The Impact Of Secondary Consolidation On The Consolidation Characteristics Of Quasi Over-consolidated Clay And The Design Of Surcharge Preloading

When building the highways and railways in soft soil the surcharge preloading method has been widely used in the treatment of the soft soil foundation for its economic and effective, but some built highways through preloading treatment showed the post-construction settlement was much bigger than theory predicts during the traffic operation period, this indicated it will lead to the post-construction settlement exceeds than expected and bring hazards if using the primary consolidation theory to predict the post-construction settlement of soft soil foundation and put forward the preloading duration. If there are high requirements on settlement it will limit the application of surcharge preloading method in the treament of soft soil foundation. Another issue that didn't get enough attention in the past was the preconsolidation phenomenon of soft soil, most soft soils in natural are quasi over-consolidated soils but previous research on it was few. therefore it is necessary to study further on the consolidation properties of quasi over-consolidated soils.This master's thesis focuses on analysising of preloading duration based on the theory that considering secondary consolidation and carrying out some work about the impact of "preconsolidation".which is formed in the sedimentary of the quasi over-consolidated soils, on the consolidation characteristic. Firstly, introducing Yin-Graham equivalent time rheological model that considering the secondary consolidation and preconsolidation phenomenon of soft soils as the theoretical basis of research of full text. Secondly, compiling the finite difference programs to numerically simulate and studying the impact of secondary consolidation on consolidation characteristics of quasi over-consolidated clay according to equivalent conversion between sand wells and one-dimensional homogeneous soil, equivalent conversion between sand wells and sand-wall, axisymmetric consolidation of sand wells separately. Finally, proposing the calculation method of determining the preloading duration on the basis of the related consolidation properties.The results show when the absolute equivalent time of preconsolidation of soft clay is large, at the early stage of the quasi over-consolidated soft soil consolidation the larger the creep coefficient is, the faster the pore pressure dissipates, the greater the degree of consolidation has and the smaller the settlement gets, to reach the same settlement the soil that has less creep coefficient will require more time, the physical reason of this phenomenon is then qualitatively analyzed. The results also show that the larger the initial absolute equivalent time is. the more obvious the preconsolidation effect of soft soil shows, the faster the consolidation has. and the greater the long-term settlement gets, therefore "preconsolidation" level of soft soil should be taken into consideration when analysising the consolidation properties of soft soil and designing the surcharge preloading, if not considering the preconsolidation effect it will bring great influences on consolidation properties and surcharge preloading design. On the basis of previous study the calculation method to determine the preloading duration for the design of surcharge preloading in quasi over-consolidated soft soil is put forward.