Consolidation Theory Of Vertical Drain Based On Continuous Drainage Boundary

As an economic,environmentally friendly,energy-saving and efficient land-building method,coastal land reclamation has become a major means of alleviating land resource tension in coastal areas,but the properties of dredger fills,such as high moisture content,high compressibility,low strength,low permeability and so on,could not be ignored.The drainage distance of the soft soil foundation can be shortened from the vertical foundation thickness to half of the transverse vertical drain spacing by the installation of vertical drains,and the consolidation rate of the soft soil foundation could be greatly accelerated.At the same time,laying sand or gravel cushion on the top can also increase the permeability of the boundary surface and apply the preloading effectively to promote the drainage consolidation of the foundation.Therefore,the research of vertical drain consolidation has practical significance.The consolidation theory of vertical drain mainly includes the vertical drain consolidation control equation,solution conditions and various assumptions.The solution conditions include initial conditions and boundary conditions,and the selection of boundary conditions has an important influence on the consolidation process.For example,if a soil sample is placed in an impervious plastic bag,and then add water into the plastic bag,the water cannot be discharged no matter how much load applied and how long it takes.Traditional consolidation theory of vertical drain regards the drainage boundary as the limit state which is completely permeable or completely impervious,that is the excess pore water pressure at the boundary surface is zero or the seepage velocity is zero.However,the drainage boundary in reality should be in a state between the two extreme cases of completely pervious and impervious boundaries.Based on this,the consolidation theory of vertical drain under continuous drainage boundary conditions was deduced,and the effects of interface parameter,well resistance,smearing effect,shape effect and soil nonlinearity on the consolidation process were analyzed.Meanwhile,under continuous drainage boundary conditions,the secondary development and analysis of examples was carried out based on the finite element software ABAQUS.The main research results are as follows:(1)The consolidation theory of vertical drain of the top continuous drainage boundary was established and solved based on the continuous drainage boundary and the corresponding analytical solution was obtained.The solution was verified by degenerating and comparing the analytic solution with the existing research results.the excess pore water pressure and average consolidation degree of the model were analyzed according to the analytical solution.The results showed that: Any status between the entire boundary process from complete pervious to impervious could be realized by changing the value of the interface parameter b.This process compensates for the limitation of the traditional theory which can only express the extreme state of pervious or impervious.The drainage capacity of foundation surface significantly affects the consolidation rate,the stronger the permeability,the faster the consolidation.Excess pore water pressure in the foundation under the continuous drainage boundary condition is not only related with the position and consolidation time,but is also influenced by the interface parameter b.The closer it is to the foundation top surface and the vertical drain center,the faster the excess pore pressure dissipates.The longer the consolidation time,the smaller the excess pore water pressure.In addition,larger value of b causes faster dissipating of the excess pore water pressure.In contrast with the traditional theory under the continuous drainage boundary condition within a certain period,excess pore water pressure exists at the foundation top surface.The excess pore water pressure on the top surface gradually dissipates with increased consolidation time.The accuracy and feasibility of the model in predicting the consolidation rate were verified thorough analysis of example.(2)The consolidation theory of vertical drain for continuous drainage on both sides of the top and bottom was established and solved,and the analytical solution is obtained.The correctness of the analytical solution was verified by the comparison of the degree of consolidation expression and the comparison of the excess pore pressure curves.Through the analysis of the excess pore water pressure and consolidation degree equation under the model,it was found that: under continuous drainage boundary conditions,when the top and bottom interfaces have different interface parameters,which means different drainage capacities,the maximum excess pore water pressure will no longer be at the middle of the foundation while the maximum excess pore water pressure interface moves toward the boundary surface with small interface parameters(weak drainage capacity),which is different from the traditional consolidation theory.When considering the gravity stress of the soil,the excess pore water pressure is higher than that when ignoring the gravity stress of soil mass.If the various water permeability of the top and bottom surface and the soil gravity condition are taken into account,the maximum pore pressure will be more inclined to the position of the foundation bottom surface,which is the superposition effect of additional stress and gravity stress and it provides an accurate reference for the location of the horizontal drainage channel of foundation.The gravity stress of soil has no effect on the calculation of consolidation degree,but when the gravity stress of soil is considered,the settlement of foundation soil under different gravity stress conditions has significant difference,which is the result of the transformation of gravity stress stress into additional stress.By comparing the consolidation curves of single-drainage and doubledrainage conditions,it was found that the water-permeable ability of the boundary surface when double drainage is lower than that of single drainage,the final consolidation rate is still faster than that of single drainage.Therefore,the drainage capacity at the bottom boundary should not be ignored in the actual calculation.For the dredger fill foundations with high construction schedules,laying horizontal drain channels(such as sand cushions)at the bottom before filling can extremely accelerate the consolidation process.(3)The nonlinear consolidation theory of foundation treated with PVD(prefabricated vertical drain)under continuous drainage boundary conditions was deduced,and the corresponding analytical solution was obtained.The correctness of the obtained model was verified by comparing with existing theory.Meanwhile,base on the specific examples,the consolidation curve was drawn,then researched on the influence of different parameters on the consolidation process of the model,and the practicability of the model was verified.By comparing the consolidation degrees of different equivalent methods,it was found that the calculation results of the existing equivalent calculation methods for prefabricated vertical drain are quite different,the perimeter equivalent method is closer to the result of the elliptical area of influence than the area equivalent method.During the compression process of the soil,the void ratio will gradually decrease and the compressive modulus will gradually increase.Therefore,the assumption of linear elasticity of the soil will have a certain error in the calculation of the consolidation settlements of the soil.And compared with the example in Chapter 2,the settlement calculation results of the nonlinear consolidation theory model are similar to the experimental results,which further verifies the accuracy of nonlinear calculation.(4)Based on the finite element software ABAQUS,the secondary development on the consolidation theory of vertical drain model under continuous drainage boundary conditions was carried out.The practicality and the comprehensive advantages of the finite element software were combined with the continuous drainage boundary conditions.The subroutine was used to simulate and analyze the actual project.The practicability of the program was verified by comparing analysis results with the on-site monitoring data,and the program was used to predict the settlement of soft soil subgrade.