Analysis of soil compression curves from uniaxial confined compression tests

The compressive behaviour of soil, the compression curve, is typically expressed in a semi-logarithmic diagram in the relationship between the logarithm of applied stress, sigma, and the void ratio, e. The compression curve yields three important soil compression properties: the swelling index. C-s, the compression index, C-c, and the precompression stress, sigma(pc). The compression index is considered an indicator of soil resistance to compaction, C-s is used as a measure of soil mechanical resilience, and sigma(pc) is considered the soil load support capacity. The objective of this paper was to investigate the impact of soil particle and aggregate size distribution on the characteristics of the soil compression curve and on C-s, C-c, and sigma(pc) for a range of Swedish soils. We found no or only weak impacts of particle size and aggregate size distribution on soil compression properties. Instead, soil compression properties were largely controlled by initial void ratio, e(0), in such a way that Q and C, decreases, and sigma(pc) increases with decreasing e(0), although the correlation between sigma(pc) and e(0) was not statistically significant for subsoil samples. We show that the soil compression properties (sigma(pc), C-c and C-s) and their dependency upon e(0) are partly a consequence of the compression curve being analyzed in a semi-logarithmic (log sigma-e) diagram. The use of log sigma( instead of sigma) forces the compression curve to bend at a certain value of log sigma, and this bend is associated with sigma(pc). We showed that a precompression stress, as commonly defined, can be obtained for an ideal linear-elastic material. Therefore, the use of a logarithmic stress scale may lead to misinterpretation of soil mechanical behaviour. Our empirical results and theoretical exercise emphasize the need for further studies of physically-based expressions for soil strength for use in compaction modelling. (C) 2011 Elsevier B.V. All rights reserved.