before moving on to focus on a specific subject area.
At this stage, the book “Frozen Ground Engineering” (Andersland & Ladanyi, 2004) has provided a good
introduction to the topic. The book initially describes the concept of frozen ground, including seasonally
and perennially frozen ground thoroughly. Subsequently the properties and behaviour of frozen ground
that is exposed to different nature forces are systematically reviewed in “Frozen Ground Engineering”.
These reviews include heat flow in soils, construction ground freezing and different earthwork in cold
regions. “Frozen Ground Engineering” will frequently be used as reference in the MSc thesis, as it emerges
as a handbook for the major issues associated with frozen ground engineering.
Likewise the book “Cold Regions” (Smith et al., 1996) has been useful in the process of establishing an
overview of the topics within frozen ground engineering. “Cold Regions” deals in general with the concepts
related to the design, construction and operation of infrastructure in cold regions. In the context of frozen
ground engineering, chapter 3 concerning geotechnical considerations appears most relevant. This chapter
addresses many different problems related to frozen ground engineering, and is therefore a good
supplement to the book “Frozen ground engineering”. Especially the review of the challenges regarding
frost heave should be highlighted, since it provides a good insight in the mechanisms which generate this
phenomenon.
Another source of inspiration has been the Norwegian experience-based guidance “Frost i jord” (Sætersdal
et al., 1976). The authors of the book were at the time of publication leading in the research within frozen
ground behaviour in Norway. The book is a guide to prevent frost damage in various engineering disciplines
and is thereby useful in relation to understanding the complexity within frozen ground behaviour. “Frost i
jord” supports the theoretical theory reviewed in the two previous books with experiential knowledge.
Perusing above books has provided an overall understanding of the field within frozen ground engineering.
The obtained knowledge has inspired to explore new areas within the topic. However, in order to improve
the effectiveness of my searches, an information retrieval course at DTV was attended by undersigned.
Besides teaching how to perform advanced searches in databases related to DTU, the literature survey
course dealt with searching databases worldwide. Experience from the course has already early in the
project proved to be very useful.
Further literature survey led to the identification of two additional books. The first book titled “The
mechanics of frozen ground” (Tsytovich et al., 1975), the second titled “Permafrost” (Johnston, 1981).
The first part of “The mechanics of frozen ground” gives a combined experimental and theoretical
exposition of the mechanics behaviour of frozen ground. The second part of “The mechanics of frozen
ground” is a practical application of frozen ground mechanics. Though the book is still relevant today,
additional research can be found in literature of more recent date.
6
The next book examined is titled “Permafrost” (Johnston, 1981). Chapter 3 in the book concern engineering
characteristics of frozen and thawing soils and appears as useful inspirations source. This part also
examines the issues related to thaw settlement and consolidation, which deals with the theory behind
several of the project topics which were discussed at the initial collaborative meeting at GEO. Especially the
review of the challenges regarding consolidation of frozen ground provides a good understanding of the
topic within artificial freezing. In the further literature study a large number of articles dealing with frozen
ground and consolidation have been identified and studied. Among these articles the following have
provided most inspiration:
•
•
•
The effect of freezing on the strength of silty-clay-sand mixtures (Anagnostopoulos &
Grammatikopoulos, 2005)
Determination of the stress field during thawing of frozen ground (Stepanov & Kholin, 1979)
Over consolidation effects of ground freezing (Chamberlain, 1981)
Last mentioned article concerns artificial ground freezing, and describes several interesting aspects.
Therefore this subject has been studied in an effort to define a problem statement for the MSc thesis.
Many of the processes taking place during freezing and thawing of soil are identical whether the
surrounding temperature change is caused by artificial mechanism or by the climate change. This also
applies to the settlement of soft clay soils after freezing and thawing, which is a result of the suction forces
that draw pore water to the freezing front (Chamberlain, 1981).
Chamberlain (1981) describes several interesting considerations regarding possible effects of thaw
settlement on ground freezing projects, including the process of over consolidation of soils during freezing.
An example of thaw settlement related to artificial freezing is illustrated in Figure 1. A very interesting point
in relation to the keyword frozen ground and consolidation is mentioned in the following way by
Chamberlain (1981): “Particular care must be taken in selecting the coefficients of consolidation, because
these factors can be greatly affected by freezing.”
Figure 1. Possible effects of thaw settlement on ground freezing projects (Chamberlain, 1981).
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Focus has been on artificial ground freezing in the subsequent literature survey. The earlier mentioned
book “Frozen Ground Engineering” provides a thorough review on this topic, which has given a good overall
understanding. Knowledge through literature concerning artificial ground freezing, the international
symposium on ground freezing, appears to be quite essential. The symposium is held every second to third
year and brings together all leading researchers within the field. In 1978, during the first international
symposium regarding ground freezing, the basic principles of ground freezing and the process of ice
segregation and frost heave were explored. Furthermore attempts were made to determine the freezing
performance of full-scale structures (Chamberlain, 1981).
