Evaluation of the USDA soil texture triangle through Atterberg limits and an alternative classification system

Abstract

Since the texture triangle developed by the U.S. Department of Agriculture (USDA) has traditionally been a basic tool in soil classification, a thorough examination of its suitability has been carried out in this research. A total of 256 soil samples from 21 different countries and 35 publications containing both textural (sand, silt and clay percentages) and Atterberg limits data have been analyzed. The soils were classified according to traditional particle size criteria using the USDA texture triangle. On the other hand, the Plasticity index / Liquid limit ratio, a parameter called clay factor (CF) in this research, was calculated and plotted against the percentage of sand to classify the soil according to the texture-plasticity system proposed by the authors in 2018. The evaluation through CF and comparison with the authors' proposal has shown the limitations of the USDA classification, which only determines soil texture in terms of particle size, but not how this texture affects actual soil properties, an aspect that could be very useful. It has been demonstrated that a significant percentage (20%) of soils classified with the USDA system actually exhibit plastic behavior that is completely unrelated to the assigned name (e.g., non-plastic soils classified as clay). However, the fact that about 80% of the classified soils with both systems present the same group designation (∼37%) or are adjacent groups (∼43%), shows that the authors' classification does not break sharply with the textural criteria accepted to date, which is positive if this new proposal is intended to be used as an alternative. Therefore, the proposed classification conciliates plastic and textural properties, allowing to classify the soil in a more accurate way, adjusting it to its real behavior.