Estimating unconfined compressive soil strength of forest roads using continuum removal technique

Understanding the soil resistance and soil strength are essential for the construction and maintenance of forest roads. Unconfined compressive strength (UCS) test approach is used to determine the strength of fine-grained and cohesive forest soils. Spectroscopy is a fast and non-destructive method that can be used to understand, analysis, and assessment of the soil properties. The present study was aimed to investigate the capability of spectroscopy data to estimate UCS. To do so, a continuum removal technique, narrow band (i.e., hyperspectral) indices, and a combination of both methods were used. Three absorption ranges with wavelengths of 1400, 1900, and 2200 nm are effective in estimating the unconfined strength of the soil. The continuum removal technique was applied on the selected absorption regions and its indices were calculated for 45 soil sample plots. In addition, NINSON and NSMI hyperspectral indices were calculated. The capability of these data to estimate unconfined soil strength was evaluated using multiple stepwise regression analysis. The results of this study showed that NINSON and NSMI indices had an R2 = 0.75 and an rRMSE% of 11.29%. Continuum removal index (i.e. NBDI) gained an R2 = 0.78 and an rRMSE% = 10.16 which shows a better result compared to the individual index. The results of the present study (i.e., a reasonable rRMSE%) showed that the spectroscopy data and continuum removal techniques can be used to estimate soil strength.