Analytical Study On Consolidation Theory Of Composite Ground Under Complicated Conditions

Comparing with sand drain well, the columns in composite ground have a higher compression modulus and a larger diameter together with the characteristics of stress concentration from soil to column and the lateral expansion of column after loading applied. Therefore, although the consolidation theory for composite ground is similar to that for sand drain ground, it is also different from the latter. Aiming at the difficiency that the previous consolidation theory can not consider the abovementioned characteristic of composite ground in a comprehensive and rational way, a new initial condition more suitable for the consolidation of composite ground is derived based on the principle that the external load is always undertaken by the column and the surrounding soil together. Furthermore, detailed studies are made on the consolidation of composite ground under a series of complicated conditions, such as the variation of horizontal permeability coefficient of soil due to the column construction, the depth-and time-dependent characteristic of stress increment, the consolidation and compression of column, the non-linearity of soil and the later difromations of soil and column in the process of consolidation et al. The main original works and innovative achievements are as follows:1. A general analytical solution is obtained for the consolidation of composite ground considering simultaneously the variation of horizontal permeability coefficient of soil, the coupled variation of stress increment with depth and time together. Furthermore, detailed solutions are obtained for excess pore water pressure (EPWP) and average degree of consolidation (ADC) under the particular situation that the stress increment is linearly changed along the column depth and the erternal load is applied in a single-stage way. Finally, the consolidation behavior of composite ground and the development of the column-soil stress ratio are analyzed, respectively.2. In light of the contradiction between the tradional flow continuity assumption at the column-soil interface and the equal strain condition, the traditional flow continuity assumption is modified to eliminate this contradiction. New equations governing the consolidation of composite ground are proposed to account for the column consolidation and compression. Solutions are obtained for EPWP and ADC with an instantly applied and uniformly distributed stress increment along column depth. Then the present solution is compared to some previous available solutions and the consolidation behavior of composite ground is investigated. The results show that ADC predicted by the present solution is less than that predicted by the previous solution using the traditional flow continuity assumption. In addition, the difference in ADC predicted by these two types of solution increases with a reduction of radius ratio.3. To incorporate the radial seepage within the column due to the large diameter of column, new governing eqations for consolidation are proposed to consider the combined flows in radial and vertical directions within the column. The solutions of the new governing equation are obtained by introducing two new boundary conditions. The results by comparisons show that the present solution can be degenerated to five simple particular cases in a rational way. Ignoring the radial flow within the column will over-estimate the consolidation rate. Furthermore, the less the radius ratio is, the greater the over-estimated value is.4. By incorporating the well-known logarithm relationship in effective stress and void ratio as well as that in void ration and permeability coefficient, the characteristics of the nonlinear increase in the soil's compressive modulus and the non-linear decrease in the soil's permeability during consolidation are considered. Based on the non-linear characteristics of soil, an analytical solution for the consolidation of composite ground is developed. Then the non-linear consolidation behavior of composite ground is analyzed and the results show that ADC in terms of stress is not equal to that in terms of deformation and furthermore the latter is always greater than the former. When the soil's compressive indices is less than the permeability indices, ignoring the soil's non-linearity will under-estimate the consolidation rate; however, when the soil's compressive indices is greater than the permeability indices, the reverse is true: ignoring the non-linearity will over-estimate the consolidation rate. For ADC based on stress, when the soil's compressive indices is less than the permeability indices, the increase in the stress increment within the ground leads to an acceleration of the consolidation rate; however, when the compressive indices is larger than the permeability indices, the increase in the stress increment causes a reduction in the consolidation rate. For ADC in terms of deformation, whether the compressive indices is less or larger than the permeability indices, an increase in the stress increment within the ground always accelerates the consolidation rate.5. Under the condition of equal vertical deformation, an analytical solution is derived for the consolidation of composite ground by incorporating the lateral deformations of column and soil occurring in the process of consolidation. The present solution is compared to the solution ignoring the lateral deformations of column and soil. The results show that considering the lateral deformation will reduce the consolidation rate. Finally, analyses are performed on the influence of Poisson's ratios and Young's moduli of column and soil to the consolidation behavior of composite ground and the development of column-soil stress ratio is investigated. The results show that the less the Poisson' ratios of column and soil are and the larger the Young' modulus ratio of column to soil is, the greater the consolidation rate is. When the lateral deformations of column and soil are considered, the column-soil stress ratio will keep increasing during the earlier stage of consolidation, but unlike that predicted by traditional theory, it will not increase up to the value of compression modulus ratio of column to soil.The present work makes consolidation theory of composite ground closer to the practical engineering.