Articles published in connection with the international ground freezing symposiums are assembled in book
form and published. It has been possible to obtain the assembled book from the first, second and fourth
symposiums held respectively in 1978, 1980 and 1985, which have been studied (Jessberger, 1979, Frivik,
1982, Kinosita, 1985). In addition, proceedings from the seventh symposium are available through the
Technical Information Centre of Denmark, however, with a processing time up to several weeks, this option
hasn’t been compatible with present literature survey.
In the subsequent literature survey, interest has arisen for an earlier consultant project in Citronen fjord,
Greenland. The project is prepared for a customer at GEO and is therefore confidential. Undersigned have
reviewed the present project material where different triaxial and oedometer tests have been performed
on frozen soil samples. In addition the geotechnical engineers in charge of the project; N. Foged and J.
Christensen have been consulted. The opportunities to explore this area further aroused great interest.
As an introduction to the above mentioned test of frozen soil, the main factors related to this subject will
be described. These include the geological history of Greenland and the influence of permafrost. Likewise
the practical issues related to construction in perennially frozen regions will be described.
In-situ permafrost samples from Ilulissat in Greenland have been available through Arctic Technology
Centre, DTU. The subsequent focus of the report will be on permafrost. Further literature survey regarding
tests of frozen soil will also be described. Additionally the geology around the town of Ilulissat will be
described, in order to understand the geological history of the available samples.
8
2 GEOLOGICAL DESCRIPTION
A large percentage of Greenland consists of frozen ground and permafrost. In this chapter the overall
geological structure of Greenland is introduced. In addition to the geology around the town of Ilulissat will
be described. The geological focus on Ilulissat is due to the fact that frozen and unfrozen soil material from
boreholes is available from this location.
2.1 THE GEOLOGY OF GREENLAND
Greenland is the largest island in the world with a surface area of 2415100 km2 and approximately 75 % is
covered by the Inland Ice (Humlum, 2000). The ice free area consists mainly of bedrock. The U-shaped
valleys which form the characteristic fjord landscape in west Greenland indicates the glacial erosion which
has taken place in the area. The geological structure of Greenland reflects a history that spans nearly four
billion years. Fjords and valleys are formed by flood erosion and glacial erosion due the movement of the
inland ice (Geus, 2010). The ice-free mountainous rim of well exposed bedrock is up to 250 km broad and
the highest peak is 3733 m. The largest part of this area is composed of crystalline rocks of the Precambrian
shield. The Precambrian shield acted as a stable block under which sediments accumulated throughout the
Precambrian and were deformed and metamorphosed (Escher, 1976).
Besides the Precambrian shield, the geological structure of Greenland is composed of the Palaeozoic fold
belt. Two major mid-Palaeozoic belts occur in Greenland, the Caledonian fold belt and the North Greenland
fold belt. The Precambrian shield is made up of four main structural provinces, among these the Archaean
block, the Nagssugtoqidian mobile belt, the Rinkian mobile belt and the Ketilidan mobile belt (Escher &
Watt, 1976). At Figure 2 the location of the main geological provinces of Greenland is illustrated.
Figure 2. The main geological provinces of Greenland (Ingeman-Nielsen, 2002)(Escher & Watt, 1976).
9
Regarding the west cost of Greenland, a sedimentary basin developed during the Mesozoic age. This basin
is today represented onshore by Cretaceous and Tertiary sediments. During the Tertiary, considerable
volcanic activity occurred at both West- and East Greenland (Escher & Watt, 1976).
2.2 GEOLOGY IN AND AROUND ILULISSAT
Ilulissat is located on west Greenland on the East coast of the Disko Bay, see Figure 2. Christiansen &
Humlum (2000) reported that Ilulissat is located in the discontinuous permafrost zone. Foged (1979)
describes an average temperature in Ilulissat of -5,7 °C. The bedrock in the Ililissat area is part of the
absolute northernmost part of the Nagssugtoqidian mobile belt close to the Rinkian mobile belt and the
Greenland volcanic province at Disko Island. The Rinkian mobile belt is located less than 50 km from
Ilulissat in a northerly direction and the west Greenland volcanic province is located across the Disko bugt
(Escher & Watt, 1976).
The Nagssugtoqidian mobile belt exists within a 300 km wide belt across Greenland from east to west as
illustrated in figure 2. The majority of the composition is Archaean gneisses with amphibolitic bands and
pegmatic veins that often appear weathered and fractured (Ingeman-Nielsen et al., 2007).
