Assessment of the effects of the structure on the compression behaviour of a young alluvial silty soil

A comprehensive series of tests was performed to investigate the effects of the in-situ structure and sample preparation technique on the compression behaviour of a shallow young alluvial silty soil. Two different particle size distributions were investigated by testing intact, compacted, and slurry samples in an oedometer. The compression of the slurry samples, created at different initial water contents, gave a unique intrinsic compression line for one of the two distributions. This was used as a reference for analysing the effects of the structure of the compacted and intact specimens. The compacted samples were prepared by means of static and dynamic compaction in one or more layers with vertical holes simulating those created by plant roots in natural samples. The results showed that a unique normal compression line is defined regardless of the number of layers or the number of holes, with a clear effect of the structure when compared to the slurry samples. The significant effects of the in-situ structure of the intact samples were observed even at high stress levels. An important finding was the non-uniqueness of the compression line for the second distribution. For the case of the slurry samples, the compression lines remained parallel to each other even at high stress levels, whereas a unique line was found for the compacted samples. These results are contrary to those often given in the literature, namely, that the sample preparation technique can create very robust initial structures resulting in a mode of behaviour that has been called “transitional”. The research also emphasises how the quantification of the natural structure is critically dependent on the way in which the intrinsic samples are prepared.