Cementation effects in two lacustrine clayey soils

This paper describes the main findings of a laboratory study on the mechanical behaviour of cemented geologically normally consolidated lacustrine clayey soils from two sites, Bacinetto (BA) and Avezzano (AZ), in the Fucino basin (Italy). Onedimensional and triaxial compression tests were carried out in order to investigate the effects of the presence and of the progressive degradation of the interparticle cementation bonds. The two tested soils showed quite different physical and mechanical properties, the more apparent ones being plasticity and yield stress values. The experimental results allowed the gross yield curves and the critical state conditions to be identified for both soils (BA clay and AZ silt). A number of typical features generally exhibited by cemented soils were clearly apparent: yield stresses greater than the in situ stress states, both soils being geologically normally consolidated; high values of compressibility index after yielding, which gradually reduce with increasingly applied stresses; strength reductions associated with a globally contractive behaviour. A convenient normalisation of the experimental results, in which the critical state conditions are assumed as a reference state, allowed the effects of cementation bonds and of their progressive degradation to be highlighted. In particular, BA samples were found to be characterised by different structures related to different degrees of cementation. Furthermore, despite the larger values of the yielding stresses exhibited by AZ silt, stronger effects of cementation are apparent in BA soil. Experimental results seem to indicate that at high values of the applied stress and strain paths, when bonds are largely damaged, the structures of the natural and parent reconstituted BA soil continue to be different.