The main phase of Nagssugtoqidian deformation and metamorphism is dated 2700 m.y. ago although the
belt was active until about 1700 m.y. ago. The Nagssugtoqidian bedrock has undergone great deformations
due to horizontal shear movements associated with over thrusting of the Nagssugtoqidian block onto the
Archaean block to the south. Subsequently sediments have settled on the Nagssugtoqidian shield and
undergone deformation and metamorphism during the Precambrian (Escher & Watt, 1976). During the
Quaternary the area was exposed to strong erosion from glaciers (Ingeman-Nielsen et al., 2007).
During the Weichsel-Wisconsin, which indicates the glaciations, the entire area was covered by ice
(Ingeman-Nielsen et al, 2008). When the ice retreated approximately 11000 years ago, a larger part of the
coastal landscape was below sea level due to consolidation from the icecap. The area was then covered by
sludge which originated from thaw sediments of the ice cap. The area covered by thaw sediment was then
covered by seawater in areas below sea level, and marine sediment was deposited together with the thaw
sediments. Therefore the oldest marine sedimentation is typically thick layer of clay, while the younger is
more typically marine environment. Younger layers consist generally of sand and shell particles (Foged,
1979)
Ingeman-Nielsen et al (2007) describes the two above mentioned sedimentation types in the following way;
glaciomarine clay deposited in a coastal environment by the sea often at the glacier front, and younger
marine deposits of gravel and sand fragments deposited during the retreat of the sea level.
Ingeman-Nielsen et al (2007) describes the process as follows;
The interaction between isostatic depression of the bedrock due to the load form the icecap combined with
eustatic variations in the global sea level has resulted in a change in the relative sea level during the period
of the deglaciation.
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This is the reason why marine sediments today occur above sea level along the West coast of Greenland.
The upper marine limits in the area of Ilulissat are illustrated in Figure 3.
Figure 3. Upper marine limits around Ilulissat (Weidick, 1976).
In some areas these marine deposits have been covered by freshwater deposits (Ingeman-Nielsen, 2007).
In the period around 3000-7000 years ago the area of Ilulissat was exposed to isostatic uplift and the
marine environment retreated. Leaching of glacial and marine deposits occurred when the seawater was
suppressed by landmasses. Many of those sediments where later eroded away into the sea (Foged, 1979).
Immediately after the isostatic uplift of the area, permafrost was formed in the area due to climatic change.
Earlier deposits where thus exposed to freezing and thawing, which resulted in consolidation and cracked
formation. This occurred mainly in the fine grained sediments due to formation of ice lenses (Foged, 1979).
The mapping performed by Weidick (1966) and GEUS (2004) around Ilulissat is illustrated in figure 4 and 5.
Most of the landscapes are represented in the form of gneisses reworked in the early Proterozoic.
Figure 4. Glacial geological observations by Weidick (1966).
Figure 5. Geology mapping of the Ilulissat area (Geus, 2004).
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3
PERMAFROST
Frozen ground is defined as soil or rock with a temperature below 0 ˚C. The definition is based on
temperature and independent of the water and ice content of the soil or rock (Andersland & Ladanyi,
2004). Soil or rock frozen continuously from year to year is defined as perennially frozen ground or
permafrost. However, Brown & Kupsch (1974) defined that the ground at least must remain frozen for two
consecutive winters and the intervening summer to be classified as permafrost.
In cold regions where freezing beneath the exposed land surface occurs, the regions are classified either as
seasonally frozen ground, discontinuous or continuous permafrost. Frozen ground exposed to thawing in
the entire depth during the summer is defined as seasonally frozen ground. Perennially frozen ground is
divided into continuous and discontinues areas. An area where permafrost occurs everywhere and is
continuous in a given depth is defined as a continuous permafrost area, while an area with scattered
permafrost zones is defined as discontinuous permafrost as illustrated in Figure 6 (Andersland & Ladanyi,
2004)(Johnston, 1981). Sporadic permafrost occurs where small patches of permafrost are scattered in
generally unfrozen areas (Pidwirny, 2006).
Figure 6. Vertical distribution and thickness of permafrost in the northern hemisphere (Johnston, 1981).
The vast majority of the worldwide perennially and seasonally frozen ground is found in the Northern and
Southern hemispheres. For instance permafrost is found in about 25 % of the Earth`s non-glaciated land
surface. The common depth of permafrost is several 100 meters. The thickness of this layer varies from 15
cm in the far north to 1 m or more to the south. The active layer above the permafrost is subjected to a
cyclic thaw during the summer season, since heat and moisture movements between the permafrost and
the atmosphere take place in the active layer (Pidwirny, 2006)(Andersland & Ladanyi, 2004).
12
The mean ground and air temperature in the three permafrost zones are illustrated in table 1.
Table 1. Yearly mean air and ground temperatures in permafrost zones (Johnston, 1981)(Mortensen et al, 2002).
