Time Domain Reflectometry Probes Research Articles

Application of time domain reflectometry to triaxial shear tests on hydrate-bearing sediments

Various triaxial shearing apparatus are used to quantify mechanical properties of hydrate-bearing sediments which are basically required to the assessment of risks posed by dissociation of natural gas hydrates. However, the key factor, hydrate saturation, has not been well measured during triaxial shearing tests. This study aims to show how a time domain reflectometry (TDR) method nondestructively measures hydrate saturation for triaxial shear tests on hydrate-bearing sands. Hydrate saturation is determined based on the difference of volumetric water contents measured by using a flexible TDR probe with two rods after calibration. The results suggest that values of hydrate saturation acquired by the TDR method are accurate, and the flexible TDR probe can monitor the kinetic process of hydrate formation and avoid any reinforcement effects on triaxial shearing properties of test specimens.

Time domain reflectometry coil probes for measuring thin surface layer soil moisture: Field tests over 21 years (2002–2022) under highly variable climate conditions in Mongolia

A coil probe (CP) for time-domain reflectometry (TDR) with a sensor length of 40 mm (CP40) was developed for long-term field soil moisture measurements in a thin surface soil layer (0–3 cm depth). In laboratory, soil moisture measurements of CP40 were nearly identical to those of a traditional two-rod type TDR (2RTDR) probe (rod length: 15 cm, diameter: 0.3 cm, spacing: 3 cm). The CP40 measurement accuracy was between 0.01 m3/m3 and 0.03 m3/m3. For long-term field soil moisture measurements, five CP40 units were installed in the highly wetted soil at the Sanzai site (SS), which is in the permafrost area of the Taiga, and dry soil at the Mandalgobi site (MGS) in the semi-arid area of Mongolia. Four units accurately measured soil moisture at both sites over six years (2002–2007and 2008–2009). Three units succeeded in conducting precisely continuous soil moisture measurements between 2008 and 2022 at the MGS. Two units successfully measured soil moisture over 21 years at both the sites. The representativeness of the CP40 soil moisture measurements in highly wetted soils was low because of heavy rainfall and soil heterogeneity. The accuracy of CP40 soil moisture measurements in highly wetted soils was slightly lower than that of the traditional 2RTDR probe. However, the accuracy of the CP40 soil-moisture measurements in the dry soil was comparable to that of the traditional 2RTDR probe. CP40 units are durable and their field soil moisture measurements demonstrate stable and precise performance (bias, RMSE), even under severe environmental conditions.

Evaluation of electrical resistivity of cement-based materials using time domain reflectometry

Time domain reflectometry (TDR) is a widely used electromagnetic wave–based technique for evaluating the volumetric water content of soils, although its use for moisture measurement in cement-based materials is limited. This paper presents a comprehensive analysis of the electrical properties of cement pastes and cement paste–slime mixtures using TDR. Both TDR and electrical resistivity probes are used to estimate the electrical resistivities of various cement-based materials over their curing period. The study reveals that these properties are significantly influenced by curing time, water–cement ratio, and water–solid ratio. The experimental results demonstrate a consistent increase in electrical resistivity with curing time, underscoring the significant role of hydration-induced water evaporation and microstructural changes. Furthermore, the interpretation of characteristic voltages on the TDR waveform significantly influences resistivity estimates. Thus, this study highlights the potential of TDR in accurately evaluating the electrical resistivity of cement-based materials.

Curing process monitoring of cementitious grout using time domain reflectometry (TDR) system

Curing process monitoring of the cementitious grout is necessary for its successful use and to ensure the serviceability of the constructed, repaired, and stabilized infrastructures using that material. In this study, time domain reflectometry (TDR) system is applied to monitor the curing process of the cementitious grout based on the changes in relative permittivity (εr). To utilize the TDR system to electrically conductive cementitious grout, a TDR probe with coated core electrode (coated probe) is manufactured, and a calibration is performed to evaluate the formal εr by using the coated probe. The coated probes are installed in the middle and at the bottom of the cementitious grouts with a length of 950 mm, and the εr have been evaluated with curing time using both middle probe (εr(middle)) and bottom probe (εr(bottom)). In addition, compressive strength tests are performed on separately prepared specimens for comparative analyses. Experimental results show that the εr(middle) and εr(bottom) decrease with curing time and converge from the fully cured time, and the curing process of cementitious grout monitored by the TDR system is consistent with the compressive strength developments of the specimens tested with curing time. Therefore, the TDR system with a coated probe may effectively be used for monitoring the curing process of the cementitious grouts based on the changes in electromagnetic property.

