Thermo-time Domain Reflectometry Research Articles

In-situ assessment of soil shrinkage and swelling behavior and hydro-thermal regimes with a thermo-time domain reflectometry technique

The shrinkage and swelling behaviors of clayey soils could lead to cracks and erosions inducing severe damages to agricultural lands and engineering projects. Thus, improving the understanding of shrink-swell characteristics is crucial to nonrigid soils. However, it remains difficult for in-situ monitoring of soil shrink-swell processes continuously and nondestructively. The objective of this study was to present a thermo-time domain reflectometry (thermo-TDR) technique for determination of soil shrinkage and swelling dynamics and hydro-thermal regimes in-situ during wet-dry cycles. A field experiment on two clayey soils (a paddy soil and an alluvial soil) was conducted to evaluate the performance of the thermo-TDR method for in-situ determination of soil volume changes and associated hydraulic and thermal properties during wet-dry cycles. The results showed that both soils experienced considerable shrinking and swelling processes with maximum vertical deformations of 2.4 and 1.8 cm for the paddy soil and alluvial soil, respectively, and with maximum horizontal deformations of 370 and 280 cm2 for the paddy soil and alluvial soil, respectively, during a 36-day period. The thermo-TDR method provided volume change values (shown as the bulk density) agreed well with independent core-sampling measurements with an average RMSE of 0.15 Mg m−3 and a R2 of 0.46. A better agreement was observed between thermo-TDR measured bulk densities and values from the ruler-image processing method with an average RMSE of 0.07 Mg m−3 and a R2 of 0.77. Besides, the thermo-TDR method gave consistent in-situ soil shrinkage curve and water retention surface measurements by combining with soil matric potential sensors. Unlike rigid soils, the paddy and alluvial soils showed complicated hydro-thermal regimes in response to the shrink-swell processes. We conclude that the thermo-TDR technique is a reliable tool for continuous in-situ determination of soil shrink-swell processes. Future studies should focus on development of comprehensive soil heat and water transport models accounting for both moisture content and volume changes.

A four-parameter-based thermo-TDR approach to estimate water and NAPL contents of soil liquid

In-situ determination of soil non-aqueous phase liquid content (θNAPL) is necessary for early detection of soil NAPL contamination and preventing the spread of the contamination. Thermo-time domain reflectometry (thermo-TDR), which can simultaneously measure dielectric constant, electrical conductivity, volumetric heat capacity, and thermal conductivity, has the potential to estimate θNAPL. The objectives of the study are i) to establish a relationship between the four thermo-TDR measured soil properties and soil water content (θw) and θNAPL values and ii) to evaluate the sensitivities of the thermo-TDR measured properties to θw and θNAPL, and iii) to develop a four-parameter based approach to simultaneously determine θw and θNAPL. Thermo-TDR measurements were performed on sand and glass beads containing various amounts of water and Canola oil as a NAPL. In all cases θw rather than θNAPL dominated all four thermo-TDR measured properties. A sensitivity analysis also indicated that all four properties were more sensitive to θw than to θNAPL. Among the four properties, the dielectric constant, electrical conductivity, and volumetric heat capacity were somewhat sensitive to θNAPL, while thermal conductivity was not sensitive. A new approach using all four thermo-TDR measured properties (four-parameter-based approach) to estimate θNAPL was found to be more accurate than the existing two property-based approaches, i.e., dielectric constant and volumetric heat capacity-based. The root mean square error (RMSE) values for θNAPL estimation with the four-parameter-based approach were 0.066 and 0.042 m3 m−3 for sand and glass beads, while the two property-based approach had RMSE values of 0.180 and 0.220 m3 m−3. The four-parameter-based approach enabled suppression of the effects of measurement errors by the optimization processes and allowed the high sensitivity parameters to cover for shortcomings in the low sensitivity parameters. Use of thermo-TDR sensors with the four-parameter-based approach to determine θNAPL can contribute to various NAPL soil contamination studies as a NAPL content quantifying approach.

