Soil Freezing Characteristic Curve Research Articles

Effect of freeze–thaw cycling on the soil‐freezing characteristic curve of five Canadian soils

AbstractThe frozen soil processes and their interaction with the environment in the vadose zone of cold regions is vital in both agricultural and engineering practice applications. In a frozen soil, unfrozen water and pore ice coexist. The relationship between the unfrozen water content and subzero temperature is widely known as the soil‐freezing characteristic curve (SFCC). The SFCC is a valuable tool for predicting the hydromechanical properties and for modeling the coupled thermal–hydraulic–mechanical–chemical process in frozen soils. In spite of its importance, the effect of freeze–thaw (F–T) cycling on SFCC has not been well investigated or understood. In this technical note, the effect of F–T cycles on the SFCC of five soils from cold regions of Canada were investigated. The SFCC (including both freezing and thawing branches) of the five soils for different F–T cycles were measured using frequency domain reflectometry (FDR) technique. The experimental results suggest that the effect of F–T cycles on the SFCC of the five soils is not significant. Such a behavior may be attributed to the destruction of soil structure during the saturation process. However, all the five soils’ SFCC exhibited hysteresis behavior for all the F–T cycles. The results of the study are valuable and contribute towards better understanding of the fundamental behavior of SFCC of various cold region soils.

Parameterization of soil freezing characteristic curve for unsaturated soils

The soil freezing characteristic curve (SFCC) describes the relationship between the temperature and unfrozen water content in a soil. The SFCC is indispensable in modelling the hydro-mechanical behaviour of frozen soils, but is less understood for the unsaturated soils. A series of SFCC tests of unsaturated silica sand, silt and red clay are preformed based on a newly developed nuclear magnetic resonance (NMR) apparatus, which can precisely control the sample temperature in the magnetic field. The experimental results show that the measured SFCC varies significantly for different initial water contents, and that a lower initial water content leads to a slower increase in unfrozen water content, proving that the SFCC is closely related to the initial unsaturated state. It is found that the thawing curve is better to represent the SFCC, in contrast the freezing curve is significantly affected by the supercooling phenomenon. A new parameterization of the SFCC is presented for unsaturated soils by combining the Clapeyron equation and the model for Soil Water Characteristic Curve (SWCC). A number of test results from the literature and this study are used to validate the new SFCC model. By inputting the parameters for the SWCC and initial state into the proposed model, the predicted SFCC can agree well with the measured results. The new model has a theoretical basis and simple form and is applicable to both saturated and unsaturated soils.

Estimating soil-water characteristic curve from soil-freezing characteristic curve for mine waste tailings using time domain reflectometry

The unsaturated properties of a soil are required to predict the rate of dewatering and magnitude of strength gain of a mine waste tailings deposit during desiccation dewatering. This prediction requires the soil-water characteristic curve (SWCC), which is time-consuming and challenging to attain and may take anywhere from weeks to months to complete a single test. As a result, alternative methods are needed to estimate the SWCC. Past research has indicated that the soil-freezing characteristic curve (SFCC) can be used to estimate the SWCC in some soils. An experimental method and apparatus were developed to measure the SFCC to estimate the SWCC for different mine waste tailings, including copper tailings, gold tailings, and oil sands centrifuge cake. The experimental method involved using a resistance temperature detector to measure the temperature and time domain reflectometry to determine the unfrozen water content of the soil. The results showed that the SFCC could be used to estimate the SWCC for tailings from metal mines (gold tailings and copper tailings) with a high portion of sand-sized particles and a small amount of clay-sized particles, but was not able to estimate the SWCC for oil sands tailings.

Comparison of Soil‐Freezing and Soil‐Water Characteristic Curves of Two Canadian Soils

