The laboratory calibration of a soil moisture capacitance probe in sandy soils

Abstract

Determining and mitigating landslide risk is a technical-scientific objective, particularly for the protection and proper territorial management and planning. The slope stability depends on the pore pressure distribution, which is influenced by the saturation front propagation through the unsaturated zone, whose monitoring is useful to understand any possible instabilities. Such monitoring may be undertaken by sensors based on the measurement of the relative dielectric permittivity. Reliable relationships between the measurement and the soil moisture are necessary. The main objective of this study is to assess a laboratory calibration protocol for a specific capacitance sensor (Drill & Drop, Sentek Sensor Technologies). Two monogranular sands have been selected for the calibration purpose. The laboratory tests were performed under three relative density values (D<sub>R</sub> equal to 40%, 60% and 80%) for seven volumetric water content values (θ<sub>v</sub> ranging from 0.00% to 36.26%). Based on the experimental measurements, the soil-specific calibration curves were determined at an assigned relative density value; in particular, a simple power law is adopted to describe the probe’s reading as a function of the volumetric water content. The results point out that the relative density values slightly affect the tests, thus, the soil-specific calibration curves are derived based on a simple regression analysis fitting the whole set of the laboratory tests validated for each sand. The calculated coefficient of determination (R<sup>2</sup> = 0.96÷0.99) and root mean square error (RMSE = 1.4%÷2.8%) values confirm the goodness of fit. In order to propose more general fitting curves, suitable for both the investigated sands, multiple linear regressions are performed by considering θ<sub>v</sub> and the mean grain size, D<sub>50</sub> as independent variables; again, the R<sup>2</sup> and RMSE values equal to 0.97 and 2.41%, respectively, confirm the suitability of the calibration curve. Finally, the laboratory calibration curves are compared with the manufacturer-supplied curves, thus, enhancing the need for the soil-specific calibration.

Field Measurements of Soil Water Content at Shallow Depths for Landslide Monitoring

“…The Drill&Drop probe (Figure 5) is a multi-capacitive probe (it has, in fact, one sensor per 10 cm of its length), which allows the measurement of the water content along a vertical alignment in the soil. It was previously studied in the laboratory [36], but being rather expensive, it was not used for monitoring the project sites.…”

“…The Drill&Drop probe (Figure 5) is a multi-capacitive probe (it has, in fact, one sensor per 10 cm of its length), which allows the measurement of the water content along a vertical alignment in the soil. It was previously studied in the laboratory [36], but being rather expensive, it was not used for monitoring the project sites. Over a period of about three months, the two sensors (Drill&Drop and a set of WaterScouts) were installed close to each other, to monitor the evolution of the soil moisture profiles in a test field, up to a depth of about 90 cm below ground level.…”

Geotechnical Analysis and 3D Fem Modeling of Ville San Pietro (Italy)

“…They provide real-time water content profiles in the shallow layers (typically in the upper meter) of the slope. The capacitance sensors (more specifically, WaterScout SM100 sensors, manufactured by Spectrum Technologies, Inc., Plainfield, IL, USA) are appropriately calibrated by laboratory tests in order to define soil-specific calibration curves [4,5].…”

Development and Validation of a New Calibration Model for Diviner 2000® Probe Based on Soil Physical Attributes

“…In the last decades, Frequency Domain Reflectometry (FDR) sensors have been extensively used to monitor soil water content [14][15][16][17]. Real-time measurements of soil moisture allow, among others, quick and accurate determinations of irrigation scheduling variables, such as irrigation timing and amounts of water to apply, reducing the need of extensive computations to determine the actual crop water use [1,16,18,19].…”

“…The relationship between soil dielectric permittivity and volumetric water content is well known, as well as that other soil factors affect the measurements, such as soil texture, bulk density, salinity, and organic matter [17,28,29]. On the other hand, not deeply investigated are the calibration procedures valid for swelling/shrinking clay soils, for which the changes of bulk density with soil water content affect the apparent soil dielectric permittivity [30].…”