Determining water content and bulk density: The heat-pulse method outperforms the thermo-TDR method in high-salinity soils

Heat-pulse (HP) and thermo-time domain reflectometry (thermo-TDR) methods have been used to determine soil thermal properties, water content (θ) and bulk density (ρb) simultaneously. Their performances on salt-affected soils, however, remain unknown. This study investigated the effect of salinity on HP signals and thermo-TDR measured electromagnetic waveforms, and the derived θ and thermal property values of packed soil columns with various textures, saturations and bulk electrical conductivities (σa). The thermo-TDR and HP-based methods for estimating ρb values were also evaluated. The results showed that: (1) at σa values lower than 1.0 dS m−1, the TDR method provided reliable θ with relative errors within 5%; salt effects became apparent at σa values greater than 1.0 dS m−1 due to the distortion of TDR waveforms; the TDR method failed to estimate θ at σa > 2.71 dS m−1 because the 2nd reflection point on the waveform was undetectable; (2) salinity had negligible effects on soil thermal property values in the studied range (σa < 7.59 dS m−1), and the HP-based approach was able to derive θ and ρb values from thermal property measurements, with root mean square errors within 0.02 m3 m−3 for θ and within 0.12 Mg m−3 for ρb. Thus, the HP-based approach outperformed the thermo-TDR approach for determining θ and ρb values in soils with σa > 1.0 dS m−1.

Root influences on soil bulk density measurements with thermo-time domain reflectometry

The thermo-TDR (time domain reflectometry) technique has been applied for measuring soil bulk density (ρb) in-situ. However, the accuracy of thermo-TDR measured ρb data, as influenced by plant roots, has not been studied. In this study, we applied the extended de Vries heat capacity model to examine the influences of roots on thermo-TDR sensor performance for measuring ρb dynamics in the root zone. Soil samples were collected at multiple depths and horizontal positions over time during a maize growth period, and ρb values were determined gravimetrically and indirectly from thermo-TDR measurements. Results showed that by using the extended de Vries model, the thermo-TDR measured ρb agreed well with the gravimetric values. Ignoring root contribution to bulk soil heat capacity introduced 6.7%, 13.8% and 13.9% errors in thermo-TDR measured ρb data on the loamy sand, sandy loam, and clay loam soils, respectively. A critical root density of 0.037 g cm−3 was determined beyond which roots may induce ρb errors greater than 0.1 g cm−3 with the thermo-TDR technique.

Estimation of soil water retention curves from soil bulk electrical conductivity and water content measurements

Measurement of soil water retention curves (SWRCs) is time consuming, and there is no single laboratory device available to measure a SWRC over an entire range of relevant pressures. The van Genuchten (vG) model is commonly used to characterize the shape of the SWRC. Bulk soil electrical conductivity as a function of water content, σ(θ), has been used to estimate hydraulic properties of unsaturated soils, thus making it possible to relate σ(θ) and SWRC. The saturated and residual water content values, θs and θr, can be estimated from soil bulk density and particle size distribution. In this study, we present an approach to estimate vG parameters m and α from σ measured at saturated and residual soil water contents, as well as σ(θ) values measured at intermediate water contents. A thermo-time domain reflectometry (thermo-TDR) sensor is used to measure σ and θ of the same soil sample volume. SWRCs for three soils (Glassil 530 sand, Tennessee silt loam and Illinois clay loam with bulk densities ranging from 1.52 to 1.67 g cm−3, 1.05 to 1.25 g cm−3 and 1.05 to 1.2 g cm−3, respectively) are estimated from σ(θ) measurements and compared with direct SWRC measurements obtained with a tension table and pressure plate extractors. Additional comparisons are made using data obtained from the literature. The proposed method to estimate SWRCs performs well when compared to direct SWRC measurements (with an average RMSE and an average bias of 0.041 cm3 cm−3 and 0.008 cm3 cm−3, respectively). Results indicate that the new σ(θ) based method accurately estimates SWRCs.

