Effect of time on the evolution of soil behaviour: new advanced triaxial creep cell experiments

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Extending the life of existing infrastructure is one of the main aspects of sustainable construction in current civil engineering projects. Examples include the re-use of foundations in urban re-development projects, or raising the height of existing flood embankments to account for sea-level rise due to climate change. The design requirement in such projects is to quantify how much higher load the foundation soil can take in order to enable the construction of a higher flood embankment or a heavier new structure on existing foundations.In both foundation re-use and raising flood embankments the existing structure will have been in place for a long period of time (several decades) during which the foundation soil has been subjected to evolving anisotropy and to time-related processes of consolidation (dissipation of excess pore water pressures within the soil generated by the applied load) and creep (continued straining of the soil with insignificant change in effective stress). Both consolidation and creep processes cause additional ground movements dependent upon time. The consequence of these ground movements is the reduction in the void ratio which leads to the strength and stiffness increase in the soil with time. It is this enhancement of soil mechanical properties that enables an increase in applied load on existing foundation systems. However, there is currently little guidance on the degree or magnitude of increase in stiffness or strength of the founding soil with respect to the duration of creep or loading. This aspect of the time-dependant framework of soil behaviour is poorly defined, but has significant implications in terms of the sustainability of civil structures.The proposed research is very timely in addressing design needs for sustainable construction and redevelopment of existing infrastructure. It relies on the development of new experimental equipment consisting of an advanced triaxial creep apparatus, and will enable the apparatus to be properly commissioned for the first experiments to be performed. Long term experiments exploring the effect of creep on the strength and stiffness of clay have not been performed before and the lack of experimental data on this topic is hindering the ability of both research and industry to account for or quantify improvements in engineering properties due to creep processes.Project partners, Arup, have a strong interest in supporting and collaborating in research into geotechnical engineering issues and are providing soil samples in addition to settlement records and load estimates from some structures founded on the same soil that have experienced very large and long-term settlement behaviour. This input will provide am active and pertinent focus for the project.The database of experimental behaviour which can be generated by a cell designed to monitor soil sample creep accurately and stably in the long-term will enable constitutive models to be calibrated to aid design using numerical analysis methods. The principal aim of the project is to quantify the improvement to strength and stiffness from periods of creep to gain a better understanding of the effect of time on the evolution of soil behaviour which has been notably absent from previous studies. This is a novel approach in the study of the time dependant behaviour of soil and it is of note that previous studies have been unable to study the effect of significant periods of creep, due to the lack of time and capability for accurate and stable long term measurement of sample volume change. The Imperial College Geotechnics Laboratory are unique in their expertise and facilities to be able to tackle this challenge.