Development and Application of an IoT-Based System for Soil Water Status Monitoring in a Soil Profile.

Soil water content (θ), matric potential (h) and hydraulic conductivity (K) are key parameters for hydrological and environmental processes. Several sensors have been developed for measuring soil θ-h-K relationships. The cost of such commercially available sensors may vary over several orders of magnitude. In recent years, some sensors have been designed in the framework of Internet of Things (i.e., IoT) systems to make remote real-time soil data acquisition more straightforward, enabling low-cost field-scale monitoring at high spatio-temporal scales. In this paper, we introduce a new multi-parameter sensor designed for the simultaneous estimation of θ and h at different soil depths and, due to the sensor's specific layout, the soil hydraulic conductivity function via the instantaneous profile method (IPM). Our findings indicate that a second-order polynomial function is the most suitable model (R2 = 0.99) for capturing the behavior of the capacitive-based sensor in estimating θ in the examined soil, which has a silty-loam texture. The effectiveness of low-cost capacitive sensors, coupled with the IPM method, was confirmed as a viable alternative to time domain reflectometry (TDR) probes. Notably, the layout of the sensor makes the IPM method less labor-intensive to implement. The proposed monitoring system consistently demonstrated robust performance throughout extended periods of data acquisition and is highly suitable for ongoing monitoring of soil water status.

A permittivity-conductivity joint model for hydrate saturation quantification in clayey sediments based on measurements of time domain reflectometry

Hydrate saturation (Sh) is one of the key parameters for resource assessment of hydrate reservoirs and production optimization of natural gas. There are still significant challenges in determining the Sh in clayey formations. Both dielectric and resistivity logging tools have been used for identifying and evaluating hydrate-bearing formations; however, there is little work on a joint analysis and modelling of the permittivity and resistivity for quantifying the Sh. To bridge the knowledge gap, we have proposed a novel permittivity-conductivity (P–C) joint approach based on TDR (time domain reflectometry)-derived parameters (i.e., apparent permittivity Ka and bulk conductivity σdc) in this work. The proposed P–C joint approach can provide a theoretical basis for the joint interpretation of dielectric and resistivity geophysical measurements on hydrate-bearing formations in the field. First, the basic theory for deriving the Ka and σdc from the TDR responses of hydrate-bearing sediments was formulated based on the dielectric polarization and electrical conduction mechanisms. Second, an experimental campaign was carried out including the development of experimental system, calibration of TDR probe and design of experimental scheme. Third, the influences of hydrate saturation, clay mineralogy and clay content on the TDR responses of unconsolidated sediments were examined. Then the Ka and σdc were related to Sh respectively, and finally a novel P–C joint model for the quantification of Sh in clayey sediments was established and verified. It has been demonstrated that: (1) the Ka of the clayey samples with hydrates decreases almost linearly with an increasing clay content up to 20 %, while the σdc of the smectite-bearing samples decreases nonlinearly in contrast to the linear trend for illite; (2) the power-law mixing formula incorporating an empirical exponent is a preferable permittivity model for hydrate-bearing clayey sediments due to its merits of empirical and theoretical nature, while the Simandoux equation is effective to account for the clay effects on the conductivity of hydrate-bearing sediments with smectite and illite; (3) the P–C joint model can be established by utilizing the porosity of hydrate-bearing sediments as a bridge parameter between Ka and σdc. The variation behavior of Ka and σdc with different types and contents of clay minerals can be explained by the difference of the amount of bound water and swelling effects between the illite-bearing and smectite-bearing samples. The proposed P–C joint model outperforms the standalone permittivity-based and conductivity-based models especially for the clayey cases. The root-mean-squared errors of the P–C joint models are 7.339, 2.930 and 2.065 % for the clean-sand samples, clayey samples with illite and smectite, respectively.

