Studies On One Dimensional Consolidation For Soft Soils With Threshold Gradient

Up until now. Darcy’s law has been incorporated in most conventional theories of consolidation to depict the flow of water through soft soil such as clays. However, Darcy"s law was established from permeability test on sands. It has been widely agreed that deviations from Darcy’s law do exist in some clays. Some investigators have revealed evidence indicating deviations of water flow from Darcy’s law and demonstrated the existence of a threshold gradient in some fine-grained soils with low permeability. There is no flow in the soil where the hydraulic gradient is less than the threshold gradient, and little research has been reported on consolidation with threshold gradient at present. In order to further improve the theory of consolidation for soft soils with threshold gradient, systematically, studies are made on the basis of previous research. The main original works and innovative achievements include:(1) The motions of seepage are discussed and the governing equations of consolidation are derived by considering three cases, such as 1). the initial pore pressure gradient is larger than or equal to the threshold gradient;2). the initial pore pressure gradient is less than or equal to the negative value of threshold gradient; 3) the absolute value of initial pore pressure gradient is less than threshold gradient. Analytical solutions for excess pore pressure and average degree of consolidation are obtained with different distribution of initial pore pressure. A comparison is made between the present solution and previous solution for a particular case in which the distribution of the initial excess pore pressure is uniform. Base on the present solution, influences of the threshold gradient and the distribution of initial excess pore pressure on the motion of the moving boundary of seepage, the distribution of excess pore pressure and the average degree of consolidation are analyzed, and the difference between the average degree of consolidation defined by settlement and defined by pore pressure is discussed. At last, computation table describing the motion of moving boundary of seepage with time is provided for the distribution of initial excess pore pressure being uniform.(2) In light of the application of the external load during construction always takeing some time. A general analytical solution is developed for one-dimensional consolidation with a threshold gradient under time-dependent loading. Moreover, detailed solutions are obtained for the moving boundary of seepage, the excess pore pressure and average degree of consolidation in terms of stress and strain under the single-stage loading, and two different cases are discussed, such as 1). The loading time is larger and equal to the time which the moving boundary takes to the bottom of the layer; 2). The loading time is smaller than the time which the moving boundary takes to the bottom of the layer. Finally, the effect of loading time on the consolidation behaviour is investigated.(3) By assuming threshold gradients less than the initial pore pressure gradient the flow would occur throughout the whole layer instantaneously, and a analytical solution and its simplified forms are firstly obtained for one-dimensional consolidation of layered systems with threshold gradient. A relevant computation program is developed. Based on the solutions and the corresponding computation program, an example is investigated and the consolidation behavior of layered systems with threshold gradient is analyzed.(4) Performing discretizations in the time and spatialdomain, a new semi-analytical method is proposed for solving the problem of one-dimensional consolidation of layered systems considering threshold gradient. The corresponding computation program is developed. Then a comparison is made between the result attained from the semi-analytical solution and that obtained from the previous solutions for homogeneous soil. Finally, the effect of different combination of threshold gradients of double-layered ground on consolidation behaviour is discussed.(5) By incorporating the logarithm relationship between void ratio and permeability coefficient as well as that between void ratio and effective stress, a semi-analytical solution is attained for one-dimensional nonlinear consolidation considering the threshold gradient, the distribution type of self-weight stress, the nonlinear variation of soil’s permeability and compressibility. A comparison is made between the present solution and the previous solution without considering threshold gradient with the ratio of compression index permeability index being equal to 1, and the effects of the ratio of compression index permeability index, the external load, the threshold gradient and the distribution type of self-weight stress on the motion of moving boundary and average degree of consolidation are discussed.