Soil compaction by wheeling: changes in soil suction caused by compression

Abstract

Soil compaction caused by agricultural machinery has been increasingly recognized as a considerable problem facing intensive agriculture. Most of the models used to estimate soil deformation during the passage of machines are based on the concept of total stress: they have neglected an important stress variable for unsaturated soils; that is, the matric suction. The aim of the present work was to evaluate the validity of this hypothesis by studying suction variation during a static compression test. A standard oedometer cell equipped with a tensiometer was used to measure soil suction in situ for different vertical stresses. Measurements were carried out on remoulded soil samples obtained by compacting a loamy soil at different initial water suctions (<100 kPa). The results showed that the suction remained almost constant until a stress threshold value, σt, beyond which the suction decreased as the stress increased. This stress threshold increased with the initial suction. These results corroborated the hypothesis of a constant suction during deformation, which is usually assumed to model soil compaction during traffic for soils with suction greater than 20 kPa. The results obtained highlighted the effect of soil structure on the stress threshold: σt was greater for soil samples with initial aggregates <2 mm than for those with initial aggregates <0.4 mm. This was interpreted at the pore scale by comparing qualitatively the change in pore‐size distribution and the expected distribution of water in the pores. This interpretation was based on pore‐size distribution measurement by mercury intrusion.