Model For Large Strain Consolidation With Non-darcian Flow

Since Terzaghi developed the basic theory for soil mechanics,the consolidation theories have made great progress in terms of analytical analysis numerical simulations,experimental verifications,and other aspects.Most of these methods were based on the assumption that the water flow in the soils obeyed Darcy’s law;that was,regardless of the magnitude of the hydraulic gradient,the seepage velocity was always linear with the hydraulic gradient.However,Darcy’s flow law was initially established for coarse-grained soils.Recently,the inaccuracy of Darcy’s law in fine-grained soil has been confirmed by many studies.This phenomenon is known as non-Darcian flow.Therefore,it is of theoretical and practical significance to study one-dimensional large strain consolidation based on a non-Darcy flow law.Based on the widely-recognized CS2 model,several piecewise-linear models considering three different types of non-Darcian flow are proposed in this thesis,called CNDF1 model,CNDF2 model and CNDF3 model.The new models inherit all the merits of the CS2 model,accounting for vertical strain,general constitutive relationships,the relative velocity of the fluid and solid phases,variable compressibility and permeability relation during consolidation,time-dependent loading and unload/reload effects.The verification examples of the new models demonstrate excellent accuracy for both small strain and large strain consolidation.Compared with traditional numerical methods,the new models are more computationally efficient because they do not rely on any simplified assumptions or derivative calculation.In this thesis,the development of the new models is first presented,followed by several examples to illustrate the effects of the non-Darcian flow on the consolidation behavior.It can be found that:（1）For the small strain consolidation considering exponential flow,the consolidation rate will improve with the increase of n at the beginning of the consolidation,while the consolidation rate will decrease with the increase of n at the later of the consolidation.The reason for this phenomenon is that the discharge velocity will decrease with increasing the value of n when the hydraulic gradient is less than 1.However,this phenomenon is not so obvious in the large strain consolidation.It is mainly manifested when the average degree of consolidation is extremely close to 100%.（2）The consolidation rate based on the Hansbo flow is slower than that based on Darcy flow.Both a larger value of m and a larger value of i₁ can make this phenomenon more obvious.The increase of the value of m and i₁ will slow down the dissipation rate of excess pore pressure.In addition,in the consolidation theory based on the Hansbo flow,soil self-weight also has significant influence on the settlement of the layer,but it has a relatively small effect on the consolidation rate.（3）For the consolidation with threshold gradient,the value of i₀ has a great influence on the final settlement of soil layer.A larger value of i₀ will result in a larger value of residual excess pore pressure and a smaller value of final settlement.The bottom drainage boundary will contribute to the dissipation of excess pore water pressure.The final residual excess pore pressure under single-drained condition will increase along the soil layer,while the final residual excess pore pressure under double-drained condition is symmetrically distributed and reaches its maximum value at the center of the soil layer.When considering the threshold gradient,the final void ratio under single-drained condition is still linearly distributed,which is similar to the results considering Darcy’s law.However,the distribution of final void ratio under double-drained condition is totally different from the results considering Darcy’s law.（4）The consolidation rate under double-drained condition is much faster than that under single-drained condition.（5）The non-Darcian flow has a similar effect on the unload/reload process to the Darcy flow.