A new thermo-time domain reflectometry approach to quantify soil ice content at temperatures near the freezing point

Soil ice content (θi) is an important property for many studies associated with cold regions. In situ quantification of θi with thermo-time domain reflectometry (TDR) at temperatures near the freezing point has been difficult. The objective of this study is to propose and test a new thermo-TDR approach to determine θi. First, the liquid water content (θl) of a partially frozen soil is determined from a TDR waveform. Next, a pulse of heat is applied through the thermo-TDR sensor to melt the ice in the partially frozen soil. Then, a second TDR waveform is obtained after melting to determine the θl, which is equivalent to the total water content (θt) of the partially frozen soil. Finally, θi is calculated as the difference between θt and θl. The performance of the new approach was evaluated in sand and loam soils at a variety of θt values. The new approach estimated θt, θl, and θi accurately. The root mean square errors (RMSE) of estimation were 0.013, 0.020, and 0.023 m3 m−3 for sand, and 0.041, 0.026, and 0.031 m3 m−3 for loam. These RMSE values are smaller than those reported in earlier thermo-TDR studies. Repeating the thermo-TDR measurements at the same location on the same soil sample caused decreased accuracy of estimated values, because of radial water transfer away from the heater tube of the thermo-TDR sensor. Further research is needed to determine if it is possible to obtain accurate repeated measurements. The use of a dielectric mixing model to convert the soil apparent dielectric constant to θl improved the accuracy of this approach. In our investigation, application of a small heat intensity until the partially frozen soil temperature became larger than about 1 °C was favorable. The new method was shown to be suitable for estimating ice contents in soil at temperatures between 0 °C and − 2 °C, and it could be combined with the volumetric heat capacity or thermal conductivity thermo-TDR based methods, which measured ice content at colder temperatures. Thus, the thermo-TDR technique could measure θi at all temperatures.

Measuring dynamic changes of soil porosity during compaction

Soil porosity and pore-size distribution changes in response to compaction are important for heat, water, and air flow in soils. In this study, we used the thermo-time domain reflectometry (thermo-TDR) technique to investigate dynamics of in-situ soil porosity and pore-size distribution as affected by number of traffic passes, water content and soil depth. The study was conducted at a field site located near Clayton, NC, USA. A roller was dragged across the length of a 3- by 12-m plot three to five times to repeatedly compact the soil after tillage. Nine thermo-TDR probes, installed at 2.5-, 7.5-, and 12.5-cm depths (representing 0–5, 5–10, and 10–15 cm depth intervals, respectively) at three locations within the plot, were used to determine dynamic changes in soil porosity after each compaction event. Pore-size distribution changes within the top soil layer were determined for a subset of conditions by measuring in-situ infiltration at low tension using a mini disk infiltrometer. Nine core samples were also collected (considered to be a destructive method) near each thermo-TDR probe for measuring total porosity and water content after each compaction. Results showed that the thermo-TDR technique can accurately monitor the change of soil porosity during soil compaction compared to the destructive core method. Variability of replicated soil porosity measurements by the thermo-TDR technique (with a root mean square error (RMSE) of 0.011 m3 m−3 and mean standard error (MSE) of 0.010 m3 m−3) was lower than that of the core method (RMSE = 0.017 m3 m−3, MSE = 0.019 m3 m−3). As expected, total soil porosity decreased with the number of passes; a major portion of compaction (59–89% of the total porosity decrease) occurred during the first pass. The trend of topsoil (0–5 cm) compaction differed from that of subsoil layers (5–10 and 10–15 cm). Changes in porosity were highly sensitive to soil water content. For the sandy-textured soil in this study, soil porosity decreased as water content increased (during compaction period), and the maximum compaction (associated with the lowest porosity) was reached at an initial water content range between 0.08 and 0.10 g g-1. Above this range, the compaction level decreased with increasing water content. In addition, there was a shift in pore-size distribution for the surface layer. More importantly, pore-size distribution continued to change with additional traffic passes even after soil total porosity became stable.

