An investigation into wetland soil-implement mechanics

An investigation was initiated to obtain some understanding on the behaviour of soil at higher moisture content and to explore the potential of preparing paddy fields with reduced amounts of water. This investigation comprised of three separate studies. Based on existing information that water could be reduced when soil clods were initially formed prior to flooding, the effects of clod size, clod initial moisture content and confining states on the rate of water uptake were explored. The moisture gradients within clods wetted and dried for different period of times were also studied. The results of the clod wetting experiments show that· the rate of water uptake by capillarity was greatest when clods were initially very dry and smaller clods tended to absorb water faster than bigger clods when under confined conditions. Confining had no effect on infiltration when the initial condition was very wet. On drying, the smallest clod dried the fastest, reduced greater volume and increased its dry bulk density significantly. Larger clods required,longer drying period to arrive at a uniform moisture profile within as compared to smaller clods. Results from the wetting experiments were tested against the infiltration model of .Jarvis and Leeds-Harrison (1987) and a model developed based on linear flow of heat into a solid (Carslaw and Jaeger, 1959). A second project involved the study of soil deformation at high moisture contents in an attempt to produce clods with minimum draught force using simple relieved tines at various rake angles and depths in a soil tank. The principal. objective of the

• ii - study was to utilise soil implement mechanics knowledge to improve the efficiency of soil preparation for wetland crops. Aspects like the nature of soil disturbance, extent of disturbance and draught requirement were investigated. The soil was in a plastic consistency prepared to three specified density states of 940, 1000 and 1250 kg/m3• The soil disturbance pattern was monitored using implanted coloured beads and glass sided tank studies. In addition, the extent and height of heave and surface disturbance were noted. Predictive models based upon Mohr-Coulomb soil mechanics theory were developed to predict the interaction between the soil and simple implements at three rake angles. These were based on the lateral failure theory of Godwin and Spoor (1977) and the two dimensional soil failure model of Hettiaratchi and Reece (1974). Results from the single tine study were tested against the models. A sliding resistance component and crescent effect were incorporated to improve the predictions for the 45° and 90° rake angle tines. The magnitude of each mode of failure is dependent upon the critical aspect ratio which varies with tine rake angles and soil conditions. The mode of failure is considered to be lateral when the tine aspect ratio is larger than the critical aspect ratio and an upward failure when the tine aspect ratio is lower than the critical aspect ratio. The predicted results are in close agreement with the results of the experimental studies. For the backward raked tine, a model was developed based on the formation of an elliptical wedge and bearing capacity type of failure ahead and below the wedge. This failure theory was based on the bearing capacity failure for deep footings. The model
• iii - helped identify an additional parameter that influenced the draught force for a backward raked tine. This parameter is the sliding resistance component on both sides and beneath the elliptical soil wedge •. Results from multitine studies showed that draught force increased with tine spacinq but the increase was not significant. In the wet condition the tines merely cut slots and little or no interaction was noted. In an effort to find the optimum water level for soil puddlinq, a laboratory study was conducted to determine the influence of water-soil ratio on the ease of puddling air dry aqqreqates. Soil puddlinq was carried out usinq a·rotary stirrer simulatinq the rotary motion of a rotary cultivator commonly used in wetland preparation •. The results obtained showed that· the fastest dispersion of particles resultinq in a minimum wet bulk density of 1.23 Mg/m3, was achieved at a water-soil ratio of 1.2. (A supersaturated condition equivalent to a moisture content of 120% dry basis). Increasing the water-soil ratio above this value did not change the wet bulk density value for all stirring times.