1. Introduction

1.1 Soil freezing
When soil is cooled under 0 °C, soil water solidifies with decreasing
temperature.

This is known as soil freezing.

Large areas of land that undergo soil

freezing are found worldwide and frozen soil is harnessed in various fields.
Soils at high altitudes and latitudes can remain frozen for long periods.

Soilthat remains at or below 0 °C for at least two years is termed permafrost (InternationalPermafrost Association, 1998).

Permafrost underlies about 14 per cent of the Earth’s

land surface (21 × 106 km 2) and the permafrost is over 1-km thick in some regions (e.g.
Washburn, 1980).

Even at middle latitudes, most of the ground surface is subject toseasonal or short term freezing.

In Japan, the ground is frozen seasonally in Hokkaido,

Tohoku, and some mountain areas.

Short-term frozen ground is even found in theKanto plains and the western part of Japan, where the temperature is below 0 °C on
winter mornings.

In this manner, soil freezing is estimated to occur in almost 70 per

cent of the Earth’s land surface (Kinosita, 1984).

Furthermore, soil near highways,

bridges, and tunnels sometimes freezes in areas where soil freezing generally does notoccur. Soil freezing is not limited to the Earth.

Frozen soil water is reported on theMoon (Feldman, 1998) and Mars (Carr and Schaber, 1977). Moreover, most soil in thesolar system is in the frozen state.
Soil is sometimes frozen artificially for use, since the mechanical strength ofsoil increases with freezing and its permeability decreases dramatically.

For example,

cold storage for farm produce (Tuchiya and Ryoukai, 1996) and barriers against
contaminant migration (Mohamed et al., 1995) have been constructed from soil kept
frozen using refrigeration pipes.

Artificially frozen ground is also utilized for-1-

underground tunnel construction in urban areas and for soft ground construction incaves and underwater (artificial soil freezing method).

The soil around LNG (LiquidNatural Gas) storage tanks (Akiyama and Iguno, 1981) and pipelines under construction
in cold regions is also kept frozen to maintain a stable construction base (Nixon, 1987).
On the other hand, frozen soil is used as a source of cold energy because it contains alarge amount of ice. Thus, there are many situations in which soil freezes.

1.2 The importance of studying soil freezing
If soil expands when it freezes, the volume change is called frost heave.
particles in soil migrate as a result of frost heaving and thawing.

ClayThe migration of clay

particles can change water paths and micro-pores in soil and damage irrigation anddrainage systems in farmland.

Disturbance of soil and changes of aggregated structureand hydraulic conductivity due to the action of frost-thaw are also reported (Tuchiya,1986).

The ground surface sometimes expands dozens of centimeters due to frost

heaving and the heaving pressure can exceed 10 kPa (Japanese Society of GeotechnicalEngineering, 1994).

The heaving and subsidence of the surface causes much damage

to paved roads (Photograph 1), airports, railways, foundations, and so on.
When heaving occurs, some of the ice formed near the ground surface ispresent as seams of pure segregated ice; this ice is called an ice lens.

The growth ofice lenses induces soil water to flow from the unfrozen zone to the surface zone.

Inthis case, some solutes illuviate to near the ground surface with the water flow, so thatsalinization occurs.

If the ice lenses thaw and water collected near the ground surface

runs off, soil loss can occur. Water can collect near the ground surface of farmland andremain until early spring, hindering plowing and seeding (Yakuwa, 1961).
thawing of ice lenses softens the ground.

The

Furthermore, plant roots are sometime cut or

extracted by frost heaving and ice lensing, so that plants wither in the field.

-2-

Photograph 1. Frost damage to paved roads (Kushiro, 1997).

-3-

With artificial soil freezing, unless the freezing is well controlled, frost heaving also
occurs and affects the surrounding soil.

Frost heave is also reported in concrete (Wanget al. 1996) and can damage the bases of dams and bridges.

To overcome such frost-action damage, it is important to clarify the mechanism of frost heave.
We can identify various features associated with frost action, such assolifluction and patterned grounds in cold regions, where the ground undergoes repeatedfreezing and thawing (e.g. Washburn, 1980; Fukuda et al., 1992; Mackay, 1993; Soneand Strelin, 1997).

Pingos occur in Siberia, Alaska, and northern Canada.

These areperennial frost mounds, 10 m or more in height, consisting of surface soil and a core of
massive ice (e.g. Yoshikawa and Harada, 1995).

Soil freezing and frost heave play

important roles in the formation of these landforms.
Knowledge of soil freezing is also required in the field of climate change andglobal warming.

The surface layer of ground above the permafrost thaws in summer

and freezes again in winter (active layer).

