Most previous research focused on the soil’s consolidation within the treatment depth of the prefabricated vertical drains (PVDs). Vacuum preloading is an effective ground improvement technique for treating soft soils. It was concluded that after preloading, soil settlement approached acceptable settlement values, which indicates the efficiency of this method for large-scale soil improvement projects. Moreover, optimum spacing and depth of PVDs were evaluated, and soil settlement was compared before and after the preloading operation. The numerical model results indicated that the settlement value using modified parameters obtained from the back-analysis procedure was lower than the calculated settlement based on the assumed parameters in the initial estimation, which can be attributed to the conservative determination of initial geotechnical parameters. To this end, the numerical simulation was carried out using Settle3D software. First, the ultimate settlement values were predicted using observational methods, then, to determine the modified preloading design parameters, the involved parameters such as compression index (Cc) and preconsolidation pressure (P'c) were back-analyzed by comparing measured data with the numerical simulations. This study deals with the performance evaluation of the soil improvement using surcharge preloading techniques with prefabricated vertical drains (PVDs) for the Mahshahr oil storage tanks project as a case study. By measuring the Excess Pore Pressure Gradients (EPPGs) during cyclic loading, the test results convincingly reveal the promising performance of PVD-geocomposite combination under different loading conditions. Laboratory experiments show that the continuous dissipation of EPWP and the substantial reduction in drainage path lengths by PVDs can prevent subgrade fluidisation at shallow depths, while geocomposite can provide adequate surficial drainage and effective confinement at the ballast/subgrade interface. The primary objective of this paper is to evaluate the integrated role of PVDs and geocomposite in preventing subgrade fluidisation using Dynamic Filtration Apparatus (DFA).
Although numerous laboratory experiments on the performance of Prefabricated Vertical Drains (PVDs) and geocomposites have already been carried out in the past, how effectively this combination can mitigate the potential for subgrade fluidisation under repeated (cyclic) loading is still not properly understood. Instability of subgrade soil sometimes associated with soil fluidisation can lead to uncontrollable deformation and failure at a critical number of loading cycles for a given cyclic deviator stress and frequency. The recommendations were provided after applying the proposed method to a case history. The 1-D compression method was modified by introducing two factors to consider the effect of soil lateral displacement and to correct the constrained modulus due to the soil disturbance caused by PVD installation, respectively. Such degradation could be due to the soil disturbance caused by PVD installation. The back-analyzed soil modulus of the natural soft clay in Shanghai Disneyland Resort project showed considerable degradation as compared to the corresponding constrained modulus from the laboratory test.
The observed small overall lateral-to-vertical strain ratio at the perimeter of the PVD-improved zone soil showed that the vertical compression of PVD-improved zone was close to one-dimensional (1-D) state, but 1-D compression method yielded smaller predictions than the measurements from the case histories. A theoretical equation was developed to evaluate the effect of lateral displacement on the soil vertical compression. In this paper, the soil vertical compression within the prefabricated vertical drain (PVD)-improved zone under vacuum pressure was investigated based on the theoretical analysis, reported laboratory tests and the case histories.
Reliably estimating soil vertical compression under vacuum loading is still challenging for geotechnical engineers.
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