In order to accelerate the rate of consolidation settlement and to gain a required shear strength for a given soft clay deposit, the preloading technique combined with a vertical drainage system has been widely applied. Vertical drains are especially effective in stratified soils in which the horizontal permeability is larger than that in the vertical direction. In this study a practical and efficient method of nonlinear consolidation analysis was proposed to predict the behavior of drainage-installed deposits.
However, even if a sophisticated numerical analysis technique is applied to solve the consolidation behavior of drainage-installed soft deposits, the actual field behavior is often different from the behavior predicted in the design state due to several uncertainties involved in soil properties, numerical modeling, and measuring system. Therefore, two back-analysis schemes such as the modified simplex and BFGS methods have been implemented to characterize the in-situ properties based on field measurements..
In order to improve the prediction of magnitude and rate of settlement and pore pressure dissipation under embankment type of loading on drainage- installed deposits, the following two major parts are intensively studied: practical and efficient nonlinear consolidation analysis and optimization of in-situ properties by using in-situ measurements.
First, as for a more practical and simpler method, the theory of axisymmetric consolidation in which considers the variation of compressibility and permeability during the consolidation process has been developed. A computer program named AXICON for the analysis of axisymmetric nonlinear consolidation is developed by adopting an explicit finite difference method. Smear and well resistance effects are also considered. The AXICON is capable of analyzing the consolidation behavior of multi-layered deposits and simulates time dependent loading sequence. The results of AXICON are validated with analytical solutions of Hansbo and Barron, and they are compared with in-situ settlements and pore pressures measured in a soft clay deposit.
The in-situ consolidation behavior of drainage system-installed deposits has three dimensional characteristics. Therefore, for an approximate 2-D plane strain consolidation analysis, it is necessary to convert the 3-D spatial flow of actual cases into the laminar flow simulated by the 2-D plane strain model. As an efficient and accurate method in order to properly model the effect of three dimensional characteristics, an equivalent and efficient model has been applied in a finite element technique to analyze the drainage system-installed soil deposits. The equivalent two-dimensional model involves an equivalent permeability concept and an adjusted width of drainage. To validate the equivalent two dimensional model, three-dimensional analyses were performed by using the ABAQUS program and the results of 3-D analyses were compared with those of the 2-D analyses. It shows that, by using the proposed equivalent model, one may be able to appropriate predict the consolidation behavior of drainage system-installed soft deposits.
Secondly, since the predicted consolidation behavior of in-situ soft clay is quite different from the measured one mainly due to the approximate numerical modeling techniques as well as the uncertainties involved in soil properties and geological configurations, two back-analysis schemes such as modified simplex and BFGS methods have been implemented in both an axisymmetric nonlinear consolidation program(AXICON) and a general plane-strain consolidation program(SPIN2D).
The soil parameters, such as nonlinear constitutive parameters and permeability used in both AXICON and SPIN2D programs, which have an effect on the process of consolidation, are back-analyzed by the two back-analysis schemes on the basis of settlements and pore pressures measured in real sites. Utilizing these programs, one might be able to appropriately predict the subsequent consolidation behavior from the measured data in an early stage of consolidation of drainage-installed soft deposits.
In conclusion, by combining the developed method of nonlinear consolidation analysis and the back-analysis scheme, it has been found that one may be able to appropriately predict the consolidation behavior of drainage- installed soft deposits.