Trunk dielectric permittivity correlates with irrigation based on soil water content in fruit trees

Time-domain reflectometry (TDR) is an electromagnetic technique that measures the dielectric permittivity (K) which is a surrogate property influenced by water content. Advances in nanoelectronics have enabled the development of a TDR probe (TDR-305 N) to monitor changes in K, bulk electrical conductivity (ECbulk) and temperature (T) in a porous medium, such as a tree trunk. The main objective of this study was to assess the effectiveness of the TDR-305 N sensors for real-time monitoring changes in water content in the trunk of nectarine trees. Throughout the summer of 2022, irrigation was automatically managed with threshold values of soil water content (θv-soil) measured with capacitance probes. Different management allowed depletion (MAD) values were set to trigger irrigation: 50 % in July (moderate water deficit), 100 % in August (severe water deficit), and recovery to well-irrigated conditions in September. Discrete measurements of midday stem water potential (Ψs,md) and leaf gas exchange were made frequently. The results showed a progressive reduction of the measured physiological parameters, as well as of K and ECbulk and θv-soil decreased. Notably, Ψs,md reached a critically low value of -2.03 MPa, coinciding with pronounced and severe stomatal closure. Both K and Ψs,md, were able to explain the variations of θv-soil by more than 75 %. Daily, a positive relationship of K and ECbulk was observed, although ECbulk exhibited a stronger dependence on Ttrunk compared to K. Furthermore, K did not return to its initial values prior to the onset of water stress, possibly influenced by xylem cavitation and a reduction in leaf area during its senescence stage. The findings suggest that trunk permittivity measurements obtained using TDR-305 N sensors could be a reliable indicator for monitoring tree water status. However, further research is needed to determine the threshold values of trunk water content under non-limiting soil water conditions for accurate irrigation scheduling.

Transient infiltration tests in pyroclastic soils with double porosity

Fallout volcanic deposits of Somma- Vesuvius (Campania, southern Italy), characterized by the presence of layers with contrasting textural and hydraulic properties, are frequently affected by shallow landslides during rainwater infiltration. The soils of the stratigraphic sequence present intra- particle pores, originated by the gases escaped during magma decompression in the volcanic conduit, thus are characterized by double porosity (i.e., intraparticle and interparticle pores), which is expected to affect their hydraulic behaviour, and to play a key role in rainwater infiltration through layered deposits. To understand the effect of double porosity on the hydraulic behaviour of the involved soils, controlled experiments have been carried out in an infiltration column. The experimental apparatus is provided with newly designed non-invasive Time Domain Reflectometry (TDR) probes, not buried in the investigated soil layers so as to minimize disturbance to the flow, allowing water content measurement during vertical flow processes. Specifically, transient flow experiments are carried out through reconstituted specimens of black scoriae and grey pumices, both loose pyroclastic granular soils from fallout deposits of Somma-Vesuvius, featuring double porosity with different pore size distributions, that were estimated by X-ray tomography and Mercury Intrusion Porosimetry. The experimental results highlight the effects of the double porosity and clearly indicate the different behaviour of the two soils during wetting and drying processes, mainly related to the different dimensions of intraparticle pores.

