Abstract
The Atterberg limits display soil mechanical behavior and, therefore, can be so important for topics related to soil management. The aim of the research was to investigate the spatial variability of the Atterberg limits using three most common digital soil-mapping techniques, the pool of easy-to-obtain environmental variables and 85 soil samples in central Iran. The results showed that the maximum amount of liquid limit (LL) and plastic limit (PL) were obtained in the central, eastern and southeastern parts of the study area where the soil textural classes were loam and clay loam. The minimum amount of LL and PL were related to the northwestern parts of the study area, adjacent to the mountain regions, where the samples had high levels of sand content (>80%). The ranges of plasticity index (PI) in the study area were obtained between 0.01 to 4%. According to the leave-in-out cross-validation method, it should be highlighted the combination of artifiial bee colony algorithm (ABC) and artifiial neural network (ANN) techniques were the best model to predict the Atterberg limits in the study area, compared to the support vector machine and regression tree model. For instance, ABC-ANN could predict PI with RMSE, R 2 and ME of 0.23, 0.91 and -0.03, respectively. Our fiding generally indicated that the proposed method can explain the most of variations of the Atterberg limits in the study area, and it could be recommended, therefore, as an indirect approach to assess soil mechanical properties in the arid regions, where the soil survey/sampling is difficult to undertake.
Highlights
The Atterberg limits are a criterion for explanation of soil moisture content
The study area which is a part of Yazd-Ardakan region or plain has a total area of 15,950 km[2] and geographic coordinates are between 31°48' and 32°13'N and 52°57' to 54°59'E
The results showed that the ANFIS model produced more accurate results than the artificial neural network (ANN) model in terms of liquid limit and plasticity index, but not plastic limit
Summary
The Atterberg limits are a criterion for explanation of soil moisture content. Based on this criterion, three limits are defined for moisture in the soil, including shrinkage limit (SL), plastic limit (PL), and liquid limit (LL) (Atterberg 1911). Fine-grained soils can show different status according to the amount of their water absorption These three boundaries are among the four states of soil behavior, which are solid, semi solid, plastic, and liquid. Zolfaghari et al (2015) predicted the Atterberg limits by some soil characteristics and bio-environmental data using an ANN method. Their results showed that the soil organic matter, clay content, calcium carbonate equivalent, and terrain attributes could explain most of the variances of the Atterberg limits in the region. Keller and Dexter (2012) reported that the PL had significant correlation with the soils having more than 35% clay content in agricultural soils in different countries. The similar positive correlation between soil organic carbon and PI was reported by Abdi et al (2018), who investigated the Atterberg limits in forest soils of Hyrcanian forest in northern Iran
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