Core IdeasThe similarity between SFCC and SWCC, and hysteresis of SFCC are reviewed.The SFCC and SWCC of two fine‐grained soils are measured and analyzed.No quantitative similarity is found between the measured SFCC and SWCC.Several concerns regarding the similarity between SFCC and SWCC are discussed.The drying–wetting and freezing–thawing cycles significantly influence the soil pore water in the vadose zone in permafrost and seasonally frozen regions. The soil‐freezing characteristic curve (SFCC) describes the relationship between unfrozen water content and subzero temperature in a soil at frozen condition. Several studies suggest that the SFCC of a frozen saturated soil is similar to soil‐water characteristic curve (SWCC), which describes the relationship between water content and suction for a soil under unfrozen unsaturated condition. In the present study, the similarity between SFCC and SWCC, and possible reasons for the hysteresis of SFCC are succinctly reviewed. The SFCC and SWCC of two Canadian soils were measured and critically interpreted to understand the fundamental behavior of SFCC in comparison with the SWCC. The observed hysteresis of SFCC for the two soils was mainly associated with the supercooling of pore water. The measured SFCC and SWCC of the two soils show quantitative dissimilarity rather than similarity. This may be attributed to the experimental limitations and possible fundamental differences between drying–wetting and freezing–thawing processes. In addition, several concerns regarding the similarity between SFCC and SWCC are discussed. The present study highlights that rigorous investigations are required for better understanding the SFCC to facilitate its use for cold‐region engineering practice applications.

Stress Effects on Soil Freezing Characteristic Curve: Equipment Development and Experimental Results

Core IdeasA triaxial apparatus is developed to measure stress‐dependent SFCC.Unfrozen water content of clay and sand increases with increasing confining stress.Stress effects on the SFCC of clay are more significant than sand.The soil freezing characteristic curve (SFCC) defines the relationship between soil temperature and unfrozen water content. This curve is important for predicting water flow and heat conduction in frozen soils, as well as freezing heave and thawing settlement. So far, SFCCs reported in the literature were usually determined at zero stress. To investigate stress effects on SFCC, a stress‐ and temperature‐controlled triaxial apparatus was developed in this study. The unfrozen volumetric water content was measured using a newly developed noninvasive time domain reflectometry (TDR) probe. The new apparatus was used to measure SFCCs of two typical soils (i.e., clay and sand) at different stress conditions (i.e., 30, 100, and 200 kPa). Each specimen was subjected to compression, and then a cycle of freezing and thawing. As expected, for both soils, the saturated water content prior to freezing was smaller at higher stresses because of compression. During the subsequent freezing and thawing, the soil specimen at a higher stress was able to retain more liquid water than that at a lower stress. The higher unfrozen water retention capacity at higher stresses is mainly because the pore size of a soil specimen becomes smaller during compression. Hence, more water can retain a liquid state due to the enhanced capillarity. On the other hand, stress effects on the SFCC were found to be more significant for clay than for sand. This is likely because the stress‐induced change in pore size distribution is larger in clay due to its higher compressibility.

Soil freezing process and different expressions for the soil-freezing characteristic curve

The soil-freezing characteristic curve (SFCC), which represents the relationship between unfrozen water content and sub-freezing temperature (or suction at ice-water interface) in a freezing soil, can be used for understanding the transportation of heat, water, and solute in frozen soils. In this paper, the soil freezing process and the similarity between the SFCC of saturated frozen soil and soil-water characteristic curve (SWCC) of unfrozen unsaturated soil are reviewed. Based on similar characteristics between SWCC and SFCC, a conceptual SFCC is drawn for illustrating the main features of soil freezing and thawing processes. Various SFCC expressions from the literature are summarized. Four widely used expressions ( i.e. , power relationship, exponential relationship, van Genuchten 1980 equation and Fredlund and Xing 1994 equation) are evaluated using published experimental data on four different soils ( i.e. , sandy loam, silt, clay, and saline silt). Results show that the exponential relationship and van Genuchten (1980) equation are more suitable for sandy soils. The simple power relationship can be used to reasonably best-fit the SFCC for soils with different particle sizes; however, it exhibits limitations when fitting the saline silt data. The Fredlund and Xing (1994) equation is suitable for fitting the SFCCs for all soils studied in this paper.

Prediction of resilient modulus of frozen unbound road materials using soil-freezing characteristic curve

The resilient modulus is a key parameter required in the mechanistic design of pavements. Experimental determination of the resilient modulus requires elaborate equipment for testing and requires trained personnel; for this reason, it is expensive. There are several models for predicting the resilient modulus for unbound road materials that take into account the influence of wetting and drying conditions. However, well-established models are not available for the prediction of the resilient modulus of these materials in a frozen state. In this paper, a semi-empirical model, which uses a soil-freezing characteristic curve as a tool, is proposed for predicting the variation of the resilient modulus with subzero temperature and the associated cryogenic suction for frozen soils. Experimental data on seven different pavement unbound materials were used to validate the proposed model. It is shown that the model can reasonably predict the resilient modulus of the investigated soils that are in a frozen state. More investigations on different types of soils would be useful to better understand the strengths and limitations of the proposed model.

