Assessing Structure of Aged Natural Sedimentary Clays

Abstract

Soils become stiffer as they age. In this paper, attempts were made to assess the in-situ structure of natural clays by calculating the current quasi-elastic shear modulus from shear wave velocity measurement, Gmax. The Gmax value of reconstituted clays was employed as a yardstick against which the corresponding characteristic of natural sedimentary clays is examined, implying that the intrinsic properties of a non-aged reconstituted clay provide a basic frame of reference for assessing the in-situ structure of a natural clay and the influence of ageing (secondary compression and interparticle bondings) on its in-situ properties. In a series of bender element (BE) tests on a reconstituted clay, it was first manifested that the relationship between the void ratio and Gmax as subjected to one-dimensional compression exhibited the state boundary (SB) curve applicable to the short-term migration of effective stress. Second, it was demonstrated that the SB curve was violated when the clay aged by drained creep, and that the state of the aged clay rejoined the SB curve in a gradual manner on subsequent stressing applied over a short period. In the light of this characteristic time-dependent behaviour of Gmax, a measure termed metastability index MI (Gmax) has been newly proposed for assessing the structuration/destructuration of in-situ natural sedimentary clays which undergo effects of a long-term insitu ageing. The performance of MI (Gmax), together with the non-dimensional soil constant reflecting structure, S (Jamiolkowski et al., 1994), was carefully examined in the BE tests using various natural and reconstituted clays. It was successfully demonstrated that : i) the proposed index MI (Gmax) as well as the S-value were both equally capable of expressing structuration/destructuration of clays as examined in tests on Ariake (Japan), Bangkok (Thailand) and Louiseville (Canada) clays, each comprising a pair of a natural and its reconstituted sample ; ii) two different formations of soil structure, i.e., an on-depositional structure formed at sedimentation and the evolving post-depositional structure enriched due to ageing, should both be addressed for a proper understanding of the current structure of natural clays ; iii) the S values were similar among a total of nine non-aged reconstituted clays, despite a wide spread of plasticity index as well as compresssibility ; and iv) the S-value of each reconstituted clay is the lower bound against which the in-situ structure of the natural clay can be quantified at the relevant effective stress. A time-dependent soil structuration was also predicted by introducing a soil constant for metastabilization, Cβ.