Retention curve. Collapsible soils. Expansive soils. Adjustment methods. Forecasting methods

In unsaturated soils, understanding water distribution, retention and release under different conditions is crucial. The soil-water relationship can be described by the retention curve, it is fundamental to understanding the water behavior of the soil and very important in several areas, such as agriculture, geology and geotechnical engineering. There are several proposals for adjustment and prediction equations for the soil retention curve. In this study, the adjustments proposed by Van Genuchten (1980), Gitirana Jr. and Fredlund (2004) and the retention curve prediction method based on particle size distribution developed by Mohammadi and Vanclooster (2012) were used. Retention curves obtained from the literature on collapsible and expansive soils are objects of analysis in this work. The adjustments made were compared with published experimental data to verify which of the adjustments best represents these points. In this research, 60 soils were analyzed, 30 collapsible and 30 expansive. In the search for academic works, keywords were used in both English and Portuguese, as follows: Collapsible soil, Characteristic curve, Retention curve, Expansive soil, Suction, combined by the Boolean operators “AND” and “OR”. With the aim of identifying behavior patterns in collapsible and expansive soils, an analysis of the physical characteristics of the soils obtained in the works consulted and the data extracted from adjustments and predictions of retention curves was conducted. This analysis was carried out using the ORANGE software, which made it possible to identify the influence of the variables studied in the identification of collapsible and expansive soils. Van Genuchten's (1980) fit was accurate in describing the unimodal curves of collapsible and expansive soils, but failed to adequately capture the bimodal shape of the curves in both soils. The method proposed by Gitirana and Fredlund (2004) performed well in both unimodal and bimodal soils, whether collapsible or expansive. The prediction of the retention curve by the method of Mohammadi and Vanclooster (2012) is better adjusted in sandy and silty soils, in other soils the characteristic shape of the retention curves were not well represented, for this method the coefficient of determination R2 was not shown. a representative statistical measure. The adjustment indices of Van Genuchten (1980), Gitirana and Fredlund (2004), the suction capacity and the slope of the granulometric curve presented higher values for collapsible soils than for expansive soils. Collapse potential values were similar for unimodal and bimodal soils, but bimodal soils presented a wider range of values. The void index in collapsible and expansive soils was higher in bimodal soils than in unimodal soils.