Experimental comparison of thermal and solute dispersion under one‐dimensional water flow in saturated soils

Thermal dispersion, which is similar to solute dispersion, results mainly from microscopic differences in pore‐scale water velocities in porous media. In this study, heat and Cl− were used as tracers to perform a systematic and quantitative comparison of the characteristics of coupled heat and solute dispersion through three types of saturated packed media (a sand, silt loam and sandy clay loam soil). An inverse model was used to calculate the thermal dispersion coefficient (Dh) and solute dispersion coefficient (Ds) by fitting a heat and solute transport model to the measured temperature and solute breakthrough curves obtained using the thermo–time domain reflectometry (thermo–TDR) technique under the same experimental conditions. Results showed that Ds and Dh were both described by a power function of water flux. Most importantly, heat and solute dispersivities had the same dimensions of length , the magnitudes of which were maintained almost unchanged with water flux densities and depended considerably on soil texture. Fine‐textured soil had a greater dispersivity than did coarse‐textured soil, and solute dispersivity was 220% greater than heat dispersivity in sandy clay loam, about 92% greater in silt loam soil and 36% greater in sand. We suggest that the magnitude and dimension of heat dispersion were essentially comparable to those of solute dispersion, which provides a better understanding of the relation between heat and solute dispersion. It has also been shown to be advantageous in practice for using heat dispersion to quantify easily solute dispersion approximately (or vice versa).Highlights Thermal and solute dispersion coefficients were power functions of water fluxes. Heat and solute dispersivities had the same length and order of magnitude. Thermal dispersion was experimentally comparable to solute dispersion. Solute dispersion may be predicted quantitatively by heat dispersion values (or vice versa).

A design of experimental apparatus for studying coupled heat and moisture transfer in soils at high-temperature conditions

ABSTRACTThe objective of this paper is to present a design of an experimental apparatus for studying coupled heat and moisture transport phenomena in soils at high temperatures up to 90°C and to present some preliminary testing results from the apparatus for understanding the extent of its measurement uncertainties. A soil cell was designed and constructed for enclosing a vertical soil column for experimental studies of one-dimensional heat and moisture transfer in the soil column. The design of the experimental apparatus was realized based on the worst condition of achieving one-dimensional heat flow within a soil cell filled with dry soil. The soil cell, which is of cylindrical shape and contains five thermo-time domain reflectometry (T-TDR) probes, was sandwiched between a hot plate on the top and a cold plate at the bottom for studying coupled heat and moisture transfer in soils. The existence of one-dimensional heat flow from the top to the bottom of the soil column can be indicated by two conditions. In an ideal steady-state condition, first, the amounts of inflow and outflow rates of heat transfer at the top and bottom of the soil column should be the same, and secondly, the heat flux distribution at any cross-section of the soil column should be uniform. In the present design, these conditions were met within ±5% of their variations, as a design criterion. With respect to the design criterion, the general integrity and reliability of the design models were numerically assessed using COMSOL, based on realistic geometries and boundary conditions. Once the apparatus was built, a preliminary test of the apparatus was performed to check the existence of one-dimensional heat flow through the soil column in the soil cell.

An in situ probe‐spacing‐correction thermo‐TDR sensor to measure soil water content accurately

SummaryTo reduce the possibility of probe deflections, conventional thermo‐time domain reflectometry (T‐TDR) sensors have relatively short probe lengths (≤4 cm). However, short probes lead to large errors in TDR‐estimated soil water content (θv). In this study, two new 6‐cm‐long probe‐spacing‐correction T‐TDR (CT‐TDR) sensors were investigated. Compared with conventional 4‐cm‐long T‐TDR sensors, the 6‐cm‐long CT‐TDR sensors reduced errors in TDR‐estimated θv. Errors in heat pulse (HP)‐estimated θv because of probe deflections were reduced when linear or nonlinear probe‐spacing‐correction algorithms were implemented. The 6‐cm‐long CT‐TDR sensors provided more accurate θv estimations than do the conventional 4‐cm‐long T‐TDR sensors.Highlights Short thermo‐TDR sensor has shortcomings in determining soil water content. Changes in thermo‐TDR probe spacing caused by deflections can be determined in situ. Correcting changed thermo‐TDR probe spacing determined soil water content accurately. The 6‐cm‐long thermo‐TDR sensors determined soil water content more accurately than 4‐cm‐long sensors.

