Reconstitution of sensitive clays

In this thesis a method is developed to regain the geotechnical behaviour of undisturbed quick clay by reconstitution of remoulded natural quick clay samples. Quick clay is characterised by a high sensitivity; the ratio between the undisturbed and remoulded strength is very high. Because of the high water content they behave fluid-like after remoulding. As the change of material structure during remoulding is irreversible, a special method is required to rebuild this structure. With such a reconstitution method physical model testing on quick clay can be simplified. Firstly, samples can be reconstituted in any shape without the need of sample disturbing techniques, like sample handling and cutting. Secondly, it decreases the need for expensive in-situ undisturbed sampling, and natural quick clays can be reused for reconstitution. Finally, such an material could be a suitable material to investigate sample disturbance. In this study firstly the geological genesis and geotechnical characteristics of quick clays are discussed, the concept of structure is introduced, and several models to describe the development of sensitivity in quick clays are presented. Furthermore, it is shown that approaching quick clays from a colloid chemical perspective provides a useful framework to understand development and breakdown of structure and sensitivity. The theoretical framework is used to set up an experimental programme. Firstly, the used natural quick clay material is characterised. Hereafter the influence of the depositional environment (influence of mixing methods, water content and varying salinity) and consolidation under self-weight in a geotechnical centrifuge (with varying loading rates and varying sample thickness) are studied. Finally, the influence of leaching (both by diffusion and by application of a hydraulic head in a geotechnical centrifuge) is investigated. Initially, tests are performed on a small scale 14 mm diameter tubes to establish what is the most promising method. This reconstitution method is thereupon tested with 50 mm samples. It is shown that increasing salinity increases the sediment void ratio, both after settling and consolidation at 1-g and 87-g. The loading rate during consolidation appears to be unimportant, because the salt provides structural integrity. This reconstituted salt material has significant strength and behaves only slightly sensitive. However, when compared to the natural material as equal stresses, the undisturbed strength is lower and the remoulded strength higher. As a result of leaching, even when the reconstituted material is carefully leached with an hydraulic head, the undisturbed strength decreases with almost 90%. However, because the remoulded strength is reduced to immeasurably small strengths the sensitivity is increased drastically. It is proven that the flux of water itself does not affect the strength. Therefore these changes should be attributed to the change in salinity. Leaching with a hydraulic head is preferable over leaching by diffusion, because hydraulic head leaching is less time-consuming and the effectiveness of diffusion for development of sensitivity could not be proven in this study. The result of this reconstitution method is a very sensitive and weak clay (c_u = 1.2 kPa, when e = 2.4), which behaves like a quick clay upon remoulding. This low undisturbed strength is the main limitation of the proposed method. However, based on the experimental results it is expected that by adjusting the consolidation conditions the method can be improved. The developed reconstitution method is not tested on physical model scale samples. However, there are no major differences in slurry composition and stress conditions between the performed experiments and reconstitution of such large sample, except for time scaling effects. Therefore is concluded that the method will be feasible for physical model scale samples. The results of this thesis can be used as a starting point for future reconstitution studies. It provides a useful framework, and shows both in a theoretical and experimental way which methods and procedures might be feasible to improve the results.

Subject

reconstitution
quick clay
sensitive clay
structure
sensitivity
flocculation
leaching
centrifuge

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http://resolver.tudelft.nl/uuid:ede4af12-d290-4307-b504-a74020e6bf88

Embargo date

2012-10-30

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Student theses

Document type

master thesis

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GJMeijer_MSc_Thesis.pdf

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