When soil water in this layer melts, themelting water is not drained by infiltration because of the high hydraulic conductivity ofpermafrost (Bridges, 1978).

Consequently, wetlands are a major feature in permafrost

areas.

Wetlands are a major source of atmospheric greenhouse gasses (Nakayama.

1995).

The surface soil of permafrost contains large quantities of stored carbon.

If

the permafrost soil thaws, this carbon may be released as carbon dioxide and methane(Kane, 1997).
Recent research suggests that water and heat circulation in the permafrost area
play important roles since the physical environment of the permafrost undergoes large
seasonal swings in temperature, duration and intensity of solar radiation, surface albedo,
and surface energy balance (e.g. Kane, 1997).

Ground surface conditions and soil

freezing/thawing in permafrost areas affect water and heat circulation and the climate
system (Kanae et al., 1998).

-4-

Recently, to observe, understand, and model the hydrological cycle and energy
fluxes in the atmosphere, at the land surface, and in the upper oceans, an international
program, the Global Energy and Water Cycle Experiment (GEWEX), was instituted.
Cold region phenomena and the role of frozen ground are major topics of this program.
In order to predict global climate change and its impact on permafrost, it is important toclarify the mechanism of soil freezing and to understand the nature of frozen ground.
Natural gas hydrate is found in the ground buried under the permafrost (e.g.
Uchida, 1999).

Knowledge of soil freezing is also important for exploiting thisresource and investigating its distribution and amount (Matumoto, 1996).
In space, the Mars Pathfinder mission is part of the long-term Mars exploration
program being conducted by NASA.

In July 1997, Sojourner, the rover carried on

Pathfinder, investigated the Martian surface (Kerr, 1997).

Lunar Prospector, launched

in 1998, has brought about many new insights into soil freezing on the moon (Feldman,1998).

The construction of the International Space Station (ISS), which is a hugemanned construction, located about 400 km above the Earth, is just beginning.
Extraterrestrial soil will be utilized for engineering and agriculture in the near future,and knowledge of soil freezing will be of great importance.

1.3 Ice lensing and the purpose of this study
When water freezes in situ during soil freezing, the expansion of soil volume isless than 9 per cent of the volume of soil pores.

When soil freezes, however, water issometimes induced to flow through the unfrozen zone, towards a freezing front, andform regions of ice that are almost devoid of soil particles (Figure 1).
ground surface and causes a great amount of frost heave.
ice lens because of its shape.

This raises theThe region of ice is called anPhotograph 2 shows ice lenses observed in natural soiland Figure 2 shows artificial ice lenses made in our laboratory in both soil and a porous-5-

medium consisting of glass particles (Watanabe et al., 1997; Muto et al., 1998).

Asshown in these figures, a feature of ice lensing is the formation of intermittent layers,which results from the repeated generation, growth, and jumping of the ice lens near thefreezing front.
The major factor producing frost heave is ice lensing, which changes themicrostructure, water, and heat regimes of soil.
impermeable layers in soil.

Ice lenses form intermittent

Ice lensing complicates water and heat migration, making

the process difficult to understand and predict. To clarify the mechanism of ice lensing
is not only the main theme of soil freezing science, but also an important theme invarious fields, such as agriculture, engineering, geology, hydrology, meteorology, andspace science.
Ice lensing is not a phenomenon limited to soil.

It has also been observed intuff (Miyata and Akagawa, 1996, 1997), tofu (Fuchigami, 1996), living cells (Mazur,1970), sawdust and Bengala (Japanese Society of Geotechnical Engineering, 1994), andin some fine particles and powders.

Recent progress in science and technology has

increased the opportunities to handle porous media and micro particles under freezing
conditions. Knowledge of the mechanism of ice lensing will be applied to fields thatinvolve these frozen materials, including physical chemistry, biology (Mazur, 1970;
Wisniewski et al., 1997), material science (Tada, 1996), food processing (Thyholt andIsaksson, 1997), and medicine.
In this study, we perform unidirectional freezing experiments to clarify themechanism of ice lensing, and propose a model of ice lensing.

-6--7-

Fig.1. Schematic diagram of soil freezing and ice lensing.
When soil is frozen unidirectionally, water is sometimes induced to flow toward the freezing front and forms region of ice
that is almost devoid of soil particles. The region is called ice lens because of its shape. The ground is heaved upward
cased by the ice lensing (frost heave).

Photograph 2. Ice lenses in actual soil (James Ross Island, 1998).

-8--9-Fig. 2. Ice lensing observed in soil (a) and in porous medium consisting uniform sized glass particles (b) (fromWatanabe et al., 1997; Muto et al., 1998). Intermittent layers are shown in the frozen region.