High Resolution Soil Extensometer and Its Applications in Soil Physics.

An extensometer has been developed to measure high-resolution, vertical soil displacement for interpreting hydrologic processes and evaluating soil properties. The extensometer is 2 m long and 25 mm in diameter. The upper and lower segments of the sensor, which have a slightly larger diameter, function as anchors to form a friction fit with the soil. Displacement is measured between the two anchors with resolution of 10 nm.

Eleven extensometers have been installed in four different regions with varying soil types: saprolite near Clemson, SC; clay in the Texas panhandle; loess near Holdrege, NE; and silt loam near Pullman, WA. In all cases the instrument responds to body weight or vehicle loads at the ground surface, which indicates proper installation in each soil type. Extensometers have been installed at depths from 3 to 9 m. Sensors respond to surface loads applied within radial distances up to two times the installation depth.

Two applications for this technology are estimates of average changes in the mass above a sensor and characterization of in-situ soil properties near a sensor. Changes in mass related to soil water content are of primary interest. The extensometer could be used to monitor changes in soil moisture on a scale of 1 to 10⁵ m², depending on installation depth. Extensometers installed at a depth of 6 m measure 0.13 to 0.29 μm of compression per mm of rainfall. Displacements are also caused by subsurface changes in temperature and pressure near the sensor, so these components must be removed from the measured signal to isolate the contribution from soil moisture. Additionally, the poroelastic response from surface loads can be evaluated using displacement in saturated soils. Displacements from body weight loads were used to estimate Young’s modulus, permeability, and air entry depth using a numerical COMSOL model. The breadth of possible applications for this technology are still being explored.