Investigation of shear strength parameters of unsaturated soils

Determining the behavior of unsaturated soils is of great importance for accurately evaluating the mechanical properties of soils. Traditionally, saturated soil parameters are used in soil mechanics, which simplifies calculations and reduces costs. However, this approach may lead to excessively conservative designs by causing unnecessary material usage in field applications. Moreover, due to climate change and the decreasing annual precipitation in Turkey, the proportion of unsaturated soils is increasing, which necessitates a more precise understanding of the mechanical behavior of such soils. One of the fundamental parameters in unsaturated soil mechanics is soil suction, which defines the relationship between water content and matric potential. This relationship is represented by the Soil-Water Characteristic Curve (SWCC) and is a critical tool for predicting the hydro-mechanical behavior of soils. In this study, the filter paper method used for measuring soil suction, its calibration characteristics, and the relationship between soil suction values and soil parameters were experimentally investigated. For filter paper calibration, three test series were conducted in accordance with ASTM D5298 standard, and calibration curves were derived using NaCl solutions. Subsequently, five sets of three different compacted soil samples (one sample at optimum, optimum-saturated, and optimum-dry conditions) were left to dry at room temperature for 0, 3, 6, 18, 24, 48, 72, and 168 hours. Based on the total and matric suction values of these samples, soil-water characteristic curves were determined using the filter paper method. Unconfined compression tests were also conducted on the same samples, and the relationship between the mechanical properties of the soils and their suction values was examined. As a result of the experimental study, it was observed that calibration of the filter paper method is significantly effective and necessary, and that soil suction values are correlated with the unconfined compressive strength of the soils. The obtained results are consistent with studies in the literature.