Comparing penetrometer and shear vane measurements with measured and predicted mouldboard plough draught in a range of Swedish soils

Models for draught requirement are in most cases developed from foundation engineering, and generally include cohesion as the main parameter to define soil strength. There are also empirical models which include penetration resistance as a soil strength factor. This study examined specific draught during mouldboard ploughing as a function of soil strength and related the results to existing models. Draught requirement was measured on 37 different occasions at 15 sites with clay content ranging from 13 to 56%. Four of these sites were ploughed on several occasions in the autumn to obtain a range of water contents. Shear vane measurements were carried out on all occasions and were used to derive soil cohesion. Penetration resistance was measured on 21 occasions. Soil strength proved much more sensitive to changes in water content than draught requirement. Consequently, model predictions of specific draught were much higher than measured values in soil with high cohesion values. A possible explanation is that specific draught mainly depends on inter-aggregate strength, while the intra-aggregate strength may be considerably higher. Over all tillage occasions, specific draught was much more strongly correlated to cohesion than to penetration resistance. There was also a clear correlation between draught and soil clay content. Multiple regression including cohesion and soil water content resulted in a much higher coefficient of determination than regression with cohesion only. The results indicate that neither shear vane nor penetrometer measurements are sufficient to predict draught requirement, and it is desirable to develop a simple yet reliable field method that can be used for this purpose. More general estimates of fuel consumption can probably be made based on the soil clay content only. (C) 2010 Elsevier B.V. All rights reserved.