Temperature Dependent Behavior Of Soft Soils

With the development of the energy engineering,the challenges are arisen from nuclear waste landfill,high voltage cable,energy pile,high temperature subsea pipeline and heating pipeline.The usage of these buried thermal geo-structures will alter the temperature of the surrounding soil and change the stiffness and strength of the soil,leading to possible deformation and potential instability of the structures.Despite the potentially significant influence of the temperature change on the mechanical behaviours of soft clay,them thermal effect is rarely considered in practice.To understand the thermo-mechanical behaviours of soft clay,extensive studies have been carried out during the past five decades.The influences of undrained heating and heating-cooling cycle on the strength and stiffness of soft clay,however,receive relatively little attention.In view of the aforementioned limitations,the following issues are addressed in this thesis:1.The correlation between a thermal related parameter θ and the pre-consolidation pressure of soft clay has been established.By implementing the thermal related parameter θ into the framework of critical state soil mechanics,a new thermos-mechanical model for anisotropically consolidated soft clay has been proposed.Compared to the Modified Cam-Clay Model,the new model proposed herein only include two additional model parameters,with one featuring the thermal effect while the other capturing the stress-induced anisotropy.2.A new temperature-controlled cyclic loading apparatus has been developed,by implementing a refrigerated&heating circulator into a GDS triaxial testing apparatus.Based on the newly developed temperature-controlled GDS triaxial testing apparatus,several series of experiments were carried out to study the effect of drained heating,undrained heating and drained heat-cooling cycle on mechanical behaviours of Zijingang clay and Kaolinite with varying over-consolidation ratios.3.It is revealed from the experimental data that the critical state stress ratio M of kaolinite(PI=28)is temperature-dependent,while the M of Zijingang clay(PI=21)appears to be insensitive to the temperature.By considering temperature dependent M in the constitutive model developed in this study,the undrained shear strength of kaolinite subjected to drained heating and drained heating-cooling cycle are reasonably well predicted.4.The excess pore pressure due to undrained heating of soft clay with varying over-consolidation ratios has been formulated.By implementing this equation into the constitutive model developed in this study,the change of the undrained shear strength of soft clay following undrained heating can be reasonably simulated.5.It is revealed that the worst case scenario occurred while the pipeline was heated under the undrained condition.Under this circumstance,the temperature induced excess pore pressure in the soil surrounding the thermal pipe can be up to 40%of its initial vertical effective stress,leading to a 20%reduction in the axial capacity of the pipe and a 40%increase in the major influence zone of the soil around the axially loaded pipe.