A major design issue in the implementation of a Subsurface Drip Irrigation (SDI) system for extensively crops such as alfalfa (i.e. crops that cover the entire surface as opposed to row crops), is the determination of the appropriate depth of placement of the drip line tubing. It is important to allow necessary farming operations with heavy equipment at harvesting times while still providing adequate water to meet the crop water requirements. It is also a need to ensure appropriate spacing between the dripline laterals to assure reasonable lateral irrigation uniformity for plant germination. In this study, the program HYDRUS-2D was used to determine the wetting pattern above and laterally from a subsurface drip emitter of an SDI system, for three soils typically found in Southern California and Arizona, a Sandy Clay Loam (SCL), a Clay Loam (CL) and a Loam (L). The design and management conditions from an experimental alfalfa field with an SDI system located at Holtville CA were used and analyzed. The first irrigation design was with a drip line depth of placement of 30 cm and the second design with an installation depth of 50 cm. The two different irrigation management schemes utilized by the farmers and producers in that area were: one with a running time of six hours and a frequency of every three days and the second one with an irrigation running time of twenty-four hours with a frequency of seven days or irrigation every week. After having carried out the analysis and studies of the irrigation designs and management schemes mentioned above, a new model with its corresponding management was proposed to meet the alfalfa water requirements under that particular field and weather conditions while we ensure a sufficiently dry soil surface at harvesting time for each soil case. This irrigation management includes twelve hours or irrigation every three days, for each of the three soils analyzed. It was found that the vertical rise of water above the emitters on the day of the cut, for our recommended SDI management was 26 cm, 29 cm, and 27 cm, with a moisture content at the soil surface of 14.9%, 24%, and 13% for the SCL, CL, and L soils respectively. Then, through the utilization of classical soil mechanics theory, an analysis to calculate the increase in stress on soils at any depth due to a load on the surface from a conventional tractor used during harvest operations was made for the proposed SDI system. The results from the increase in stress were then used together with soil strength properties such as shear strength as a function of soil moisture content to determine the minimum allowable depth of placement of the drip line tubing to ensure that soil failure does not occur. The load increase from a 3,300-kg four-wheel tractor was found to be 0.59 kg/cm2 under a rear tire at 10 cm below the surface and 0.07 kg/cm2 at 70 cm below the surface. To ensure that shearing failure does not occur, a stress analysis using Mohr’s circle indicated that the soil moisture content at 10 cm below the surface should be no greater than 26.8%, 32.7%, and 27% in the SCL, CL, and L soils respectively. The mimimum moisture content of 26.8% occur at 10 cm above the drip line for a SCL soil, which means that the minimum depth placement to avoid failure would be 40 cm below the surface. A similar analysis for the CL and L yielded minimum installation depths of 35 cm and 40 cm respectively. This type of analysis is useful in determining the depth of placement of SDI drip line tubing to ensure adequate trafficability of soil irrigated with subsurface drip irrigation systems. An additional outcome of the modeling study was the determination of the lateral extent of the wetted zone which can be used to determine the appropriate lateral spacing between drip line tubing. Thus, to ensure adequate spatial coverage by a subsurface drip system, the maximum horizontal spacing should be of 80 cm for SCL and L soils and 90 cm in CL soils.

