Browsing by Author "Dresser, M. L."

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    Design of an automated tension infiltrometer for unsaturated hydraulic conductivity measurement
    (Cranfield University, 2006-09) Špongrová, Kamila; Kechavarzi, Cédric; Dresser, M. L.
    A tension infiltrometer for field use, where both water level changes measurement and tension settings could be automated, was built in-house. Differential pressure transducers were used to automate the water level measurement in the reservoir. The Mariotte bottle was automated by a set of solenoid valves which were connected via tubing to pre-defined depths in the Mariotte bottle. Based on design parameters tested in the laboratory (sensor sensitivity, water reservoir diameter, and bubbling rate) three identical tension infiltrometers connected to a single Mariotte bottle were built. A new reservoir system made of two plexiglass tubes of different diameter slotted in each other was found to reduce the measurement fluctuations caused by the disturbance created to rising bubbles in the reservoir. The new system was tested on a uniform sandy loam profile prepared in the soil bin laboratory and different analytical and numerical data analysis methods were compared. The measured steady state data were used to determine K(h) at different suctions using the analytical method proposed by Reynolds and Elrick (1991). The K(h) points obtained were fitted with the van Genuchten’s equation (van Genuchten, 1980) using the RETC program to calculate the best fit parameters Ks,  and n. These parameters were used as initial estimates of the soil hydraulic parameters in the numerical models HYDRUS 1D and 2D, in which transient cumulative flow data was used to determine the soil hydraulic functions via inverse modelling. The analysis of variance determined significantly higher K(h) values calculated by HYDRUS 1D while the other methods did not differ from each other. Finally, the tension infiltrometer was used in the field on a sandy loam soil to characterise five different tillage treatments (conventional plough, shallow plough, minimum tillage, direct drill, and no-tillage). The effect of wheel traffic was also evaluated by measuring the infiltration rates in the wheel-marks. The fully automated system allowed the measurement of infiltration rates for 8 tensions in triplicate per day with hardly any human intervention apart for refilling the reservoir. The results show that the tillage practices and wheel-traffic have a significant influence on the soil hydraulic function K(h).
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    The effects of tyre systems on the depth and severity of compaction
    (Cranfield University, Cranfield University at Silsoe, 2006-11-01T15:09:33Z) Stranks, Simon Nicholas; Godwin, R. J.; Dresser, M. L.
    High value crops such as peas for the frozen vegetable market have to be harvested at the optimum point for quality, regardless of field conditions. Six wheeled pea harvesters with a gross weight of 27 t, giving a wheel load of 4.5 t are required to move from field to field with optimum timeliness. In order to achieve road speeds of 25 km/h an inflation pressure of 2.2 bar is required for the specific tyre load speed rating. Typically, in field conditions, this inflation pressure is not reduced and therefore the likelihood of soil damage is increased. This study was undertaken to examine the effects of tyre section width and inflation pressure on rolling resistance, rut depth and sub-surface soil deformation. Under controlled laboratory conditions three tyres, at three inflation pressures, with a load of 4.5 t were passed over a soil at three different initial bulk densities. Measurements of dry bulk density, rut depth, rolling resistance, cone penetrometer resistance and soil deformation through the profile were taken before and after the passage of the tyres. The results show that by increasing the tyre size and reducing the inflation pressure the depth at which compaction occurs and rut depth decrease by 44%. The 800 mm section tyre causes less compaction than any of the other tyres tested especially when inflated to 1.6 bar. Rolling resistance is reduced when the tyre is inflated to the optimum for each tyre. The change in bulk density when plotted against either initial bulk density or penetration resistance results in a set of curves which can be used by the manufacturer, farmer or operator to select the correct tyre section width and inflation pressure for field conditions. As a result of this study the harvester manufacturers are investigating the engineering requirements of increasing tyre section width to 800 mm and are now equipping pea harvesters with central tyre inflation systems. The benefits of which would be to significantly reduce the amount of soil compaction, reduce rolling resistance and save the operator £455 000 per annum in fuel costs.