The role of calcium carbonate in the compressibility of Pliocene lacustrine deposits

The results of a study on the influence of calcium carbonate content on the physical characteristics and compressibility parameters of Pliocene lacust rine deposits are discussed. The origin of CaCO3 has been investigated and it is thought to have been precipitated, after consolidation, as cement whi ch partially filled the void spaces between soil particles. Average CaCO3 c ontent was about 22% along a 116.5 m deep vertical profile. The CaCO3 in th e clay and silt fractions (CaCO3*) was considered the best available estima te of that acting as cement. The relationships reported in the literature b etween Atterberg limits and CaCO3 content have been statistically confirmed by the tests performed on decalcified laboratory samples, but not by those performed on natural samples due to lithological heterogeneities. Yield st ress, constrained modulus, swelling index and microstructure resistance wer e affected by CaCO3* content in the stress range lower than yield stress. I n the stress range higher than sigma'(vy), no relationship was observed bet ween C-c and CaCO3, while an inverse relationship between C-s and CaCO, was observed in both the natural and remoulded samples and thus is thought to be due to the presence of inactive calcite. Moreover, the remoulded samples also show an increase in C-s with respect to the natural ones. From these findings it follows that the yield stress for cemented materials correspond s to the partial disruption of the structure.