A fuzzy classification routine for fine-grained soils

Soil classification is one of the most important stages in preliminary studies for design applications in geotechnical engineering. The classification of fine-grained soils is mostly determined using Casagrande's plasticity chart. However, owing to the factors affecting determination of liquid and plastic limits, uncertainties can arise in fine-grained soil classification when using this approach. The uncertainty increases particularly when the points on the chart fall on the lines that separate certain soil classes (A-line and/or 50% liquid limit line) or very close to these lines. In this study, a fuzzy classification routine is proposed to minimize these uncertainties. For this purpose, the spatial distances of the evaluation points on the chart from the lines were used as a controlling unit. The membership degrees that define the fuzzified soil (clay and/or silt), and plasticity (low and/or high plasticity), were evaluated by considering sigmoid functions. As a consequence, the soil types were established by aggregating fuzzified soil and plasticity using the fuzzy operators.