In the continuous permafrost zone, permafrost may reach a thickness up to 1000 m, while the
discontinuous zone can vary from few meters up to 100 meters close to the continuous permafrost zone.
The thickness of the sporadic permafrost zones is limited to approximately 12 m (Andersland & Ladanyi,
2004). In practice permafrost has been recorded in depths up to 2600 meters (Ingeman-Nielsen, 2011).
The ground (surface) temperature is mainly determined by the above air temperature, heat flow from the
interior of the earth and soil thermal properties (Andersland & Ladanyi, 2004).
The depth of freezing is dependent on the surface freezing index and creates a temperature distribution as
shown in Figure 7. The figure illustrates the temperature variation and geothermal gradient affecting the
depth of permafrost and active layer. In the Northern hemisphere, the active layer will decrease as the
mean annual temperature is decreased as one goes from south to north. Below the level of zero annual
temperature, the ground temperature increases with the depth and the geothermal gradient (Andersland
& Ladanyi, 2004)(Johnston, 1981). Therefore the figure is essential in understanding the existence of
permafrost.
Figure 7. Temperature profile in perennially frozen soil (Andersland & Ladanyi, 2004).
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4 PRACTICAL ISSUES RELATED TO FROZEN GROUND ENGINEERING
Two main issues are related to construction in cold regions. The frost heave due to freezing, and the loss of
soil bearing capacity due to thawing. The problems are common in all areas where seasonally freezing and
permafrost occurs. In the perm frozen areas, especially the active layer is affected by the above mentioned
mechanism (Johnston, 1981).
Construction in frozen ground can be divided into two cases. The difference is whether or not the frozen
foundation soils are thaw-stable or thaw-unstable. Permafrost conditions can be neglected and
conventional designs and construction method used when foundation materials are stable upon thawing.
Where thaw-unstable foundation materials are present, the most desirable approach for building
permanent structures is to design and construct them so as to preserve the frozen ground condition
(Johnston, 1981).
At present there is no generally accepted criterion that characterizes a non-frost-susceptible earth material.
Johnston (1981) states that the most commonly used criteria are based on grain size. According to Johnston
(1981), the Casagrande (1932) frost criterion is one of the most appropriate methods to determine where
damaging frost action may occur. Casagrande (1932) proposed the following, widely known, rule-of-thumb
criterion for identifying potentially frost susceptible soils:
“Under natural freezing conditions and with sufficient water supply one should expect considerable ice
segregation in non-uniform soils containing more than 3% of grains smaller than 0.02 mm, and in very
uniform soils containing more than 10 percent smaller than 0.02 mm. No ice segregation was observed in
soils containing less than 1 percent of grains smaller than 0.02 mm, even if the groundwater level is as high
as the frost line” (Casagrande, 1932).
Figure 8. Warm interior of a building causes the permafrost
to thaw (NSIDC, 2011).
Figure 9. Uneven thawing and freezing causing damage to
a road (NSIDC, 2011).
There is often a need to build structures in the low-lands where frost susceptible fine-grained materials
occur. There has been much damage to structures over time in case where the thawing and freezing cycle
haven’t been taken into account for frost susceptible soils. In this context, it is also important to think
14
through the impact of the foundation on the future temperature of the underlying ground. Buildings that
are heated from the inside give off heat. During time the heat is transferred into the ground and will thaw
the permafrost underneath the building. Once the permafrost thaws, it sinks, damaging the building it
supports as illustrated in Figure 8 (NSIDC, 2011).
Infrastructure such as roads, bridges and railroads have been affected worldwide by frozen ground or
permafrost. If the ground thaws and freeze unevenly, it will cause damage to the construction. For road
construction the pavement will be exposed to frost heave during the vintner. Further, the pavement may
even crack due to the excessive tensile stress produced under the effect of extremely low temperatures as
illustrated in Figure 9. The thaw during summer accompanied by increased activity of traffic cause
deformation and thus considerable damage to the pavement (Kachroo, 2011).
Frost heave of pile foundations is a great challenge for engineers in charge of foundations in cold regions.
The American geologist, Stephen Taber, defined an essential feature of the frost heaving phenomenon.
Taber noted that while frost heave can result from the freezing of water present in the voids of a soil,
“excessive” heave only occurs when water is pulled up through soil to build up layers of segregated ice
(Vinson, 2006). Ice segregation is illustrated in Figure 14. A bridge exposed to excessive frost heave and pile
jacking over several winters is illustrated in Figure 10. A less fatal situation of frost heave is illustrated in
Figure 11, where utility poles are jacked out of the ground by frost heaving.
Figure 10. Bridge damage caused by frost heaving and pile
jacking (Vinson, 2006).
Figure 11. Utility poles are jacked out of the ground by
frost heaving (Vinson, 2006).
