Soil compaction and soil tillage - studies in agricultural soil mechanics

This thesis deals with various aspects of soil compaction due to agricultural field traffic, the draught force requirement of tillage implements and soil structures produced by tillage. Several field experiments were carried out to study the mechanical impact of agricultural machines. It was shown that the stress interaction from the different wheels in dual and tandem wheel configurations is small and these wheels can be considered separate wheels with regard to soil stress. Hence, soil stress is not related to either axle load or total vehicle load. At high wheel load, tyre inflation pressure affected subsoil stresses. The maximum stress at the soil-tyre interface was greater than the tyre inflation pressure. Furthermore, the distribution of stress beneath tyres and rubber belts was highly non-uniform. This was shown to have a great influence on stress propagation in soil. Therefore, with regard to soil compaction modelling, a uniform stress distribution (as often used) is too poor an approximation of the real stress distribution and can result in underestimation of soil compaction. A model for predicting the distribution of stress below tyres using readily-available tyre parameters is proposed. With a more realistic approximation of the stress distribution at the soil surface, simulated stresses generally agreed well with measured stresses. Both field and laboratory measurements rejected the concept of precompression stress as a distinct threshold value between reversible and irreversible compressive strain. Irreversible strain was measured at applied stresses that were lower than the precompression stress. The precompression stress was dependent on the nature of the compression test and the method of analysis. The draught requirement of tillage implements could be related to shear vane strength for specific soil-implement combinations. Draught force and aggregate size distribution produced by tillage were strongly affected by soil water content, with the optimum tillage results being produced at water contents close to the water content at the inflection point of the water retention curve. Specific draught was calculated for comparison of the tillage efficiency of different implements. The chisel plough often worked below its critical depth, which strongly increased the energy requirement without any benefit in terms of soil break-up. Therefore, the specific draught was higher for the chisel plough compared with the disc harrow and the mouldboard plough.