Response of five types of soil to simulated compaction in the form of confined uniaxial compression tests

Abstract

In most of the Spanish dry-farming regions seedbed preparation for both winter and spring cereals occurs when the soil exhibits a high resistance to the forces applied by machinery traffic. Soil compaction resulting from this traffic has scarcely been studied to date. For this reason the objective of this work was to assess the response of five types of soil from different Spanish agricultural regions to compaction at different moisture contents in the form of confined uniaxial compression. The soils considered belonged to the sandy-loam, sandy-clay-loam, loam, clay and silty-loam textural classes. Soil samples were used to prepare cylindrical probes (50 mm diameter×20 mm height) that were compacted at different moisture contents (5, 10, 15, 20 and 25% w/w) in a universal testing machine. Water content in the sandy-loam and clay soils was increased to 35% (w/w) and 30% (w/w), respectively. For compaction, the probes were strained at a constant rate (0.83 mm s −1) and at the same time the normal stress they withstood was registered. The process of compaction ended when this stress reached values of 50, 100, 200 and 400 kPa. The response of the soils to compaction was analysed in terms of void ratio, air permeability and tensile strength. The slope of the plot of void ratio and the logarithm of the compression stress applied was found to be dependent on the moisture content at compaction for all the soils. In the case of the sandy-clay-loam, loamy and clay soils, the highest values of this slope, 0.68, 0.65, and 0.74, respectively, were reached at a moisture content of 20% (w/w), whereas for the sandy-loam and silty-loam soils these values, 0.68 and 1.02, were obtained with 25% (w/w) moisture content. Both the moisture content and the stress of compression greatly affected the air permeability of the soils. Air permeability limiting value of 10 μm 2 was in no case attained with the 50 kPa normal stress. However, with the other normal stresses this limiting permeability was obtained with moisture contents which were lower with greater stress applied. The highest permeabilities were for the clay soil, 57.35 μm 2, followed by the loamy soil, 13.55 μm 2 and the sandy-clay-loam soil, 10.78 μm 2. The lowest permeabilities were those of the sandy-loam and silty-loam soils, 3.68 μm 2 and 8.81 μm 2, respectively. For a given stress of compaction tensile strength increased with soil moisture content. At constant moisture content, greater the tensile strength, higher the compression stress. The relative importance of water content and applied normal stress in the compaction process is affected by the soil texture. The higher the clay content, or the presence of a loam texture, the higher the soil capacity to bear higher normal stresses at higher water contents without reaching a severe state of compactness.