| Summary: | 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
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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
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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.
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