Water content imaging during capillary rise within a rammed earth structure

The capillary rise in earthen buildings is a factor of risk since it changes the mechanical properties of rammed earth. A laboratory-scale experiment is performed to see the ability of time-domain induced polarization to non-intrusively monitor the change in the water content during a capillary rise experiment in a rammed earth wall (150 cm in height and 75 cm in width). A total of 64 self-adhesive Ag/AgCl electrodes are placed at the surface of the rammed earth wall. The contact between the electrodes and the wall is insured by a conductive gel. The distance between the electrodes is 4.5 cm. An impedance meter is used to perform time-domain induced polarization measurements at 7 specific dates in July 2022. The raw data (apparent resistivity and chargeability) are inverted to produce tomograms of electrical conductivity and normalized chargeability through the wall. These tomograms are presented along the plane drawn through the position of the electrodes. A petrophysical model recently developed to interpret electrical conductivity and normalized chargeability data is used here to recover the water content distribution over time. The results are compared to local measurements of the water content performed thanks to a TDR (Time Domain Reflectometry) probe placed within the wall. In addition, the wet front associated with the capillary rise is monitored using a camera. The data shows a consistent position of the capillary rise between the various datasets. A methodology is described to obtain the distribution of the water content over space and time in the rammed earth structure.

Uniplanar multirod TDR probe for soil water content measurement over a larger spatial scale

AbstractThis study developed a uniplanar multirod time domain reflectometry (TDR) probe and evaluated it in laboratory experiments for soil water measurement over a larger spatial scale. The sampling volume of the probe was determined by simulating the electric potential distribution around the probe rods by using the HYDRUS‐2D software package and verified by comparison with the measured dielectric constant in saturated sand. The performance of the probe was evaluated by comparing its dielectric measurements in wet sand over a wide range of water content during evaporative drying with that measured by CS640 probes. Additionally, the dependency of visible light reflection on the water content of the sand was evaluated. The results revealed that the average effective thickness of the probe's sampling volume was 5–6 cm on both sides of the plane of the probe rods. The probe accurately measured the soil water content over a 0.88 m2 spatial area. The visible light reflectance of the variably saturated sand surface and the corresponding water content of the sand follow a soil water retention function‐type relationship. The usage of the multirod TDR measurement system at a larger spatial scale carries practical implications, especially in increasing the accuracy of remote sensing data.

Routing stemflow water through the soil via preferential flow: a dual-labelling approach with artificial tracers

Abstract. Stemflow and its belowground funnelling along roots and macropores may play an important role in the soil moisture redistribution in forest environments. In this study, a stemflow experiment on Pinus sylvestris L. (Scots pine) used artificial tracers to view and quantify preferential flow after stemflow infiltration into the soil. A total of 41 L of water labelled with enriched deuterium and brilliant blue FCF were applied at a flow rate of 7 L h−1 to the stem of a pine tree, which corresponds to the stemflow caused by about 50 mm of rainfall. Time domain reflectometry (TDR) probes were installed around the tree trunk to measure the high-resolution volumetric water content. A total of 1 d after the stemflow discharge, soil pits were dug in the different cardinal directions and at varying distances from the tree. Photographs were taken for imaging analysis to quantify preferential flow metrics. Soil samples were taken from the different profiles to analyse the dye concentrations and isotopic compositions. We found that stemflow infiltrated through an annulus-shaped area around the tree base. We observed a heterogenous spatiotemporal soil moisture response to stemflow and the occurrence of shallow perched water tables around the tree trunk. Dye staining demonstrated that stemflow infiltrated primarily along the surface of coarse roots and through macropores. The dye coverage was less extensive close to the soil surface and increased with depth and with proximity to the tree trunk. Lateral flow was also observed, mainly in the shallow soil layers. Our analyses demonstrate the prevalence of preferential flow. Deuterium and brilliant blue FCF concentrations were significantly correlated. The tracer concentrations decreased with increasing distance from the tree trunk, indicating dilution and mixing with residual soil water. Macropores, coarse roots (living or decayed) and perched water tables produced a complex network regulating the preferential flow. Our results suggest that stemflow affects soil moisture distribution, and thus likely also groundwater recharge and surface runoff. Our study provides insights into the soil hydrological processes that are regulated by stemflow belowground funnelling and improves our understanding of forest–water interactions.

