1990 Volume 59 Pages 7-16
Increasing use of heavy machinery in intensive arable production has increased the probability of compaction and structural degradation of agricultural soils. Examples are given in this paper to show that relatively small increase in soil bulk density and small loss of air - filled porosity upon compaction could result in appreciable increase in mechanical resistance and serious deteri-oration of gas diffusion in the soils. Volume change characterictics of soils under compressive stress are affected by soil water con-tent as well as structural stability of the soils. It is stressed that the magnitude of the change in total porosity (or bulk density) of soils depends upon the previous history of compression and is not always a useful index of the soil physical state after the compression. Determination ofs oil water characteristic curves for compressed and uncompressed soils has revealed that even when soils with the same initial suction are compressed with the same magnitude of compressive stress, the size of the pores lost or collapsed differs significantly among the soils. Compression susceptibility x of soil is introduced to explain the different response of the soils in the alteration of pore size characteristics and defined as x=upf/σt where upf and σt are the overburden water pressure at the end of compression, and the applieds tress, respectively. Soils which exhibit normal shrinkage over a wide range of water content havei n general greater values of x, and are expected to be liable to structural damage by compression. The need for research on the destruction of soil structure by shear forces as well as on the envi-ronmental consequense of soil compaction is briefly discussed.