Another issue is to construct pipelines on frozen ground. The Alaska pipeline illustrated in figure 12 carries
warm oil across a permafrost area. The oil in the pipeline must be kept above 60 °C in order to flow easily,
so major challenges are associated with protecting the ground from thawing conditions. Air-cooled devises
are constructed at the top of the piles in order to avoid heat from the pipe to enter the ground (NSIDC,
2011).
15
Johnston (1981) recommends the passive foundation method when foundations are performed in the
continuous permafrost zone. The passive method is to preserve the ground frozen and examples are
illustrated in Figure 12 and Figure 13.
Figure 12. The Alaska pipeline constructed on piles (NSIDC).
Figure 13. Thermosyphons cooling the pavement
construction (Vinson, 2006).
16
5 SOIL FROST ACTION
The problem of frost-action damage is spread worldwide. Frost action occurs in all temperate zones
wherever there is seasonal soil freezing as well as in the active layer of more northerly permafrost regions
(Anderson et al., 1978). The concept of frost action in soil is a combination of frost heave during the winter
and then the loss of bearing capacity during the summer due to thaw.
5.1 EFFECTS OF FROST HEAVE
Constructions in frost-susceptible soil can be affected by large uplift forces due to frost heave. The uplift
forces may cause structural distress and frost jacking at the foundation. Size order of the heave exposed to
foundations is, among others, dependent on the water available for the freezing process of new ice lenses.
The heave forces increase in the frost-susceptible soil during the downward advance of the freezing front in
winter. Heave occurs during the phase change when water in the soil freezes and forms ice lenses, since
water expands approximately 9 % when it freezes. Frost heave is generated by ice segregation during the
freezing process and of the formation of altering bands of soil and ice (Andersland & Ladanyi, 2004).
The background of frost heave during freezing, is the volumetric expansion of in situ pore water combined
with the ongoing process of water transported to the freezing front (ice segregation). The last mentioned
part has the greatest influence on the phenomenon frost heave. The process of ice segregation is
illustrated in figure 14. Overall, three different conditions must exist simultaneously for frost heaving to
occur. Those are the freezing temperature in the soil, frost-susceptible soil and availability of water. The
simultaneous occurrence and interaction process of these mechanisms result in frost heave. The maximum
frost heave occurs if enough water is available for the process of ice segregation, which moves water to the
freezing front due to capillary rise. The necessary supply of water is normally obtained from the ground
water table or the surrounding soil (Phukan & Andersland, 1978).
Figure 14. Ilustration of ice segregation. Capillary forces attract water to the freezing front where
the water content is increased and ice lenses are formed (Tsytovich, 1975).
In permafrost areas the amount of moisture in the active layer is often limited, whereas in seasonal frost
areas an unlimited source may be provided from the groundwater table. In principle, this means that
seasonal frost areas are in greater risk of experiencing frost heaving than permafrost areas (Eranti & Lee,
1986).
17
The amount of heave by ice lensing depends on the frost-susceptibility characteristics, principally the
hydraulic permeability of the soil. Ice lenses usually form parallel to the freezing front, while heaving always
occurs in the direction of the heat flow, usually perpendicular to the ground surface. The heat-flow rate and
the balance between moisture and heat flow determine the ice-lens spacing and thickness. While water is
available for freezing at the freezing front, the ice-lens will continue growth at one side indefinitely. If the
heat-removal rate at the freezing plane exceeds the moisture supply, the temperatures at the ice-water
interface decrease. When the accessibility of water to the freezing front increase subsequently, the freezing
plane advances and a new location for ice-lens developments may be established (Anderson et al, 1978).
Capillary forces that attract moisture to the freezing front, and pressures that cause heave are inversely
proportional to the void size. It should be mentioned in this context that the permeability of very fine
grained soils is low and water movement to the freezing front are thus limited (Eranti & Lee, 1986).
Aitken (1963) stated that frost heave in fine grained soils is reduced by a surcharge load while the soil
pressure is increased; a condition existed whereby the water would no longer migrate toward the freezing
front but was expelled from it.
5.2 THAW SETTLEMENTS AND CONSOLIDATION
Thaw settlement is the generally uneven downward movement of the ground surface due to thaw
consolidation (ISSMFE, 1989). The relative influence of the rate of thaw, compared with the rate of
consolidation is referred to as thaw consolidation (Johnson et al., 1984)
Frozen ground contains ice in several forms, ranging from coatings on individual soil particles and small
lenses to large deposits. During thawing ices melt and water is released from the frozen ground. Especially
if the ground has heaved, there will be excess water present which has to drain away. Simultaneously the
soil skeleton must adapt itself to a new equilibrium (Andersland & Ladanyi, 2004)(Harris, 1995).
