Factors affecting soil permittivity and proposals to obtain gravimetric water content from time domain reflectometry measurements

Abstract

Time domain reflectometry (TDR) measures the apparent relative dielectric permittivity (ARDP) of a soil and is commonly used to determine the volumetric water content (VWC) of the soil. ARDP is affected by several factors in addition to water content, such as the soil's electrical conductivity, temperature, and density. These relationships vary with soil type and are very soil-dependent, and despite previous research, they are still not fully understood. A multivariate statistical approach (principal component analysis, PCA) is used to describe a range of soils from two separate sites in the UK (clay and silty sandsandy silt). The advantage of a PCA is that it considers several variables at a time, giving an immediate picture of their underlying relationships. It was found that for the studied soils, ARDP was positively correlated with VWC and bulk electrical conductivity, but did not show any dependence on some other geotechnical parameters. TDR has recently been used in geotechnical engineering for measuring the gravimetric water content (GWC) and dry density. However, the current approaches require a custom-made TDR probe and an extensive site specific empirical laboratory calibration. To extend the potential use of TDR in the geotechnical industry, three relatively simple methods are proposed to estimate the GWC from VWC (derived from the measured ARDP values) and dry density depending on the amount of information known about the soil. Examples of possible applications of these methods include continuous monitoring of consolidation adjacent to a structure, the effect of seasonal weather and climate change on ageing earthwork assets, and the shrink-swell potential adjacent to trees. All three methods performed well, with between 83% and 98% of the data lying within a ±5% GWC envelope, with the data for clay soils performing better than those for silty sands -sandy silts. This is partly due to the fact that the applied relationship converting ARDP to VWC performs better for clays than silty sands -sandy silts, as well as less variation of the estimated bulk density that is needed to derive the dry density.Key words: time domain reflectometry, volumetric water content, gravimetric water content, apparent relative dielectric permittivity, principal component analysis, estimation of dry density.Résumé : La réflectométrie dans le domaine temps (RDT) mesure la permittivité diélectrique apparente relative (PDAR) d'un sol, et est communément utilisée pour déterminer la teneur en eau volumique (TEV) de ce sol. La PDAR est affectée par plusieurs facteurs en plus de la teneur en eau, comme la conductivité électrique du sol, la température et la densité. Ces relations varient selon le type de sol, et malgré les recherches antérieures, elles ne sont pas encore bien comprises puisqu'elles dépendent beaucoup du sol. Une approche statistique multivariée (analyse en composantes principales, ACP) est utilisée pour décrire une variété de sols provenant de deux sites différents au Royaume-Uni (argile et sable silteux -...

Time domain reflectometry (TDR) potential for soil condition monitoring of geotechnical assets

“…Previous studies showed the potential of measuring both w and d with TDR (Siddiqui and Drnevich 1995;Lin et al 2000;Siddiqui et al 2000;Yu and Drnevich 2004;Drnevich et al 2005;Thring et al 2014;Jung et al 2013aJung et al , 2013bCurioni et al 2018), but these methods have not been tested in field monitoring applications. Recently, Bhuyan et al (2017) reported an interesting application of the TDR method for monitoring the condition of granular pavement materials.…”

Formation of Post-Fire Water Repellent Layers on Nothofagus glauca (Hualo) Forests, After the Historical “Las Máquinas” Wildfire in South-Central Chile

“…Though no significant differences were found on soil moisture (Fig. 3), The result obtained for bulk electrical conductivity were the same for both sites, near 0.0 dS m −1 , this is expected when soil water content is under 0.1m 3 m −3 (Thring et al, 2014) and reflects the limitations to assess the EC b when the soil is closer to dry through dielectric methods like GS3 sensor.…”

Extending TDR Capability for Measuring Soil Density and Water Content for Field Condition Monitoring

“…However, due to the heterogeneous nature of soils, a universal relationship has not been found that can produce accurate results for every soil. For projects where accuracy is of primary importance, it is therefore still advisable to perform a soil-specific calibration (Thring et al 2014). TDR has also been shown capable of measuring the low frequency bulk electrical conductivity (BEC, S=m) from the attenuation of the signal after reaching a steady-state level (Giese and Tiemann 1975;Topp et al 2000).…”

“…More recently, TDR was shown capable of measuring the soil ρ d and w and therefore making it more appealing to geotechnical engineers. Thring et al (2014) proposed simple methods for converting θ to w by using the information contained in the soil description and other available soil data. Although quick and inexpensive, these methods only provide estimates of these parameters and are unlikely to be as accurate as direct measurements.…”

“…A number of authors have demonstrated that the inclusion of a density term improves the relationship between K a and θ, indicating that this relationship is also affected by the soil density (Ledieu et al 1986;Roth et al 1992;Dirksen and Dasberg 1993;Jacobsen and Schjønning 1993;Malicki et al 1996;Gong et al 2003;Thring et al 2014). For this reason, Eq.…”