Microstructural strength of four subtropical soils evaluated by rheometry: properties, difficulties and opportunities

Abstract

ABSTRACT:The structural strength of soils has been extensively described by physical and mechanical properties evaluated on the mesoscale for different soils and management conditions. However, changes in the organization of the soil matrix at the microstructural level, which can be obtained by rheometry, are still seldom used in soil mechanics. Our aim was to use the amplitude sweep test, a rheometry technique, to investigate the microstructural strength of four subtropical soils (two Oxisols, an Ultisol and a Vertisol) and to discuss difficulties with respect to the samples, water content, soil density and vertical force. The various rheological properties which reveal the soil microstructural strength were determined: deformation at the end of the linear viscoelastic range, LVE range (γ LVE ), shear stress at the end of the LVE range (τ LVE ), deformation at yield point, YP (γ YP ), storage and loss moduli at YP (G'G" YP ), maximum shear stress (τ max ), and integral z. In general, soil elasticity (γ LVE and γ YP ) and microstructural strength (τ LVE and τ max ) were greater in the Oxisols and the Vertisol, which both possess high clay content, while the latter also contains expansive clay minerals. The lowest structural strength was observed in the Ultisol which had a high sand content. As rheological properties are related to soil properties such as particle size distribution and carbon content, they can be applied in the evaluation of the microstructural strength of clayey and sandy soils and allows for inferences regarding interparticle shear strength. However, the test is not applicable to very dry soil samples and sample preparations can affect the results. We suggest a number of approaches to find solutions for these difficulties/problems.

Rheological evaluation of soil aggregate microstructure and stability across a forested catena

“…At the same time, the Btg2 horizon in the Alfisol and the Btg1 horizon in the Mollisol had moderate to dominant amounts of 2:1 smectite and vermiculite phyllosilicates, which can increase shear resistance of the soils as indicated by larger LVE ranges (Holthusen et al, 2010). Previous work in southern Brazil indicated that Vertisol soils with high smectite content had increased γL compared to nearby Oxisols and Ultisols (Pértile et al, 2018). Therefore, proper understanding of rheological properties may require assessing both the type and overall content of clay minerals.…”