Time effects on artificially cemented kaolin

Abstract

With the aim to achieve a better understanding of the time-dependent behaviour of structured clays, especially cemented clay, a critical review of the literature and a programme of laboratory testing have been carried out. In order to investigate the time effects on different structured clays, a series of advanced triaxial and oedometer tests were conducted on reconstituted pure kaolin (PK), artificially cemented kaolin (ACK) with different curing periods, and remoulded artificially cemented kaolin (RACK). Strain rate effects were examined in pure kaolin with different degrees of over-consolidation in both undrained and drained shearing. The influence of bond structure on the monotonic stress-strain behaviour of cemented kaolin was investigated on specimens prepared with two different curing stress and curing periods. Tests on pure kaolin and remoulded cemented kaolin were also performed. Strain rate effects on long cured cemented kaolin under low and high confining stresses, with constant rate of straining in pre-peak stage and step-wise change rate of straining in post-peak stage were examined. This was completed by a study of the microstructure of pure kaolin and cemented kaolin specimens before and after tests via scanning electronic microscope. The coupling between stress or strain rate and curing time effects was investigated on cemented kaolin cured for shorter periods. The results have added significant data to the existing database and addressed the relationship between bond structure and time effect. The main conclusions are: ••The viscous type of PK determined is characterized as decaying positive isotache and General TESRA in undrained and drained shearing repectively. The strain rate history does not affect the critical state in both q-p’ and v-ln p’ spaces. ••The addition of cement not only creates new inter-particle bonds but also affects new fabric. The shear strength is dominated by bond structure under low confining stress, and by volume change under high confining stress. ••The viscous type of long-term cured ACK is dependent on the degree of bond structure. It is almost insensitive to strain rate changes when the specimen is stiff, and the apparent “aging” effect dominates resulting in a “negative” rate effect when bond structure is destroyed. A special rate phenomenon called stick-slip seems to occur under the lowest strain rate (0.01%/h) during the pre-peak stage. The RACK shows a TESRA type of viscosity in undrained shearing. ••The short-term cured ACK, in which structuring and rate effects are combined, shows negative rate effects due to the curing under low confining stress and apparent aging under high confining stress. Stick-slip occurs both in pre-peak and post-peak stages, due to different reasons.