Knowledge about time rate and amount of thaw settlements is important for geotechnical engineers. Thaw
settlements behaviour of frozen permafrost depend on a variety of factors, including ice content and
changes in thermal and moisture conditions. The thaw settlement characteristics of permafrost are
therefore best determined by test of representative samples under representative conditions. Thaw
settlement tests may be performed either isotropically in a triaxial pressure cell or in a conventional
consolidation device (Johnston, 1981).
The volume change for a fine grained material subjected to thawing is illustrated in Figure 15. Volume
change results from both the phase change and the flow of excess water out of the soil. Thawing at 0 °C is
represented by the line between bc. The line towards d from c represents the following consolidating in the
form of continued-drainage. In this range the equilibrium conditions develop in the soil skeleton for the
overburden pressure (σ0) plus any additional loads (Δσ) (Andersland & Ladanyi, 2004).
18
Figure 15. The volume change for a fine grained material subjected to thawing (Andersland & Ladanyi, 2004).
During consolidation water is squeezed out of the soil and settlements occur as the stress is gradually
transferred from the pore water to the soil skeleton. This consolidation process causes an increase of the
effective stress in the soil skeleton. Since the shear strength of soils is frictional, the strength increases as
the pore water pressure decrease (Andersland & Ladanyi, 2004)(Johnston, 1981).
Frost-susceptible soils such as silt and clay are generally ice-rich, particularly near the active layer, and can
give rise to large thaw settlements. Thawing of the ice lenses at a rate faster than the released moisture
can escape, accompanied by loading of the soil, cause a large decrease in bearing capacity and slope
stability. In this case, pore pressures will exceed the hydrostatic pressure and cause instability.
If thawing occurs at a slow rate, the water generated will flow from the soil at about the same rate as it is
produced and settlement will proceed concurrently with thawing. While permeable course grained soils
consolidate almost completely during thawing, fine grained soils are exposed to limited consolidation
during thawing. After thawing of fine grained soils, large pore pressure may still remain, which reduce the
stability, bearing capacity and shear strength (Johnston, 1981)(Mortensen, 2004).
19
6 MECHANICAL PROPERTIES OF FROZEN SOIL
Frozen soil present in permafrost areas consists of four different constituents, solid grains, divided into
mineral and organic, gases and unfrozen water. The behavior of frozen soil is very different from that of
unfrozen soil because of the presence of ice and unfrozen water films. It is important to note that the
mechanical property of frozen soil is strongly dependent on temperature change.
6.1 STRENGTH AND DEFORMATION
In much the same way as for unfrozen soils, the strength of frozen ground is due to cohesion, interparticle
friction and particle locking. The great difference in strength between frozen and non-frozen soil is caused
by the ice formation in the frozen soil skeleton. According to Berggren (1983), high strength and exposed
creep behavior are the main characteristics of frozen soil.
Frozen soils with a large percentage of ice tend to behave identically to ice. At low ice contents, especially
in fine-grained soil, the unfrozen water film plays an important role due to deformation and strength
properties (Johnston, 1981). The strength behavior of frozen soil is largely dependent on that part of the
pore ice which normally binds the grains together and fills most of the pore space (Andersland & Ladanyi,
2004).
6.2 CREEP
When a frozen soil sample is subjected to a load, it will respond with an instantaneous deformation and a
time-dependent deformation. If the load is high enough, it will display a limiting strength (Andersland et al,
1978).
Johnston (1981) stated that pressure melting occurs in frozen soil when hydrostatic or deviator pressure is
applied. During this pressure melting process the amount of unfrozen water tends to increase with
pressure, while the water flows to regions of lower stress where it freezes again. This process of ice melting
and water movement is accompanied by a breakdown of the ice and the bonds with the soil grains,
resulting in plastic deformation of the pore ice and a redistribution of particle arrangement. The result is
the time-dependent deformation called creep (Andersland et al, 1978). This reorganization process is
considered to be an important factor in creep and stress relaxation of fine grained frozen soil (Johnston,
1981).
Tsytovich (1975) stated that compression constitutes no more than 1/3 of the settlement, the remainder
being due to creep.
In order to describe the effect of time on the behaviour of frozen soil, creep tests on frozen samples have
to be carried out. In creep tests, cylindrical specimens are exposed to uniaxial stress and the testdata is
used to construct strain-time curves similar to Figure 16 and Figure 17. Several typical creep curves for
frozen soil samples are illustrated in Figure 16. Three basic creep stages are common for frozen soil
20
exposed to uniaxial stress and constant temperature. The three different periods of time are called
primary, secondary and tertiary creep and are illustrated in Figure 17.
During primary creep the creep rate is decreasing, while the creep rate remains constant during secondary
creep. Finally, the tertiary stage is characterized by accelerated creep rate, which normally leads to
ultimate failure of the specimen. For ice-rich soils under moderate stress conditions, stedy-state creep is
dominant. Various parameters influence the shape of the creep curve, including temperature, magnitude of
applied stress, soil type and density (Andersland et al, 1978)(Johnston, 1981).
