Effect of grain breakage on the compressibility of soils

The compression behaviour of fine-grained soils is mainly controlled by pore collapse, while the compression behaviour of sands is mainly governed by grain breakage. For soils containing both fine and coarse soil particles such as quartz and calcite grains, the compressibility may depend on both pore collapse and grain breakage. In this study, the compressibility of two natural soils reported in the literature was first analysed in this sense, showing the effect of grain breakage on the compressibility of soils. To further verify this phenomenon, high-pressure oedometer tests were carried out on a natural stiff clay containing a significant fraction of granular elements. The obtained results confirmed the significant effect of grain breakage. In order to better understand the role of fines in the grain breakage phenomenon, artificial sand-clay mixtures were prepared with different fine fractions and tested in oedometer. Results show that the compression behaviour of sand-clay mixtures can be roughly divided into three zones according to the fine fractions: at low fine fractions, the compression behaviour is governed by grain breakage; at high fine fractions, the compression behaviour is governed by the pore collapse of fines; and at medium fine fractions, both pore collapse and grain breakage affect the compression behaviour. In this last case, pore collapse occurs first, letting grains to be well in contact either directly (grain-grain contacts) or indirectly (grain-fine grain contacts) to activate the grain breakage phenomenon upon further loading. This is consistent with the identified compression behaviour of natural fine-grained soils.