Improved thermo-time domain reflectometry method for continuous in-situ determination of soil bulk density

Quantifying the dynamics of surface soil bulk density (ρb) is important for characterizing water, heat, and gas exchanges in agricultural and environmental applications. Unfortunately, very few approaches are available for continuous in-situ monitoring of ρb. The soil heat capacity-based (C-based) thermo-time domain reflectometry (thermo-TDR) approach has been used to measure ρb in-situ, but this approach gives ρb estimates with relatively large errors. In this study, we present a new soil thermal conductivity-based (λ-based) thermo-TDR approach for continuous and automatic determination of ρb variation in-situ. An error analysis, literature data, and field experiments were used to evaluate the performance of the C-based and λ-based approaches. The error analysis undertaken on hypothetical soils indicated that the new λ-based approach was less sensitive to errors in the measurement inputs than was the C-based approach when the same relative errors occurred, except on very dry soils. Thermo-TDR measurements reported in the literature on seven soils showed that the new λ-based approach provided more accurate and precise ρb estimates, with coefficient of determination (R2) of 0.70 and root mean square error (RMSE) of 0.103 Mg m−3, than did the C-based approach which gave ρb with R2 of 0.32 and RMSE of 0.178 Mg m−3. Two field experiments were conducted to test the performance of the new λ-based thermo-TDR approach for monitoring ρb dynamics. The results showed that following tillage surface ρb increased by about 35% within 40 days. The ρb obtained by the λ-based thermo-TDR approach agreed well with independent core sampling measurements, with an average RMSE of 0.122 Mg m−3. The C-based approach failed to give acceptable ρb estimates in most cases because of probe deflection and environmental factors. We conclude that the new λ-based thermo-TDR approach is a promising method for continuous in situ measurements of ρb.

An improved thermo-time domain reflectometry method for determination of ice contents in partially frozen soils

SummaryMeasuring ice contents (θi) in partially frozen soils is important for both engineering and environmental applications. Thermo-time domain reflectometry (thermo-TDR) probes can be used to determine θi based on the relationship between θi and soil heat capacity (C). This approach, however, is accurate in partially frozen soils only at temperatures below −5 °C, and it performs poorly on clayey soils. In this study, we present and evaluate a soil thermal conductivity (λ)-based approach to determine θi with thermo-TDR probes. Bulk soil λ is described with a simplified de Vries model that relates λ to θi. From this model, θi is estimated using inverse modeling of thermo-TDR measured λ. Soil bulk density (ρb) and thermo-TDR measured liquid water content (θl) are also needed for both C-based and λ-based approaches. A theoretical analysis is performed to quantify the sensitivity of C-based and λ-based θi estimates to errors in these input parameters. The analysis indicates that the λ-based approach is less sensitive to errors in the inputs (C, λ, θl, and ρb) than is the C-based approach when the same or the same percentage errors occur. Further evaluations of the C-based and λ-based approaches are made using experimentally determined θi at different temperatures on eight soils with various textures, total water contents, and ρb. The results show that the λ-based thermo-TDR approach significantly improves the accuracy of θi measurements at temperatures ≤−5 °C. The root mean square errors of λ-based θi estimates are only half those of C-based θi. At temperatures of −1 and −2 °C, the λ-based thermo-TDR approach also provides reasonable θi, while the C-based approach fails. We conclude that the λ-based thermo-TDR method can reliably determine θi even at temperatures near the freezing point of water (0 °C).

