If soil expands when it freezes, the volume change is called frost heave. When heaving occurs, some of the ice layer formed near the ground surface is present as seems of pure segregated ice; this ice layer is called an ice lens. The ground surface sometimes expands dozens of centimeters due to the ice lensing. It causes much damage to paved road, drainage system in farmland, foundation, and so on. The growth of ice lenses induces soil water to flow from the unfrozen zone to the surface zone. In this case, some solutes illuviate to near the ground surface with the water flow. To overcome such frost-action damage, it is important to clarify the mechanism of ice lensing during soil freezing. Furthermore, ice lensing is not a phenomenon limited to soil. It has also been observed in various porous materials. Knowledge of the mechanism of ice lensing will be applied to fields that involve these frozen materials, including physical chemistry, biology, material science, food processing, and medicine.
Numerous studies with the intention of clarifying the mechanism of ice lensing have been reported. Presently, one of the theories most often used is the secondary frost heave theory. In this theory, presence of partially frozen region near growing surface of ice lens is assumed. The intermittent formation of ice lenses is then explained from calculating stresses in the region. However, the partially frozen region has not been confirmed experimentally. And the stress partition factor, which is important for calculating the neutral stress, has not been theoretically verified. Summarizing the historical studies, there are two problems associated with clarifying mechanism of ice lensing. One is to clarify the microstructure near the freezing front, i.e. to clarify water conditions and particle migration in the partially frozen region. The other is to explain the dynamic mechanism of ice lensing, in which the generation and growth of an ice lens is repeated to form intermittent layers. In order to solve these problems, we performed three series of freezing experiment using a nidirectional freezing apparatus. In the first experiment, ice lensing in soil and porous media consisting of fine particles were microscopically observed, then, following results were obtained. Ideal ice lenses for modeling can be made using uniform sized glass beads. The growth of ice lens is dependent on supercooling of the growth surface. The freezing rate influences the ice lens growth more than temperature gradient. It is suggested that freezing rate, supercooling degree at growth surface of ice lens and particle condition near growing ice lens were important factors for considering ice lensing model. From the second experiment, in which ice interface in water with dispersed glass particles was observed, the criteria for exclusion and encapsulation of particles during ice formation with respect to particle size and freezing rate was shown. The relationship between particle size and critical freezing rate was explained by KberĠs theory. It is suggested that the critical freezing rate was important for the generation of ice lens. In the third experiment, microstructure in the vicinity of ice lens is observed using Raman spectroscopy. It is obtained that no ice was found in any pore warmer than the warmest ice lens in the porous media and the ice lens grew without penetrating the warmer pores.
Based on the experimental results, we then presented a model for simulating the formation of ice lenses during freezing of unconfined uniform porous media is presented. The main notions of the model are that generation and jump are dependent on the freezing rate, and growth is dependent on supercooling. The critical freezing rate is assumed to vary with changes in the number of particles near the ice lens as it grows. The model was demonstrated for the unidirectional freezing of a porous medium consisting of fine glass particles. The numerical results show that this model can represent the formation of intermittent layers of ice lenses in such a system. This model can be applied to ice formation in unconfined water-soaked porous media.
As mentioned above, we clarified the microstructure near the freezing front in water-saturated porous media experimentally and made a model, which can explain the formation of intermittent layers of ice lenses.
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1. | Introduction | |
1.1 |
Soil freezing |
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1.2 |
The importance of studying soil freezing |
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1.3 |
Ice lensing and the purpose of this study |
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2. | Factors and theories of ice lensing | |
2.1 |
Introduction |
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2.2 |
Main factors considered in soil freezing |
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2.2.1 |
Unfrozen water |
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2.2.2 |
Ice formation |
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2.2.3 |
Water flow during soil freezing |
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2.2.4 |
Heat transfer during soil freezing |
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2.3 |
Ice lensing theories and models |
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2.3.1 |
Capillary theory |
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2.3.2 |
Hydrodynamic model |
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2.3.3 |
Secondary frost heave theory |
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2.3.4 |
2.3.4 Osmotic model 25 |
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2.3.5 |
Takashifs theory |
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2.3.6 |
Adsorption force theory |
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2.3.7 |
Segregation potential concept |
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2.3.8 |
Kinetic model |
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2.3.9 |
Thermomolecular pressure model |
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2.3.10 |
Thermodynamics approach |
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2.4 |
Problems for theories and models of ice lensing |
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3. | Materials and apparatus | |
3.1 |
Characteristics of samples and grain-size distribution |
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3.2 |
Surface conditions |
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3.3 |
Specific surface area and distribution of pores |
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3.4 |
Unfrozen water content |
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3.4.1 |
NMR methods |
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3.4.2 |
Test procedure |
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3.4.3 |
Unfrozen water content |
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3.4.4 |
Effect of solute on unfrozen water content |
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3.5 |
Unidirectional freezing apparatus |
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4. | Observation of ice lensing and frost heaving (Exp. 1. 2) | |
4.1 |
Sample and method |
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4.2 |
Experimental results |
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4.2.1 |
Freezing experiment with zero sample rate |
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4.2.2 |
Freezing experiment with constant sample rate |
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4.3 |
Discussion |
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4.4 |
Summary |
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5. | Observation of freezing of dispersed glass beads in water (Exp. 3) | |
5.1 |
Sample and method |
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5.2 |
Experimental results |
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5.3 |
Discussion |
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5.4 |
Summary |
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6. | Observation of the vicinity of ice lens by Raman spectroscopy (Exp. 4) | |
6.1 |
Sample and method |
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6.2 |
Raman spectroscopy |
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6.3 |
Experimental results |
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6.4 |
Discussion |
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6.5 |
Summary |
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7. | A Model for the Formation of Ice Lenses in an Unconfined, Watersaturated, Porous Medium consisting of Spherical Particles | |
7.1 |
Model |
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7.1.1 |
System and conditions |
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7.1.2 |
Generation of an ice lens |
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7.1.3 |
Growth of the ice lens |
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7.1.4 |
Heat transfer |
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7.1.5 |
Number of particles near the growth surface |
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7.1.6 |
Ice lensing |
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7.2 |
Simulation |
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7.3 |
Discussion |
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7.4 |
Summary |
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8. | Summary and conclusions | |
References |