Soil Freezing and Soil Water Retention Characteristics: Connection and Solute Effects

The relation between the soil-water characteristic curve (SWCC) and the soil freezing characteristic curve (SFCC) is investigated based on the similarity between the freezing/thawing and drying/wetting behaviors of soils. The SWCCs of clay and silt are obtained by the pressure plate extractor and vapor pressure method, while the SFCCs are determined from the unfrozen water content measurement using the nuclear magnetic resonance (NMR) and temperature measurement. The pore water potential of the frozen soil is derived from a generalized Clapeyron equation, which addresses the effects of capillarity, sorption, and osmosis, and compared to that of unsaturated soils. Our experimental results show that at low water content the matric potential in the soil saturated with pore water and pore ice is generally different from that in the soil saturated with pore water and pore gas. A series of experiments on the soil samples saturated by NaCl solutions of different concentrations were performed to determine the osmotic potential and the water retention characteristics. Based on the principles of surface chemistry, a relationship between the adsorptive forces and the water film thickness is developed, which is capable of describing the soil water characteristic when the sorption effect is significant. Our results indicate that the freezing-point depression method should be applied with caution in determining the soil water retention characteristics of unsaturated soils at lower water content.

Solute and water effects on soil freezing characteristics based on laboratory experiments

Laboratory experiments were conducted to study effects of water and solute on soil freezing using TDR and temperature sensor combination methods. ANOVA methods were applied for analyzing significance for solute influences on soil freezing characteristic curve (SFCC). Results showed that higher initial water content influenced the SFCC by increasing liquid water content at the same temperature due to more water connection with soil pores, and adsorbed by soil particles. ANOVA results showed solute content and solute type all had significant effects (P<0.001 to P<0.5) on soil freezing processes. And solute in soil resulted in a lower freezing point of soil, which made more liquid water co-exist with ice at negative temperatures. And solute concentration condensing due to liquid water decline would also impede soil freezing processes by decreasing osmotic potential. Due to the physical and chemical process of soil solution, different ions also presented some differences in SFCC parameter estimation. Based on a trial and error method, a prediction model was also built, and it behaved well in predicting SFCC under different water and solute conditions.

Using soil freezing characteristic curve to estimate the hydraulic conductivity function of partially frozen soils

Abstract We propose using the soil freezing characteristic curve (SFCC), instead of the soil water characteristic curve (SWCC), to estimate the hydraulic conductivity of partially frozen soils. Shortcomings associated with the use of the SWCC to estimate the hydraulic conductivity function of partially frozen soils are discussed. The hydraulic conductivity function for partially frozen Devon Silt is derived using the SFCC and the empirical relationships for hydraulic conductivity estimation method developed by Fredlund et al. (1994). The SFCC for Devon Silt is determined from unfrozen water content measurement using time domain reflectometry and temperature measurements inside the soil sample. The results using this novel approach compare well with results presented by others that use different methods to determine the hydraulic conductivity function of partially frozen soils.

A New Method for Soil Water Characteristic Curve Measurement Based on Similarities Between Soil Freezing and Drying

Abstract The soil water characteristic curve (SWCC) is the basis to explain a variety of processes in unsaturated soils, ranging from transport phenomena to mechanical behaviors. In this paper, a new method is developed for SWCC estimation based on the similarity between the freezing/thawing process and drying/wetting process in soils. The theoretical basis for this method is first reviewed. The concept of the soil freezing characteristic curve (SFCC) is introduced to describe the relationship between the unfrozen water content and matric suction in frozen soils. SFCC is analogous to SWCC in that both of them describe the energy status of liquid water associated with liquid water content. Relationships between SWCC and SFCC are discussed. To measure the SFCC, a thermo-time domain reflectometry (TDR) sensor was developed which combines both temperature sensors and conventional TDR sensor. The TDR module and algorithm measured the bulk free water content of soils during the freezing/thawing processes, while the built-in thermocouples measured the internal temperature distribution. SFCCs were obtained from the simultaneously measured TDR and temperature data. Experiments were conducted on a few types of soils to validate this new procedure. The SFCC was obtained from thermo-TDR data collected in specimens subjected to a controlled thawing process, while the SWCC was directly measured by ASTM D5298, the filter paper method. Reasonable agreements were found between SWCC and SFCC. The experimental results implied that the SWCC could be estimated from SFCC, which also provided more evidence of the similarity of freezing/thawing processes and desorption/sorption processes.