Torque-Weighted Index Soil Strength Test (TWISST) for vegetated terrain

Improved understanding of vegetation impacts on soil strength could improve erosion and landslide assessment, stabilization and resilience efforts, and off-road vehicle mobility efforts while minimizing damage to vegetation or soil surfaces. It has been shown that belowground biomass, along with soil moisture and type, influences soil strength. Standard handheld geotechnical instruments, such as cone penetrometers, vane and torvane, typically used for measurements of soil strength were developed for civil engineering projects. These devices have been applied to studies in natural areas to determine shear strength profiles, namely wetlands, stream banks, slopes, and coastal dunes. However, in cases where soils are soft or complex, can be saturated, or where vegetation exists, such as coastal areas or wetlands, traditional geotechnical instruments provide uncertain and highly variable results, can miss vegetation contributions to soil strength, and are difficult to compare to one another. A few efforts exist to address these issues. In the case of wetlands, Sasser et al (2018) developed a Wetland Soil Strength Tester (WSST), which measures the torque required to shear the combined soil-vegetation wetland matrix using a four-pin design inserted into the wetland soil 15 cm. In our ongoing work to use remote sensing to map soil and vegetation properties, even when soil is obscured by vegetation, we found that the geotechnical measurements insufficiently capture soil strength in the presence of vegetation. We adapted the WSST design and systematically tested against traditional geotechnical measurements broadly (variety of vegetated terrain, mostly grasses and shrubs, ranging from low to high heights), not necessarily just wetlands, to investigate the utility of the measurements in applications where soil strength is a primary parameter. Soil types included sand, loam, and clay, and each were tested in wet and dry conditions. Measurements of peak torque were recorded at intervals of 90 degrees up to four revolutions with and without surface vegetation present to account for more soil properties than shear failure. Results of the analyses showed that the method can account for a greater variation in initial shear strength than the cone penetrometer-based cone index in soft to medium hard soils, such as wet and dry sands with vegetation and moist to wet clays. Analyses of torque-based measurements beyond initial shear, greater than 90 degrees, revealed a non-uniform and non-linear change in force required as sheared vegetation-soil matrix is further manipulated. Hard soils, especially dry clays were beyond the maximum limits of torque measurements and did not add valuable information beyond traditional geotechnical measurements. Based on these results, a preliminary model of the relationship between the torque-based measurement and the cone index was developed, terrain maps were enhanced, and further applications are being analyzed.  The torque weighted index soil strength test techniques have potential application in stability, erosion, and mobility studies as well as our ongoing research in using remote sensing for indirect inferences of soil properties in vegetated areas.