Analysing ground deformation data to predict characteristics of smear zone induced by vertical drain installation for soft soil improvement

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The use of prefabricated vertical drain (PVD) assisted preloading has been recognised over the last two decades as a very efficient method of ground improvement for sites with deposits of deep soft soil. One of the major parameters influencing the PVD assisted consolidation process, and consequently the required preloading time, is the formation of a smear zone around the vertical drains, and the corresponding soil properties. In this research a systematic procedure integrated with a developed numerical code is proposed to accurately back calculate the properties of the smear zone based on the consolidation data collected in the laboratory and in the field. Furthermore, an expanded back calculation method is developed to determine the minimum required degree of consolidation and corresponding time after the construction of the trial embankment that would result in accurately predicted smear zone characteristics. The explicit finite difference program FLAC 2D was used to develop the numerical code, simulate the laboratory testing and PVD assisted preloading case histories. Furthermore a comprehensive parametric study was conducted to investigate the effect of smear zone properties variations on the preloading process, and back calculated characteristics of the smear zone. A large and fully instrumented Rowe cell apparatus was used to investigate the effect of the smear zone on the consolidation process and verify the developed numerical code. The Rowe cell was filled with the intact zone, smear zone, and vertical drain materials to evaluate the permeability and extent ratios of kh/ks=4 and rs/rm=3, respectively. The back calculation procedure was used to conduct the parametric study and predict the properties of the smear zone. According to the results, the predicted properties of the smear zone were similar to the properties of the applied soil, proving that the proposed back calculation procedure integrated with the developed numerical simulation can successfully predict these properties. The developed numerical code was used to simulate five PVD assisted preloading case studies, including four trial embankments and a large scale consolidometer, while the back calculation procedure was used to conduct a parametric study to determine the extent and permeability of the smear zone. According to the results, integration of the back calculation procedure in the numerical code can be used as a reliable tool to make an accurate prediction of the smear zone characteristics in PVD and vacuum assisted preloading projects. The developed method in this research can be considered as a practical, accurate and cost effective tool, due to its capability in precise estimation of the extent and permeability of the smear zone in the early stages of constructing the trial embankment. In this study, the proposed systematic back calculation procedure was extended to determine the minimum degree of consolidation (i.e. the minimum waiting time after constructing the trial embankment), and accurately predict the properties of the smear zone. The numerical results of the simulated case studies were used to conduct the analyses. Accordingly, it is found that the extent and permeability of the smear zone can be predicted very well with the proposed calculation procedure when at least 33% of predicted final settlement has been reached (i.e. 33% of the degree of consolidation).
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