In-situ evaluation of chitosan-based materials containing fertilizer by time-domain reflectometry (TDR) technique

Population growth and resource reduction have created tremendous pressure on the current world scenario to meet the increasing food demands. To face this challenge, strategies are needed to promote the rational use of natural resources since overusing agricultural inputs such as chemical fertilizers may lead to severe environmental problems. In the present work, we evaluated (i) the nutrient release profile of macrospheres, microspheres, and microcapsules based on chitosan, montmorillonite clay, and KNO3 by TDR technique and (ii) the influence of controlled water replacement on the nutrient release in soil. TDR probes inserted in containers with sandy soil evaluated the nutrient release from materials, with and without tracking of field capacity, obtaining results for soil moisture and electrical conductivity. The nutrient release was dependent on soil water management, improving fertilization oversight. Microcapsules released the nutrient slower and for longer periods (higher than 46 days). Also, the water content corroborated the swell ability of materials, which affected and controlled the release of nutrients. These findings validate that chitosan-based materials matched the requirements for a more efficient and sustainable fertilizer, and the TDR technique effectively determined the release of nutrients.

Imaging the water content of rammed earth materials with induced polarization

Rammed earth is a traditional building material characterized by its low environmental impact. The variation of the moisture content of rammed earth can affect its mechanical properties. Therefore, the non-intrusive evaluation of the water content and its distribution inside rammed earth structures is of high interest to anticipate the risk of failure. A non-destructive geophysical method called induced polarization is used to evaluate its sensitivity to the water content distribution. Measurements are first performed on small cylindrical core samples of rammed earth in order to test a physical model developed to connect induced polarization properties to saturation. Then, the model is used to estimate the water content at 3 distinct heights in 3 cylindrical core samples undergoing capillarity rise. Finally, these samples were cut and their water contents were independently determined and compared to that estimated from induced polarization measurements. A good correlation between the two estimates was obtained. In the second part of this study, induced polarization is used to image the water content distribution and its evolution over time in a rammed earth structural element undergoing desiccation. The monitoring is done for a period of 25 days starting from saturated conditions. We assess the evolution of the 3D distribution of the water content and we demonstrate that the water content obtained through induced polarization tomography is in agreement with independent data from a Time Domain Reflectometry (TDR) probe. Such a probe provides a local measurement of the water content thanks to the measurement of the dielectric constant of the material.

A field evaluation of the SoilVUE10 soil moisture sensor

AbstractThe U.S. Climate Reference Network (USCRN) has been engaged in ground‐based soil water and soil temperature observations since 2009. As a nationwide climate network, the network stations are distributed across vast complex terrains. Due to the expansive distribution of the network and the related variability in soil properties, obtaining site‐specific calibrations for sensors is a significant and costly endeavor. Presented here are three commercial‐grade electromagnetic sensors, with built‐in thermistors to measure both soil water and soil temperature, including the SoilVUE10 Time Domain Reflectometry (TDR) probe (hereafter called SP) (Campbell Scientific, Inc.), 50 MHz coaxial impedance dielectric sensor (model HydraProbe, Stevens Water Monitoring Systems, Inc.) (hereafter called HP), and the TDR‐315L Probe (model TDR‐315L, Acclima, Inc.) (hereafter called AP), which were evaluated in a relatively nonconductive loam soil in Oak Ridge, TN, from 2021 to 2022. The HP manufacturer‐supplied calibration equation for loam soils was used in this study. While volumetric water content data from HP and AP were 82–99% of respective gravimetric observations at 10 cm, data from SP were only 65–81% of respective gravimetric observations in the top 20‐cm soil horizon, where soil water showed relatively large spatial variability. The poor performance of the SP is likely due to poor contact between SP sensor electrodes and soil and the presence of soil voids caused by the installation method used, which itself may have caused soil disturbance.