In the recent past, as more farm power is being demanded on farms, due to increased farm sizes and operating speeds, larger and heavier farm machines are deployed in various farming operations. Their cumulative negative effects have become more apparent with increased incidences of soil compaction problems. This has forced many farmers to practice deep tilling, using subsoilers to break up compacted subsoil layers. In some maize growing regions of South Africa, conventional subsoilers are used in a tandem configuration. The farmers believe that the use of subsoilers in this mode reduces the draft force per unit area tilled. This probably happens because the critical depth for the rear subsoiler is increased beyond its working depth of 600 mm. Operating in this mode necessitated this study, with the ultimate goal of testing an appropriate existing force model for a single tine in predicting the force requirements of the front subsoiler in a tandem configuration. Secondly, to develop an alternative model for the rear subsoiler based on the three-dimensional failed soil-profile and to determine the relative position of the front subsoiler at which energy utilization is optimized. To develop the proposed model, an analytical approach based on limit equilibrium analysis was used and a Matlab-based computer program was coded to solve it. Its verification was conducted through field experiments in sandy clay loam soil. The experiments consisted of a continuous measurement of the horizontal and vertical forces acting on each subsoiler by a two-dimensional force transducer system. At the same time, the three-dimensional and thus the cross-sectional areas of the disturbed soil-profiles at different sections were measured, as well as the soil characteristics. A manual method employing a pin-profile meter was used to measure the vertical cross-sectional areas of the failed soil-profiles at 100 mm intervals. Further more, a technique using an automatic penetrometer and a computer program was developed to identify and map the three-dimensional failed soil-profiles. This technique indicated that the subsoiler failed the soil beyond its maximum operating depth and width. The results also indicated that the soil-failure pattern at close spacing is in phase at both subsoilers, leading to reduced total draft force requirements. At a wider spacing, the soil-failure pattern was out of phase, thus resulting in increased total draft force requirements. At the same time, the cross-sectional area tilled per unit draft force increased with increased spacing. This was because the failed maximum cross-sectional area increased in size faster than the total draft force as the spacing was increased. The proposed model verification results show that the predicted and recorded forces at the rear subsoiler correlated reasonably well at a wider spacing. When the front subsoiler was shallow working and close to the rear subsoiler, the model under- predicted the measured forces on the rear subsoiler, whilst the Swick-Perumpral model over predicted the applied forces to the front subsoiler and this was generally the case at wider spacings. Furthermore the efficiency of the subsoilers was maximized when the longitudinal spacing was such that it allowed the soil failed by the front subsoiler to stabilize before the rear subsoiler reached it. The maximum cross-sectional area failed per unit draft force was recorded when the depth of the front subsoiler was equal to about 80% of the rear subsoiler-operating depth. The knowledge contributed by this research will not only facilitate qualitative field operations and optimize energy use, but also promote better management decisions. / Thesis (PhD (Engineering))--University of Pretoria, 2004. / Civil Engineering / unrestricted

Les machines agricoles doivent être conçues pour être optimales, fiables et résistantes à la rupture par fatigue. L’approche déterministe de la conception ne garantit pas ces exigences, elle est néanmoins traditionnellement appliquée pour la conception de machines agricoles. Cela est dû à la difficulté de modéliser la nature stochastique des forces agissant sur les machines agricoles, en particulier les machines de labour, car elles fonctionnent dans un environnement irrégulier et dans des conditions de fonctionnement variables. Le principal objectif de cette thèse est de développer un cadre général pour la conception de machines agricoles, en intégrant les outils d'optimisation, de fiabilité et de fatigue. En cela, nous visons à proposer une alternative à l'approche déterministe. Tout d'abord, cette thèse propose des méthodes et des modèles pour modéliser la variabilité des forces durant le labour en prenant en compte à la fois la variabilité des paramètres du système de labour et de rupture du sol. Deuxièmement, sur la base des méthodes d’optimisation fiabiliste et d’analyse de la fatigue, nous proposons des méthodologies pour la conception de machines agricoles. Tout au long de la thèse, les approches développées sont appliquées à la conception de la dent d'un chisel. / Agricultural machines should be designed to be optimal, reliable and have thecapacity to resist failure by fatigue. Although, the deterministic design approach does not guarantee these requirements, it is traditionally applied in the design of agricultural machines. This is due to the difficulties to model the stochastic nature of the forces acting on agricultural machines, especially the forces acting on tillage machines which work in irregular environment and under varying conditions. Therefore, the main objective of this dissertation is to develop a general framework for the design of agricultural machines by integrating the optimization, the reliability and the fatigue tools. We aim to provide an alternative to the traditional deterministic design one. First, this dissertation proposes methods and models for modeling the variability in tillage forces considering both thevariability in tillage system parameters and the soil failure. Second, based on the available methods in reliability-based design optimization and fatigue analysis approaches, itproposes methodologies for the design of agricultural machines. Throughout the dissertation, the developed approaches are applied to the design of the shack of a chisel plough.