Figure 16. Typical creep curves for frozen soil exposed to
uniaxial stress and constant temperature(Andersland et al,
1978).
Figure 17. Basic creep curve illustrating primary,
secondary and tertiary creep (Andersland et al, 1978).
The amount of strain developed during primary creep is generally large compared to the strain developed
during the primary creep. Sayles (1968) stated that primary creep appears to dominate for ice poor soils.
According to Johnston (1981), it`s important to determine if such a stress level exists for a particular soil,
that it might become stable when subjected to such stresses.
It should, however, be mentioned that a difference occurs between puce ice and ice-rich soil. Andersland &
Ladanyi (2004) stated that pure ice generally creeps faster than ice-rich frozen soil.
6.3 EFFECTS OF UNFROZEN WATER AND SALINITY
Generally the unfrozen water content is influenced by mineralogy, temperature and salinity of the pore
water (Harris, 1995). The amount of unfrozen water in fine grained non-saline soils is mainly governed by
the specific surface area and grain arrangement, which control the development of capillary and adsorption
forces (Hivon & Sego, 1995). However, the development of unfrozen water content in saline soils is more
complex.
The unfrozen water found in permafrost exists as a film of water around each soil particle. Water
movements in frozen ground occur via those unfrozen films. Unfrozen water films on soil particles reduce
the ice content and results in a more plastic behaviour during deformation (Andersland et al, 1978).
However, there is disagreement within the scientific community about the presence of such liquid film
around quartz particles. Research presented by Hivon & Sego (1995) and Hoekstra (1969) claim that the
21
development of ice occurs directly in contact with the course particles since they cool faster than the
surrounding pore fluid.
The unfrozen water is divided into free and bound water. The term free water covers surface water,
groundwater and infiltrating free water. Bound water is defined as chemically bound water, adsorbed
water and water exposed to capillary forces (Andersland et al, 1978).
Frozen soil with large unfrozen water content may also develop volumetric consolidation due to
temperature change (Johnston, 1981). Frozen clay can have as much as 5-7 % unfrozen water by volume at
-15 °C. Results from Hivon & Sego (1995) illustrate an increase of unfrozen water for a decrease in grain size
of the frozen sample.
The amount of unfrozen water in the frozen soil decreases as the temperature decrease (Kane & Boike,
2005). For temperatures down to -20 °C, the strength of frozen saline soil is less than that of salt-free soils
at the same temperatures. This fact is due to higher unfrozen water contents in the frozen saline soil
(Leonards & Andersland, 1960).
The strength in uniaxial compression of saline sand with salt concentrations up to 100 ppt increased with
increasing applied strain rates (Sego et al, 1982). Harris (1995) stated that this effect increased as the salt
concentration increased. When the pore water contains sodium chloride (NaCl), the freezing point is
depressed and the salts are concentrated in the unfrozen water. This further depresses the freezing point
and increases the local salt concentration (Harris, 1995)
Fine-grained deposits will always have a depression of the freezing-point due to capillary forces in the
pores. Therefore part of the material may remain unfrozen down to a temperature of -10 °C (Foged & BækMadsen, 1980)
22
7 PREVIOUS TEST PERFORMED ON FROZEN GROUND
Andersland & Ladanyi (2004) gives a thorough review on laboratory test on frozen soils. This review is
supported well by Johnston (1981) in the chapter concerning laboratory testing on frozen and thawing
soils. Detailed description of test methods for tests conducted on frozen saline fine-grained soil is
represented by Nixon & Lem (1984), Hivon & Sego (1995).
In the test program underpinning the article of Nixon and Lem (1984), creep tests and time-dependent
strength tests have been performed on two different types of frozen fine-grained saline soils. Tests were
performed for different freezing temperatures, varying from -2,3 °C to -25 °C, however, with the highest
percentage in the interval between -5 °C to -10 °C. The range of stress in the creep test was 30-400 kPa,
while the salinity was varied from 0-35 ppt.
Results presented by Nixon & Lem (1984) indicate a 10- to 100-fold increase in uniaxial creep rate for
frozen samples with a pore fluid with a salinity approaching that of seawater. Significant decrease in shear
strength and foundation bearing capacity might be the consequence in saline permafrost areas (Nixon &
Lem, 1984). One of the other significant results from the soil resistivity tests performed by Nixon & Lem
(1984) was a strong correlation between resistivity and pore water salinity.
The study performed by Hivon & Sego (1995) deals with the influence of salinity and temperature on the
behaviour of frozen soil. The testing program contain three different soil types, including uniform fine sand,
silty sand and very fine sand. Non-plasticity was common for the tested soil types, indicating no clay
content. Three quantities of salinity were used; including 5, 10 and 30 ppt. Tests were performed for
different freezing temperatures, among those -5 °C, -7 °C, -10 °C and -12°C.
