The freezing temperature of water in soil is not constant but varies over a range determined by soil texture. Consequently, the amounts of unfrozen water and ice change with temperature in frozen soil, which in turn affects hydraulic, thermal, and mechanical properties of frozen soil. In this paper, an Am-241 gamma ray source and time-domain reflectometry (TDR) were combined to measure unfrozen water content and ice content in frozen soil simultaneously. The gamma ray attenuation was used to determine total water content. The TDR was used to determine the dielectric constant of the frozen soil. Based on a four-phase mixing model, the amount of unfrozen water content in the frozen soil could be determined. The ice content was inferred by the difference between total water content and unfrozen water content. The gamma ray attenuation and the TDR were both calibrated by a gravimetric method. Water contents measured by gamma ray attenuation and TDR in an unfrozen silt column under infiltration were compared and showed that the two methods have the same accuracy and response to changes of water content. Unidirectional column freezing experiments were performed to apply the combined method of gamma ray attenuation and TDR for measuring unfrozen water content and ice content. The measurement error of the gamma ray attenuation and TDR was around 0.02 and 0.01 m3/m3, respectively. The overestimation of unfrozen water in frozen soil by TDR alone was quantified and found to depend on the amount of ice content. The higher the ice content, the larger the overestimation. The study confirmed that the combined method could accurately determine unfrozen water content and ice content in frozen soil. The results of soil column freezing experiments indicate that total water content distribution is affected by available pore space and the freezing front advance rate. It was found that there is similarity between the soil water characteristic and the soil freezing characteristic of variably saturated soil. Unfrozen water content is independent of total water content and affected only by temperature when the freezing point is reached.

Simultaneous measurement of unfrozen water content and ice content in frozen soil using gamma ray attenuation and TDR

Soil frost heaving is the result of the formation of ice lenses in the soil caused by a segregation of the soil water. Ice lenses are growing from below and pushed upward. Seedlings heave when they are pushed out of the ground by the ice sheet formed at the surface of the soil. Frost heaving may greatly reduce growth and survival of forest tree seedlings particularly in regions where freezing and thawing are accompanied by high soil moisture. Resistance to frost heaving increases with size of seedling as the ability of a seedling to anchor itself increases. A few methods, such as fertilizing, choice of planting spots, sowing or planting at the proper time, shading, and use of mulches, appear to be effective in controlling frost heaving. Shade from natural plant cover can greatly reduce frost heaving

Frost heaving of forest tree seedlings: a review

[1] Gas hydrate formation and dissociation in sediments are accompanied by changes in the bulk volume of the sediment and can lead to changes in sediment properties, loss of integrity for boreholes, and possibly regional subsidence of the ground surface over areas where methane might be produced from gas hydrate in the future. Experiments on sand, silts, and clay subject to different effective stress and containing different saturations of hydrate formed from dissolved phase tetrahydrofuran are used to systematically investigate the impact of gas hydrate formation and dissociation on bulk sediment volume. Volume changes in low specific surface sediments (i.e., having a rigid sediment skeleton like sand) are much lower than those measured in high specific surface sediments (e.g., clay). Early hydrate formation is accompanied by contraction for all soils and most stress states in part because growing gas hydrate crystals buckle skeletal force chains. Dilation can occur at high hydrate saturations. Hydrate dissociation under drained, zero lateral strain conditions is always associated with some contraction, regardless of soil type, effective stress level, or hydrate saturation. Changes in void ratio during formation-dissociation decrease at high effective stress levels. The volumetric strain during dissociation under zero lateral strain scales with hydrate saturation and sediment compressibility. The volumetric strain during dissociation under high shear is a function of the initial volume average void ratio and the stress-dependent critical state void ratio of the sediment. Other contributions to volume reduction upon hydrate dissociation are related to segregated hydrate in lenses and nodules. For natural gas hydrates, some conditions (e.g., gas production driven by depressurization) might contribute to additional volume reduction by increasing the effective stress.

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Volume change associated with formation and dissociation of hydrate in sediment

We review notable advances published in the year 1999 in the area of phase change and phase growth in porous media. Phase equilibria thermodynamics, particularly in micropores, and growth kinetics, emphasizing the pore-network structure, are highlighted. Advances reported include the effects of confinement in phase transitions in micropores, and of the pore microstructure in the growth and dissolution of gas and liquid phases with applications ranging from capillary condensation to drying to gas evolution to condensation.

Phase change in porous media

: Methods of determining the frost susceptibility of soils are identified and presented in this report. More than one hundred criteria were found, the most common based on particle size caharacteristics. These particle size criteria are frequently augmented by information such as grain size distribution, uniformity coefficients and Atterberg limits. Information on permeability, mineralogy and soil classification has also been used. More complex methods requiring pore size distribution, moisture-tension, hydraulic-conductivity, heave-stress, and frost-heave tests have also been proposed. However, none has proven to be the universal test for determining the frost susceptibility of soils. Based on this survey, four methods are proposed for further study. They are the U.S. Army Corps of Engineers Frost Susceptibility Classification System, the moisture-tension hydraulic-conductivity test, a new frost-heave test, and the CBR-after-thaw test. (Author)

/pdf/frost-susceptibility-of-soil-review-of-index-tests-50ocqtv8e2.pdf

Frost Susceptibility of Soil, Review of Index Tests,

Heat and mass flow associated with a freezing front

Developments and applications of frost susceptibility testing

Identification of frost-susceptible materials on the basis of their physical properties is too imprecise for many practical purposes, and direct freezing tests need to be employed. Heave is measured by two main types of test: the constant rate of penetration test and the constant boundary temperature test. The latter has the advantage of greater simplicity of operation and is easier to model mathematically. Nevertheless, its reproducibility is relatively poor and improvements are being sought. The development of a self-refrigerated unit (SRU) is outlined and likely future revisions to the constant boundary temperature test specification discussed briefly. A precise freezing cell (PFC) that uses the Peltier effect and permits unidirectional freezing with the boundary temperatures controlled to plus or minus 0.1 deg C has been developed. Specimens heave much less in the PFC than in the SRU because the heat extraction is more rapid and a constant temperature is applied to the moving boundary (top of specimen) rather than to the stationary boundary. Thus the penetration of the zero isotherm is accompanied by high suctions that favor ice penetration over segregation. The role of the PFC lies in research, not in routine testing, particularly in connection with the development and evaluation of mathematical models. /Authors/

R.H. Jones

Papers

Heat and mass flow associated with a freezing front

Developments and applications of frost susceptibility testing

Comparison of the precise freezing cell with other facilities for frost-heave testing