Soil testing of dynamic deformation processes of arable soils

Abstract

Soil compaction is an ongoing issue of concern in agricultural land use. Measuring and modeling stress–strain relationships for various loading conditions and field traffic intensities is therefore important for assessing and predicting soil deformation and estimating related changes in soil functions such as aeration and hydraulic conductivity. A well-known and widely used parameter that quantifies soil stability is the pre-compression stress ( P c). Pre-compression stress values are determined from compression curves measured in confined compression tests under static loading conditions. It reflects the stress history of the soil and separates the re-compression stress range (below the pre-compression stress), where soil deformation is assumed to be fully elastic, from the virgin compression stress range (above the pre-compression stress), where soil deformation is considered plastic. Particularly, in agricultural land use, however, soil deformation is caused by dynamic rather than static loading processes where in situ stress paths diverge from the conditions applied during standard static soil testing. While the pre-compression stress is a useful indicator for the soil's bearing capacity it neglects plastic soil responses in the re-compression stress range which may become important, especially concerning subsoil compaction, when long-term intensive field traffic inducing multiple reloading events is considered. On the other hand if volume deformation is estimated from compressibility indices derived under static loading conditions it may potentially overestimate the true compression since loading times are much shorter during real wheeling events than during consolidation tests. We developed a new programmable pneumatic multistep oedometer (PMO) in order to quantify soil deformation under dynamic loading conditions. The oedometer allows defining virtually any stress path and hence provides a flexible method for dynamic soil testing. To demonstrate the potential applications of dynamic soil testing accounting for typical loading conditions on arable soils we conducted confined compression tests under various stress paths. Examples from cyclic loading tests (100 loading cycles) and the simulation of stress paths measured in situ with stress-state transducers (SST) during a wheeling experiment are shown. Results suggest that dynamic soil loading leads to cumulative soil deformation with significant effects on soil functions shown by a reduction of air conductivity coefficients. Soil compaction studies concerning agricultural field traffic should account for the time-dependency and non-static nature of load application to obtain more realistic predictions of possible changes in soil functions.