Among the results of Hivon & Sego (1995) one should mention the temperature influence on unconfined
compressive strength, which was shown to follow a power-law relation. It could also be concluded that the
unconfined compressive strength follows a power-law relation with the unfrozen water content. It has
been noticed that the authors of the quoted article recommend a further investigation regarding strain rate
and confining pressure of frozen soil, since those aspects haven’t been studied properly.
Alternatively to the previous described test on in-situ permafrost samples, test on artificial frozen samples
have been carried out. A set of triaxial constant strain rate and constant stress tests were performed by
Arenson et al, (2004). The purpose of the investigation has been to study the effect of the volumetric ice
fraction, strain rate and also confining stress on the mobilized shear strength of artificial frozen soil.
Samples were made by mixing soil from the Swiss Alps with crushed ice and water. The mixed materials
were subsequently placed in a triaxial cell and exposed to one-dimensionally freezing and testing.
Throughout the process, the focus has been to recreate the natural conditions from the Swiss glaciers.
Arenson et al (2004) concludes that the strength increases with decreasing ice content. This does not apply
to samples consisting of 100 % ice, which showed the highest strength. Likewise the shear strength is
reported to be higher for pure ice than samples containing 29 % ice. It has been noticed that the influence
of confining pressure is reported to have insignificant effect on the strength.
23
The study prepared by Arenson & Springman (2005) reviews triaxial tests on both ice-rich permafrost
samples and samples exposed to artificial freezing. The intentions have been to compare the behaviour of
respectively in-situ and artificial frozen samples under constant stress and strain rate. Several similarities
occur between the survey of Arenson & Springman (2005) and Arenson et al (2004), including conclusions
regarding creep and shear strength.
Laboratory testing regarding strength and deformation on frozen and unfrozen samples has been carried
out at GEO during spring 2010. As mentioned earlier in this report, the material is confidential due to the
protection of clients` rights. However, experience and results from the current project will serve as
inspiration for any future test in connection with this thesis.
Knowledge of the behaviour of ice is important in understanding the behaviour of frozen soil, since ice
occupies the pore space within a saturated frozen soil (Hivon & Sego, 1995). Lade (2002) has investigated
the mechanical behaviour of sea ice as a frictional material. Lade (2002) stated that sea ice in the early
loading stages behaves viscoelastic followed by brittle fracture or ductile, irrecoverable deformation.
Failure of sea ice follows a pattern that demonstrates its highly cross-anisotropic nature and its behaviour
as a frictional material (Lade, 2002)
In addition, inspiration and knowledge regarding tests on other frozen soil types is available through
proceedings from the International Conference on Permafrost.
24
8 CONCLUSION
The purpose of focusing broadly at the beginning of the preliminary literature survey is to provide a
broad focus of the concept of frozen ground engineering have been a very instructive process.
Many different issues related to the subject, and an overall understanding of the concept has been
obtained, which is essential before moving on to focus on a specific subject area.
The process of gathering knowledge regarding the geological development of Greenland and permafrost
will also contribute constructively in prospective study. This can also be said about the obtained knowledge
of where additional relevant literature is to be found.
The practical issues related to frozen ground engineering are useful in relation to understanding the
complexity within frozen ground behaviour. The initial study of practical issues related to frozen ground
engineering provides an overall understanding of the importance of research within this field.
The theory regarding geotechnical properties of permafrozen soil has been studied extensively by Johnston
(1981), Johnson (1984), Tsytovich (1975), Andersland & Ladanyi (2004). A more detailed study of this
discipline appears as an obvious opportunity for the MSc thesis. This includes the mechanical properties of
frozen and thawed soil.
In the examination of previous tests performed on frozen ground, the research performed by Nixon & Lem
(1984), Hivon & Sego (1995) appears to be the best inspiration. The test on frozen ground described in this
report generally has a strong focus on the effect of salinity and unfrozen water content in the frozen
samples. This fact is underlined by Hivon & Sego (1995) which concludes that an increase in salinity, and
thus unfrozen water content, causes a significant loss of strength in the frozen soil. Finally Hivon & Sego
(1995) recommend further investigation regarding strain rate and confining pressure of frozen soil, since
those aspects haven’t been studied properly. These topics are, together with the influence of salinity and
thus unfrozen water content, interesting in connection with present MSc thesis.
25
9 PROBLEM STATEMENT
Which parameters influence the geotechnical properties of frozen and thawed fine grained soil, among
these the strength and deformation properties. Likewise, which parameters influence the physical, thermal
and mechanical properties of permafrozen, fine-grained soil.
Both at Arctic Technology Centre and GEO, test of frozen fine grained soils appear to be of great interest
compared to course grained material. Apart from settlements, are there other issues that affect this
choice?
26
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