Controlled vertical stress in a modified amplitude sweep test (rheometry) for the determination of soil microstructure stability under transient stresses
Controlled vertical stress in a modified amplitude sweep test (rheometry) for the determination of soil microstructure stability under transient stresses
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DOI：
10.1016/j.geoderma.2017.01.034
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发表时间：
2017-06
期刊：
影响因子：
6.1
通讯作者：
Dörthe Holthusen;Patricia Pértile;J. M. Reichert;R. Horn
中科院分区：
文献类型：
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作者：
Dörthe Holthusen;Patricia Pértile;J. M. Reichert;R. Horn
尽管有证据表明，较大尺度上的土壤物理和力学过程大多起源于微观尺度上的过程和性质，但直接研究微尺度土壤行为的方法范围有限。一种检测恒定和瞬变应力下的流动和变形行为的方法是流变法。这种方法是通过振幅扫描试验(AST)来检测土壤的粘弹性性质的，即由于车轮或踩踏而产生的瞬时应力下的应力-应变关系。该试验基于两个平行板之间的土样，其中上板随着应力或应变幅度的增加以振荡的方式旋转，而下板是固定的。根据试件剪应力反应的滞后程度，粘性应变和弹性应变的比例可以通过存储和损耗模数(G‘和G“)及其比率(tanδ)来定义。然而，AST对样品制备和上平板的垂直力非常敏感。要么施加较小的力(例如1×N)，要么排除力超过临界极限(例如12-15×N)的样品。我们使用1，3和10 kN的受控力，相当于2.04，6.11和20.37 kpa的压应力，找到运行AST的最佳设置。与在巴西南部两种未确定条件下观察到的力/应力相比，所选择的力/应力是低、中或高的(典型的Hapludox-oxisol的A层和B层，以及Oxy Aquic Hapluderts-vertisol的A层和B层)。此外，场密度和普通(高)密度被应用于均匀的土壤材料。结果的变异性一般随着压缩应力水平的增加而减小，并且在最高应力变异性低于变力/应力下。以铁氧化物和氢氧化物(假砂)为基础的氧化土的微团聚体和B层中高岭石含量较高的高岭石含量导致的可恢复弹性范围小于变性土。然而，在大应变下，氧化土的弹性较高，而变性土表现出与土壤颗粒的排列。氧化土体一般在最大剪应力作用下表现为脆性断裂，而在变质土中则是由于可膨胀粘土而发生屈服。高压缩力和高密度同时减少了弹性变形的份额，促进了塑性屈服。结果还不能确定最佳压应力。因此，我们建议进一步研究力或应力水平，并测试更多的土壤，例如高土壤有机质含量和/或高或低粘土、粉砂或砂质含量的土壤。
Although there are evidences that most, if not all, soil physical and mechanical processes on larger scales have their origin in processes and properties at the microscale, the range of methods to investigate micro-scaled soil behavior directly is limited. One method to detect flow and deformation behavior under steady and transient stresses is rheometry. This method is well established by means of the amplitude sweep test (AST) to detect viscoelastic properties of soils, i.e. stress-strain relationships under transient stresses that occur due to wheeling or trampling. This test is based upon a soil sample positioned between two parallel plates, where the upper plate rotates in an oscillatory manner with increasing stress or strain amplitude, and the lower plate is fixed. Depending on the retardation of the sample's shear stress reaction, viscous and elastic strain proportions are defined with the help of storage and loss modulus (G′ and G″), and their ratio (tan δ). However, the AST is strongly susceptible to sample preparation and the vertical force of the upper plate. Either a low force (e.g. 1 N) was applied or samples with forces beyond a critical limit (e.g. 12–15 N) were excluded. We used controlled force of 1, 3 and 10 N, equivalent to compressive stresses of 2.04, 6.11 and 20.37 kPa, to find the optimum settings to run the AST. The selected forces/stresses are either low, intermediate or high compared to those observed under undefined conditions in two South Brazilian soils (A and B horizon of a Typic Hapludox – Oxisol, and an Oxyaquic Hapluderts – Vertisol). Furthermore, field density and a common (high) density were applied to the homogenized soil material. The variability of the results generally decreased with increasing compressive stress level, and at highest stress variability was lower than under variable force/stress. The microaggregation of the Oxisol based upon iron oxides and hydroxides (pseudosand) and high kaolinite content in the B horizon resulted in a smaller range of recoverable elasticity than in the Vertisol. However, at large strains the elasticity was higher in the Oxisol, while the Vertisol exhibited alignment of the soil particles. The Oxisol generally showed a brittle rupture by means of a maximum shear stress; while in the Vertisol yielding occurred due to expandable clay. High compressive force and density simultaneously reduced the share of elastic deformation and promoted plastic yielding. The results do not allow yet defining an optimum compressive stress. Thus, we suggest investigating further force or stress levels and to test more soils, e.g. of high soil organic matter content and/or high or low clay, silt or sand content.