Caution on wet soils: trafficking can cause persistent subsidences!

The revised Federal Act on Environment Protection prescribes soil protection from physical damage, such as compaction. However, practical implementation of physical soil protection provokes conflicts, particularly for agriculture where efficient, but heavy, machinery is used for the sake of economic management. Soil compaction is caused by irreversible compression and shearing, i.e. plastic deformation. This contribution shows the results of four field tests where vertical soil displacement was assessed in situ with combined geodetic levelling on the soil surface and a new hydrostatic levelling system in different depths of the soil profile. In two field tests, excavators trafficked dry soil, in the other two experiments, sugar beet harvesters passed over wet soil. In the dry soil, elastic deformation dominated, plastic subsidence in deeper layers was only a few millimetres (max. 6 mm in 30 cm and 2 mm in 50 cm). In the contrary, in the wet soil, subsidence of 14 to 24 mm was measured even in 60 cm depth. The amount of settlement decreased from the soil surface to deeper layers. However, deformation was persistent during the whole monitoring period, which took 12 days at maximum. Additionally, the effect of multiple passes was tested with surface levelling. Four passes caused almost double sinkage than one single pass. This deformation was persistent for the whole monitoring time, too. After these field tests, it can be concluded that the measuring system has a great potential for long-term monitoring of an eventual mechanical resilience of the soil.