Investigating rheological properties of five soil orders in Chaharmahal-va-Bakhtiari Province

Rheological characteristics of soils (including their deformation and flow behaviors when subjected to external stress) can provide important information on microstructural and microaggregate stability. One of the newest methods of assessing the stability of aggregates is the use of rheometric discussions in this regard. In other words, rheometry can be used to evaluate the elastic behavior of soil versus its plastic behavior in a range of aggregate deformation and thus a more quantitative interpretation of aggregate formation or stability against stress. In the last decade, Amplitude sweep test (AST) has been used to evaluate the stability of soil microstructure and its elastic behaviors. The present study was designed to determine the aggregate stability and viscoelastic behavior of the soil using the rheometric method. In the present study, physical, chemical, clay mineralogical properties and classification of dominant soil orders in Chaharmahal-va-Bakhtiari Province were studied and compared in order to determine the stability of aggregates and viscoelastic behavior of soils using the rheometric method.

Five different soil orders were described and sampled in Shahrekord, Farsan, and Koohrang cities. After air-drying and sieving of soil samples taken from the genetic horizons, the physical and chemical properties were measured using standard methods. Besides, the type of clay minerals was determined by the X-ray diffraction method. AST was used to quantify the initial rheological parameter values, including the storage (G') and loss moduli (G"), deformation limit (γL: when the material begins to irreversibly deform), deformation at flow point (γf: when the material becomes viscous), loss factor (tan δ = G''/ G') and integral z (which summarizes the overall elasticity of the material) in order to study microstructural stability and microaggregates of the five soil orders. Finally, the correlation diagram was plotted among soil properties and rheological parameters using R software. Besides, the soil was classified based on the American Soil Taxonomy (2014) and WRB (2015).

The studied pedons classified based on the American Soil Taxonomy (2014) and WRB (2015) in five different soil orders and four reference soil groups, including Entisol (Cryosol in WRB), Vertisol, Mollisol (Kastanozem in WRB), Alfisol, and Inceptisol (Calcisol in WRB). Semi-quantitative analysis of clay minerals showed that the soils have different clay minerals assemblage including kaolinite, vermiculite, smectite, illite, chlorite, and quartz. The results of the AST indicated that the deformation limit was significantly higher in subsoil horizons than topsoil horizons and was also higher in the clayey soils compared to sandy soils. The soils with “vertic soil properties” showed the high range of linear viscoelastic region (LVE) that attributed to the samples with high elasticity behavior. Soils with higher clay contents and further evolution showed higher rheological parameters, implying that these soils had more rigid microstructures incomparable with other soil orders which in turn, indicated the higher microstructural and microaggregate stability. In contrast, the horizon, which had high sand content and little soil development, had the lowest values for all properties, thus indicating a lack of micro-aggregate stability. Pearson correlation coefficient analyses revealed that integral z was influenced by soil physicochemical properties, and was higher in soils with the clay fraction dominated by expansive clay minerals and higher cation exchange capacity.

Altogether, the rheological parameters indicated that more developed soils had greater microstructural stability than their less-developed counterparts. As a result, rheological measurements may be useful for identifying the major factors that affect soil aggregation. In addition, these parameters can be used as indicators to evaluate the relative amount of soil evolution in future studies.