Use of a thermo-TDR probe to measure sand thermal conductivity dryout curves (TCDCs) and model prediction

Thermal conductivity is a governing soil thermal property for geothermal applications. Measurements of soil thermal conductivity and moisture content are usually obtained separately by different sensors, which may lead to errors in measured thermal conductivity dryout curves (TCDCs). Use of a newly designed thermo-time domain reflectometry (TDR) probe to measure sand TCDCs and model prediction were carried out in this study. Laboratory experiments were performed on four quartz sands with different grain sizes. The calibration relationships for dielectric constant (Ka) and electrical conductivity (ECb) were first established by a multiple specimen method. The thermo-TDR probe was then used to measure sand thermal conductivity, and predict its moisture content and dry density in dryout process produced by a modified hanging column device (MHCD) for establishing sand TCDCs. Sand thermal conductivity was also predicted by three alternative soil thermal conductivity predictive models, and then compared to measured values. Statistical analyses were conducted to further evaluate the model performance and validate the reliability of the proposed method. It is concluded that the thermo-TDR probe is capable of measuring sand TCDCs satisfactorily in a holistic manner, and providing a more accurate estimation of sand thermal conductivity under varying moisture content condition. Measurement error might be increased when sand is under near dry condition due to the heat convection effect.

A new generalized soil thermal conductivity model for sand–kaolin clay mixtures using thermo-time domain reflectometry probe test

As the demand of exploitation and utilization of geothermal energy increases, more geothermal-related earth structures occur recently. The design of the structures depends upon an accurate prediction of soil thermal conductivity. The existing soil thermal conductivity models were mostly developed by empirical fits to datasets of soil thermal conductivity measurements. Due to the gaps in measured thermal conductivities between any two tested natural soils, the models may not provide accurate prediction for other soils, and the predicted thermal conductivity might not be continuous over the entire range of soil type. In this research, a generalized soil thermal conductivity model was proposed based on a series of laboratory experiments on sand, kaolin clay and sand–kaolin clay mixtures using a newly designed thermo-time domain reflectometry probe. The model was then validated with respect to k dry–n (thermal conductivity of dry soils and porosity) and k r–S r (normalized thermal conductivity and degree of saturation) relationships by comparing with previous experimental studies. The predicted thermal conductivities were found to be in a good agreement with the experimental data collected from both this study and the other literatures with at least 85% confidence interval. It is concluded that the proposed model accounts for the effects of both environmental factors (i.e., moisture content and dry density) and compositional factors (i.e., quartz content and soil type) on soil thermal conductivity, and it has a great potential in predicting soil thermal conductivity more accurately for geothermal applications.

Application of a thermo-time domain reflectometry probe in sand-kaolin clay mixtures

Simultaneous measurements of soil thermal properties, moisture content and dry density are quite challenging, but very significant to geothermal related engineering applications. Thermo-time domain reflectometry (TDR) probe is able to measure soil thermal properties by the thermos probe, and predict soil moisture content and dry density by the TDR technique. This research studies the application of a new thermo-TDR probe in measuring thermal property, moisture content and dry density of sand-kaolin clay mixtures. Laboratory experiments were performed on five sand-kaolin clay mixtures with clay content ranging from 0% to 30% by dry weight. Measured thermal properties by the thermo-TDR probe were compared with a KD2 Pro. The moisture content and dry density were predicted by four alternative methods: Topp's equation, heat capacity method, one-step TDR method and Jung's method. The results revealed that Jung's method exhibited the highest prediction accuracy among the four methods, being the relative deviations between the predicted and measured values within 1% and 5% for gravimetric moisture content and dry density, respectively. It is concluded that the thermo-TDR probe can be applied in sand-kaolin clay mixtures successfully to accurately measure their thermal and geotechnical properties.

Thermal conductivity of sand–kaolin clay mixtures

Thermal conductivity is a key parameter for the design of various geothermal structures such as geothermal energy piles, ground source heat pumps and soil borehole thermal energy storage. In this paper, the thermal conductivity of sand–kaolin clay mixtures prepared at varied moisture content, dry density and clay content was measured using a newly designed thermo-time domain reflectometry probe. The data revealed that the thermal conductivity of the sand–kaolin clay mixtures increased with an increase in moisture content and dry density. The thermal conductivity of the moist sand–kaolin mixtures decreased with an increase in clay content in two distinct regimes including a slow decrease in thermal conductivity above a critical clay content, which was found to be 10%. Five soil thermal conductivity models were selected to predict the thermal conductivity of the sand–kaolin clay mixtures, and the predictions were compared with the experimental results. Two models were modified to account for the effect of clay content on the thermal conductivity of the sand-clay mixtures. Recommendations regarding the design of energy geostructures were made based on the measured thermal conductivity data.