<p>Soils, in general, undergo both elastic and plastic deformations upon loading. Strain dependant anisotropic elasto-plastic models are required for realistic modeling for soil-tool mechanics that will address issues like stress history and soil anisotropy. Although several such models have been proposed, the science of coupled poro-mechanical analysis of an unsaturated soil has not been fully addressed.</p><p>Tillage tool modeling is primarily concerned with the analysis of soil deformation patterns and development of force prediction models for design optimization. Most of the models are based on quasi-static soil failure patterns that cause difficulty in accurately predicting soil-tool behaviour and soil forces for high speed operation. In recent years efforts have been made to improve the conventional analytical and experimental models by numerical approaches. Numerical simulations of soil-tool interactions using finite element modeling (FEM) and discrete element method (DEM) were mostly based on a solid mechanics approach. Due to limitations of constitutive relations, predictions of these numerical models have not been able to address tillage dynamics with high shear rates. The contribution of this research was to study the dynamics of soil-tool interaction using computational fluid dynamics (CFD) from the perspective of soil visco-plastic behavior.</p><p>A motorised soil rheometer was developed for evaluating soil visco-plastic parameters for CFD simulations. The apparatus was used to determine soil yield stress and viscosity at different soil moisture and compaction levels.</p><p>Three-dimensional CFD analyses were carried out using a commercial software CFX 4.4 to observe soil failure patterns around a tool and the pressure distribution on and around the tool. Duct flow as well as free-surface flow simulations of visco-plastic soil as a non-Newtonian Bingham material indicated soil deformation comprising of plastic flow and plug flow patterns. The soil failure front advancement demonstrated a critical speed range of 4 to 6.5 m s-1 where advancement of the failure front did not increase with speed. Soil pressure on the tool surface increased with the tool operating speed. Pressure distribution on the tool surface and draft requirement agreed well with the published literature based on experimental results and FEM analysis. The CFD approach, in its first attempt to tillage process, demonstrated its greater potential for dynamic modeling of soil-tool interaction.</p>

<p>Soils, in general, undergo both elastic and plastic deformations upon loading. Strain dependant anisotropic elasto-plastic models are required for realistic modeling for soil-tool mechanics that will address issues like stress history and soil anisotropy. Although several such models have been proposed, the science of coupled poro-mechanical analysis of an unsaturated soil has not been fully addressed.</p><p>Tillage tool modeling is primarily concerned with the analysis of soil deformation patterns and development of force prediction models for design optimization. Most of the models are based on quasi-static soil failure patterns that cause difficulty in accurately predicting soil-tool behaviour and soil forces for high speed operation. In recent years efforts have been made to improve the conventional analytical and experimental models by numerical approaches. Numerical simulations of soil-tool interactions using finite element modeling (FEM) and discrete element method (DEM) were mostly based on a solid mechanics approach. Due to limitations of constitutive relations, predictions of these numerical models have not been able to address tillage dynamics with high shear rates. The contribution of this research was to study the dynamics of soil-tool interaction using computational fluid dynamics (CFD) from the perspective of soil visco-plastic behavior.</p><p>A motorised soil rheometer was developed for evaluating soil visco-plastic parameters for CFD simulations. The apparatus was used to determine soil yield stress and viscosity at different soil moisture and compaction levels.</p><p>Three-dimensional CFD analyses were carried out using a commercial software CFX 4.4 to observe soil failure patterns around a tool and the pressure distribution on and around the tool. Duct flow as well as free-surface flow simulations of visco-plastic soil as a non-Newtonian Bingham material indicated soil deformation comprising of plastic flow and plug flow patterns. The soil failure front advancement demonstrated a critical speed range of 4 to 6.5 m s-1 where advancement of the failure front did not increase with speed. Soil pressure on the tool surface increased with the tool operating speed. Pressure distribution on the tool surface and draft requirement agreed well with the published literature based on experimental results and FEM analysis. The CFD approach, in its first attempt to tillage process, demonstrated its greater potential for dynamic modeling of soil-tool interaction.</p>

This study was undertaken to develop an analytical model that is capable of predicting the torque requirements of a rotavator fitted with commercially available L-shaped blades. An analytical approach based on the limit equilibrium analysis was used to develop the proposed model. The proposed model was verified by comparing the model and measured torque requirements at predetermined rotavator blade angular positions from the horizontal for a down-cut rotavator. The study findings indicated that there was an optimum set tillage depth for each rotavator configuration and operational conditions at which the resultant horizontal thrust generated was greatest. This unique depth was influenced by the bite length. The validation of the proposed model showed that the predicted and measured torque requirements, at different angular blade positions from the horizontal, correlated reasonably well for all the set tillage depths. As the depth of tillage increased, however, the curve for the measured torque requirements exhibited a cyclic behaviour after the peak torque requirements value had been recorded. The cyclic behaviour was probably due to the re-tilling and the instability of the tool-frame carrier, which increased with the set tillage depth. The knowledge contributed by this research will afford the designers of active tillage tools a better understanding of the operations of the rotavator, particularly in deep tillage. The modelling approach, and instrumentation technique used in this research, can be extended to analyze the performance of rotavators fitted with other types of commercial blades. / Thesis (PhD)--University of Pretoria, 2010. / Civil Engineering / unrestricted