Instrumented dynamic cone penetrometer incorporated with time domain reflectometry

In this study, an instrumented dynamic cone penetrometer incorporated with time domain reflectometry (IDPT) is developed to provide portable in-situ penetrometer for evaluation of ground strength and water content. The IDPT probe combines cone tip energy module with accelerometer and load cell, and time domain reflectometry (TDR) probe to simultaneously obtain transferred energy and water content. The IDPT is applied to embankment with dynamic cone penetrometer (DCP) and penetrometer incorporated with TDR probe (TDRP). In addition, the strength and water content of ground in rainfall conditions are characterized with time. Penetration indices of IDPT and DCP show similar trends in ground strength characterization, and volumetric water contents detected by IDPT and TDRP are almost identical. As water infiltrates the ground with time, the penetration index and volumetric water content increase. This study suggests that the IDPT may be effectively used for the simultaneous evaluation of ground strength and water content.

Mapping of soil moisture by time domain reflectometry and electrical resistivity tomography at Velika Gorica well field, Zagreb aquifer

Knowing the soil moisture distribution in the unsaturated zone can improve understanding the water flow through the unsaturated zone and thereby enable the calculation of aquifer recharge, which occurs through precipitation. One part of the Zagreb aquifer recharge occurs through infiltration from precipitation. In order to observe and model infiltration from precipitation through the unsaturated zone, the research polygon was constructed at the Velika Gorica well field, located in the southern part of the Zagreb aquifer, Croatia, where hourly measurements of electric conductivity (EC) and soil moisture content were carried out. EC and soil moisture data are measured by Time Domain Reflectometry (TDR) probes which are placed at different depths in the unsaturated zone. Furthermore, electrical resistivity tomography (ERT) measurements were conducted. Geophysical data, along with moisture and EC data from TDR probes, were used as input data for MoisturEC software, in order to obtain soil moisture distribution along a 2D profile. MoisturEC program offers three options for translating EC data to moisture content data which are all tested in this research. We obtained eight moisture content distributions along the observed profile and concluded that MoisturEC provides reasonable results with input data from geophysical measurements and TDR probe measurements. Soil moisture distribution in the unsaturated zone represents the initial conditions for further unsaturated flow modeling. Understanding the flow in the unsaturated zone enables the quantification of effective infiltration and can improve groundwater management.

Design, calibration, and validation of an inline green coffee moisture estimation system using time-domain reflectometry

Wet basis moisture content (MCwb) is an important quality parameter of green coffee as it affects the coffee's physical, chemical, and sensory characteristics. Accurate estimation of green coffee MCwb after dry hulling, long-term storage, and transportation is imperative to prevent quantitative and qualitative losses. Thus, this study aimed to design, develop, calibrate and validate a prototype inline system capable of accurately measuring the MCwb of green coffee beans, using a commercially available time-domain reflectometry (TDR) probe. The TDR probe was calibrated and validated with green coffee within a MCwb range of 9–21%. A calibration linear regression model correlating the TDR probe output (dielectric constant) to reference MCwb measurements obtained by a halogen moisture analyzer, yielded a high coefficient of correlation (R2 = 0.99). Model validation yielded a high R2, and a low Root Mean Squared Error equal to 0.93, and 0.9% MCwb, subsequently. Results indicate that the TDR inline green coffee moisture estimation system has the potential to be applied in real-time, industrial-scale operations.

In situ estimation of soil hydraulic and hydrodispersive properties by inversion of electromagnetic induction measurements and soil hydrological modeling

Abstract. Soil hydraulic and hydrodispersive properties are necessary for modeling water and solute fluxes in agricultural and environmental systems. Despite the major efforts in developing methods (e.g., laboratory-based, pedotransfer functions), their characterization at applicative scales remains an imperative requirement. Accordingly, this paper proposes a noninvasive in situ method integrating electromagnetic induction (EMI) and hydrological modeling to estimate soil hydraulic and transport properties at the plot scale. To this end, we carried out two sequential water infiltration and solute transport experiments and conducted time-lapse EMI surveys using a CMD Mini-Explorer to examine how well this methodology can be used to (i) monitor water content dynamic after irrigation and to estimate the soil hydraulic van Genuchten–Mualem parameters from the water infiltration experiment as well as (ii) to monitor solute concentration and to estimate solute dispersivity from the solute transport experiment. We then compared the results with those estimated by direct time domain reflectometry (TDR) and tensiometer probe measurements. The EMI significantly underestimated the water content distribution observed by TDR, but the water content evolved similarly over time. This introduced two main effects on soil hydraulic properties obtained by the two methods: (i) similar water retention curve shapes, but underestimated saturated water content from the EMI method, resulting in a scaled water retention curve when compared with the TDR method; the EMI-based water retention curve can be scaled by measuring the actual saturated water content at the end of the experiment with TDR probes or by weighing soil samples; (ii) almost overlapping hydraulic conductivity curves, as expected when considering that the shape of the hydraulic conductivity curve primarily reflects changes in water content over time. Nevertheless, EMI-based estimations of soil hydraulic properties and transport properties were found to be fairly accurate in comparison with those obtained from direct TDR measurements and tensiometer probe measurements.

