Study Of Large-strain Nonlinear Consolidation Of Soft Soils With Vertical Drains Based On Piecewise Linear Method

The soft soil needs to be treated before the construction of structures,and the vacuumsurcharge preloading combined method with plastic drainage board as drainage channel is widely used in engineering practice due to many advantages.With the increasing shortage of land resources,many projects have to be located in deep soft soil.The layered characteristics of this kind of soil are obvious,the compressibility and permeability of the soil are highly nonlinear,non-Darcy flow may occur in the soil,the initial stress level is low,even the self-weight induced consolidation is not completed,and large deformation of the soil is often observed.All of these make it difficult to accurately calculate the consolidation of soft soil with vertical drains.In view of the fact that current calculation methods have not taken the above factors into account,it is urgent to carry out the research of new calculation method with both accuracy and practicability to guide the engineering practices.Firstly,the thesis explains how to consider the large strain of soil,material nonlinearity,smear effect,partially penetrated vertical drains,equal stress or equal strain conditions,vacuum combined surcharge preloading,time-dependent loading,construction technology with or without vacuum membrane,vacuum loss,non-Darcy flow,underconsolidation and layered characteristics of soil in the method.By comparing with the existing analytical solution,numerical solution,test and engineering measurement results,the accuracy and practicability of the calculation method are proved,which also shows that the method has a wide range of use.In this thesis,systematic consolidation analysis of soft soil with vertical drains is carried out by several groups of calculation examples.The results show that there are essential differences between large strain and small strain solutions,and the difference between the two results increases obviously when the final strain of soil increases.Large strain,material nonlinearity and non-Darcy flow have a comprehensive influence on the difference between the results of large strain and small strain solutions and the consolidation process.When the final strain increases,the material nonlinearity and non-Darcy flow determine the acceleration or deceleration of consolidation.When the non-Darcy flow index increases,the consolidation will slow down when the final strain is small,and accelerate when large strain occurs.The difference between the conditions of equal strain and equal stress is significant,and how to select the right condition depends on the rigidity of the overlying objects and the construction technology.It is necessary to consider the underconsolidation of soil,and when the the specific gravity or initial thickness of underconsolidated soil is larger,the final settlement gets greater,but the consolidation rate becomes slower.When the initial void ratio is larger,the final settlement gets greater and the consolidation rate becomes faster.It is difficult to define the vertical drainage conditions at the boundaries of adjacent soil layers in layered soil,which makes it difficult to calculate the consolidation of layered soil by using the single-layer method multiple times.When the layered soil is partially penetrated by vertical drains,the multilayer method should be used.However,when the drains penetrate the soil,the single-layer method can be used to calculate the consolidation of layered soil equivalently if the thickness of each soil layer is 4 times or more than the equivalent influence radius of the drain.The above principle of equivalent calculation is also applicable if non-Darcy flow occurs.