Design and Evaluation of a Thermo-TDR Probe for Geothermal Applications

A thermo-time domain reflectometry (TDR) probe can function both as a regular probe, for measuring moisture and density, and a dual heat probe, for measuring thermal conductivity, thermal diffusivity, and volumetric heat capacity. This paper describes the design, fabrication, and evaluation of a newly developed thermo-TDR probe. The developed probe was first calibrated for its TDR function in chemicals with known dielectric constants and electrical conductivity. Then the probe was tested in three sands and Kaolin clay at different moisture contents and densities. Available methods for analyses of the thermo-TDR-acquired signals were presented and discussed. The measured thermal properties were also compared against a standard thermal probe KD2 pro. It was found that the thermo-TDR probe has satisfactory accuracy in measurements of soil thermal conductivity, moisture content, and dry density. Based on the test results, recommendations have been provided regarding the use of thermo-TDR probes for geothermal applications.

Thermal Conductivity of Quartz Sands by Thermo-Time Domain Reflectometry Probe and Model Prediction

AbstractThe heat transfer process in soil depends on its thermal conductivity property and is affected by soil compositional and environmental factors, such as soil gradation, mineralogy components, compaction moisture content, and dry unit weight conditions. In this research, thermal conductivities of three quartz sands were studied using a newly developed thermo–time domain reflectometry (TDR) probe. The test results revealed that thermal conductivity of quartz sands increased with compaction moisture content and dry unit weight. Higher quartz content also led to higher thermal conductivity. The new thermo-TDR probe provided measurements of thermal properties similar to those of the standard KD2 probe. The deviation of the two methods was within 5%. An improved thermal conductivity model was also proposed on the basis of the normalized thermal conductivity concept and present experimental data. This improved model exhibited the highest accuracy in thermal conductivity predictions of quartz sands (within...

In Situ Monitoring of Soil Bulk Density with a Thermo-TDR Sensor

Monitoring spatial and temporal dynamics of soil bulk density (ρb) can be challenging, and ideally soil ρb measurements should be continuous in situ. The thermo-time domain reflectometry (TDR) technique is emerging as a useful technique for measuring soil ρb, with the advantages of being automated, rapid, and minimally destructive. The objective of this study is to evaluate the effectiveness of the thermo-TDR technique for monitoring the temporal dynamics of soil ρb under field conditions. Thermo-TDR sensors were installed at two experimental sites with different soil textures, silt loam and sandy loam, to monitor soil ρb continuously in the 0- to 10- and 10- to 20-cm soil layers. The results showed that the thermo-TDR ρb in general agreed with gravimetric ρb from the core method, and the differences between the two methods were within 8%. Compared with the core method, the thermo-TDR technique had the advantage of capturing the in situ ρb changes over time with minimal soil disturbance. We conclude that the thermo-TDR sensors can be used effectively to continuously monitor soil ρb under field conditions.

Formulation and Characterization of Freezing Saturated Soils

The writers have derived a unified governing equation for freezing saturated soils. This equation considers the governing mechanisms with respect to individual thermal and hydraulic fields. The writers included coupling effects such as the thermodynamic equilibrium on the water-ice interface. The morphology of the solid matrix and the physical chemistry of the water-ice interface have also been incorporated. The equation is comprised of terms with clear physical meanings. Typical properties that are indicative of freezing soils, e.g., segregation potential, can be derived from this equation. The writers discuss the material properties that are required for implementation of the equation. For the conventional parameters in the equation, i.e., thermal conductivity, heat capacity, and hydraulic conductivity, the corresponding mathematical descriptions were investigated. The functions for prediction of these parameters during the soil freezing process are presented. The writers have also proposed a relationship between the temperature and unfrozen water content for characterization of freezing saturated soils. The writers propose that the measurement of this relationship be conducted with a new technique that uses a thermo–time domain reflectometry (TDR) sensor. The writers have described detailed sensor and experiment designs for measuring this new relationship and have compared the results with data that were measured with a standard method.