Comparative study on engineering properties of cement-based backfill material for sustainable urban area

• Relative permittivity of CLSM decreases with curing time during step II and III. • Compressional-wave velocity exponentially increases with curing time during step III. • Relative permittivity increases with FC because silts lead to water absorption. • Silts on force chain and pore distribution by FC affect compressional-wave velocity. • Strength can be expressed as a function of the wave velocity and the permittivity. Backfill materials used in urban areas should have suitable mechanical properties (e.g., strength and flowability) for safety and serviceability of constructed geo-infrastructures. The objective of this study was to estimate the relative permittivity and compressional-wave velocity to correlate them with the unconfined compressive strength of a controlled low-strength material (CLSM) for backfill. CLSMs with fines content (FC) of 30 %, 50 %, and 90 % were prepared in sensing molds instrumented by a time domain reflectometry probe and piezo disk element. The test results show that the relative permittivity decreases with respect to the curing time, whereas the compressional-wave velocity and unconfined compressive strength increase. In addition, the relative permittivity increases as the FC increases owing to the water content, while the compressional-wave velocity and unconfined compressive strength decrease owing to the FC effect on the force chain. In this context, the FC might disturb the load-carrying matrix, and it results in remaining water and delays the curing time within 672 h. From the correlations, the relationship between the unconfined compressive strength and compressional-wave velocity with respect to the relative permittivity can be reliably constructed at an R 2 of 0.83. For this reason, it is considered that the water content from the relative permittivity can improve the relationship between strength and stiffness. Consequently, the time domain reflectometry probe can be effectively used for reliable assessment of CLSM strength during curing.

Feasibility of soil erosion measurement using time domain reflectometry

Scientific research and control technology of soil erosion both rely on accurate and efficient soil erosion measurements. Based on the principles of locating interfaces in layered media and measuring dielectric properties by time domain reflectometry (TDR), we developed TDR-based measurement of changes in the air–soil interface along the TDR probe for soil erosion measurement, and simultaneously measuring volumetric soil moisture content (θv). Part of a three-rod TDR probe was inserted in the soil and the rest exposed to air, so changes in the air–soil interface could be denoted by changes in the length of exposed probe (Lair). TDR waveforms were analyzed using a combination of tangent line method and second-order bounded mean oscillation methods to identify time points at positions of impedance discontinuity in electromagnetic propagation, then converting these data to Lair and θv by calibrated equations. According to the laboratory experiment using sandy loam and silt loam, Lair could be effectively measured within a wide soil moisture range by the calibrated equation and the thickness of the eroded soil layer could thus be determined. The RMSE of θv measurement ranged between 0.003 and 0.025 cm3 cm−3, which enabled the identification of the appropriate soil moisture range for measuring Lair, and achieve reliable simultaneous measurement of soil erosion and soil moisture content. The simulated rainfall experiment using sandy loam and silt loam and field scouring experiment conducted in loam also indicated that the proposed method could achieve high measurement accuracy in erosion environments, for which Lair with the RMSE from 1.8 to 2.1 mm and θv with the RMSE from 0.007 to 0.024 cm3 cm−3. The proposed method has various advantages, including versatility, minimal disturbance, easy automation of measurement at multiple positions, etc., could serve as a new technical option for soil erosion measurement, and extend the utilization of TDR.