UNIAXIAL COMPACTION EFFECTS ON SOIL PHYSICAL-PROPERTIES IN RELATION TO SOIL TYPE AND CULTIVATION

Disturbed soil samples, representing different textures and organic matter contents, were compressed uniaxially by a range of stresses and at varying water contents. Air permeability was measured immediately after compaction and tensile strength was measured after oven drying. The samples were taken from two soil types which had been subjected to three tillage treatments in a long-term field experiment and from the site of an experiment which compared the compaction caused by different tractor wheels. Within the range of practical interest, specific volume (i.e. the ratio of total volume to volume of solids) decreased with the log of applied stress and tensile strength increased linearly with applied stress. Specific volume also decreased with increasing water content at the time of compaction, whereas both tensile strength and air permeability increased. Soils with higher organic matter contents tended to have higher specific volumes and lower tensile strengths at a given water content and stress. The influence of water content on these properties upon compaction was less in soils of greater organic matter content. Similarly, the rate of increase of tensile strength with applied stress was less in soils of greater organic matter content. However, organic matter did not influence the relative change of specific volume with stress. In the tillage experiment, a stress applied in the laboratory of 100 kPa produced a specific volume similar to the average for ploughed soil in the field but 800 kPa was required to produce a specific volume similar to that obtained by direct drilling. A comparison with compaction values produced by vehicles in the field showed that a higher stress, by a factor of between 3 and 24, was required to reach the same specific volume in the laboratory as in the field. The tensile strength results indicated the susceptibility of the soils to clod formation and their ease of cultivation. Air permeability measurements gave some insight into the possible mechanics of compaction.