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High-yielding agriculture leads to dense straw cover (mulching). Soil-engaging tools for conservation tillage in these scenarios demand enhanced tool performance, e.g., improved soil penetration, reduced force requirement, better straw cutting and minimised soil disturbance. These interdependent and yet conflicting performances could not be easily realised with traditional tillage tools. Instead, biomimetic tool design could provide a compromised solution. A biomimetic disc mimicking the mole-rat’s claw was fabricated and evaluated in comparison with traditional discs (i.e. a plain and a notched disc) in straw-mulched soil. Soil bin tests on the 3 discs were conducted at 3 tillage depths (i.e. 40, 70 and 100 mm). Results showed that the biomimetic disc achieved 21.4 % reduction in vertical resistance and 28.7 % reduction in draught forces, as compared with the notched disc (P

Predicting soil stress with analytical models requires proper selection of the models’ concentration factor. But due to the insufficient knowledge about the effects of soil conditions on stress transmission, little is known about how the concentration factor varies with soil states and loading conditions. Thus there is a need to supply specifically defined parameters with clearer physical meanings for soil stress transmission while easily being measured with simple laboratory setups. The function of the concentration factor was transformed and a dimensionless factor standing for soil-induced attenuation on the stress transmission between two points, ( σ 0 - σ z )/ σ 0 , was derived, which complement, σ z / σ 0 , is the soil stress transmission coefficient and is denoted as STC. Since soil stress transmission property is affected by soil states and loading conditions, a modified oedometer testing setup with a soil stress sensor was used to evaluate controlled soil properties on STCs. Totally 15 soil states were tested by controlling 5 soil water contents and 3 bulk densities. Correlation analysis were performed between measured STCs and soil state parameters, i.e. water content, bulk density and soil strength. The concentration factor was then back-calculated from the acquired STCs. The highly linear correlation between soil stress and applied surface stress indicates a stable STC for each particular soil state, suggesting that the theoretically derived STC could be used as a specific mechanical property to quantify soil stress transmission. In general, a high soil water content leads to an increased STC, meaning that wetter soils are more effective in transmitting the stress to deeper places. STC was also found both linearly decreased with dry bulk density and precompression stress. A higher soil strength imposes an improved shielding effect on soil stress transmission. The back-calculation of concentration factor from measured STCs illustrates that the proposed solution for soil stress transmission provides a means to define concentration factor for each soil state with measured result. Concentration factor varied from 2.64 to 12.39, being in agreement with the past reports. But the detail of how the concentration factor is affected by the changed states of soils is provided.

Soil failure patterns play an important role in obtaining a better understanding of the mechanical behavior of soils. Despite the large number of studies over the past few decades, a better understanding of soil failure patterns and its relation to soil and tool parameters for particular soils such as dry land and paddy has not been developed. This study investigated soil failure patterns and related draft at sticky, plastic and liquid consistency limits and the sticky point of dry land and paddy soils. A soil cutting test rig was developed to perform soil cutting at three consistency limits (sticky limit, plastic limit and liquid limit) and the sticky point of soil, three rake angles (15°, 30° and 45°), and three operating depths (30 mm, 50 mm and 70 mm). A flat triangular shaped tool operating at a constant speed of 10 mm s−1 was used in all experiments. Soil failure patterns were observed and recorded using a digital camera, and draft per unit displacement was measured by load cells attached to the soil bin. A direct relationship between soil failure patterns or draft and the consistency limits of soil was found. Brittle failure was obtained at the sticky limit, chip forming failure was observed at 15° rake angle and 30 mm depth, and bending failure with little strains of elements at 30° and 45° rake angles and 50 mm and 70 mm depths at plastic limit, while flow failure was linked to the liquid limit of the soil. At the sticky point, flow failure was observed at an operating depth of 30 mm and 15° rake angle, while flow with considerable bending and no strains of elements occurred at 50 mm and 70 mm operating depths and 30° and 45° rake angles. However, bending was more prominent at 70 mm depth and 45° rake angle. The draft at the sticky limit, plastic limit and sticky point was cyclic in nature, whereas at the liquid limit it was comparatively diverse and fading. The highest draft was found at the plastic limit, and the lowest at the liquid limit. Since the soil failure patterns may change with moisture content, soil type and particle size distribution within the same textural class, consistency limits can provide clearer and more accurate definitions of soil failure patterns than moisture content levels alone.

A laboratory test following a field experiment was conducted to investigate the performance of stress wavelets as a tool for soil structure quantification. Based on the test, comprehensive calculations of indices were performed, including per fragment surface contacting energy, fragment fractal dimensions and specific fragmentation energies. Fragment fractal dimensions were 2.46 for autumn-tilled soil and 1.86 for spring-tilled, showing the improvement of soil structure from winter process, mainly from statistical point. Specific fragmentation energy ranged from 78.72 to 1135.44 J/kg, depending both on the way of field treatment and initial size ranges of soil been fragmented. Compared with these, stress wavelets analysis was capable of distinguishing, specifically from mechanical points, differences between autumn-tilled and spring-tilled soils. Results shown that fractal dimensions provided information on soil fragment size distributions, bearing no significance on soil mechanical properties. Stress wavelets, on the other hand, permitted calculations of specific fragmentation energy and per fragment surface contacting energy, which had a uniquely straight-forward connection with soil mechanical properties. The low per fragment contacting energies possessed by autumn-tilled soil, as compared with spring-tilled soil, indicated that winter process reduced soil contacting energies significantly. But a negative value of per fragment contacting energy for autumn-tilled soil was un-interpretable. The combination of stress wavelets and fractals may serve as a fundamental quantitative index for modeling soil structures in its thorough dimensions. The difference between autumn-tilled and spring-tilled soils also indicated that tillage alone should not be taken as a sole way for the improvement of soil qualities.

Stress wavelet properties are inherently involved in the process of the drop-shatter method of assessment on soil structural characteristics. The analogies between wavelet analysis and the drop-shatter process are based on two factors: scale and resolution. By carefully following the requirements of wavelet analysis, a standard procedure of soil fragmentation and sieving is described. Following this procedure, a set of equations can be derived from which surface contacting energy between soil aggregates of a specific scale can be calculated. The resultant values in fact mirror the multi-resolutions of wavelets. Natural soil clods as well as artificial structured soil cores were used for fragmentation. Though the experiment can do well on natural soil clods, and it is precise enough in predicting structural state of a sub-dimensional clods of 8 mm, its use on artificial soil cores produced a set of data that was quite chaotic. The unique behavior experienced in the process of fragmentation of artificial soil structure indicates that there is no distinct stage between mother soil clods (cores) and its constituent primary particles. Such a state should result from an excessively large impact energy (too low a resolution in the wavelet analysis) for each drop stroke impact. With an ultimate goal for soil process simulation, the construction of the experiment for artificial structured soil core preparation brings the traditional methods of sample preparation a step forward further by creating an environment much nearer to the field conditions experienced in natural soils. Still, extensive refinements are needed, especially for the mode of water application, cycle and intensity of management.