Studies On The Theory Of One-dimensional Nonlinear Large Strain Consolidation Of Soft Clay

Due to the shortage of land resources and abundant dredged silt reserving in the southeast coastal areas of China,Dredging silt for land reclamation is one of the effective methods to solve the shortage of land in coastal cities.However,the formed dredger fill are characterized by high compressibility,high strain and low strength.In engineering design,problems such as instability of subgrade and uneven settlement of foundation may occur if the consolidation theory under the assumption of small strain is still used in consolidation calculation.At the same time,the compression and permeability characteristics of soft soils are typically nonlinear.If the nonlinear variation of soft soils are not fully considered,the calculation results will inevitably lead to large deviations.Unfortunately,there are few reports on the analytical solution of large strain consolidation which can fully consider the nonlinear variation of compression and permeability of soil.Therefore,it is of great theoretical value and engineering significance to study the theory of one-dimensional nonlinear large strain consolidation of soft soil.In this paper,it is assumed that the seepage of pore water obeys Darcy’s fiow law,and a one-dimensional nonlinear large strain consolidation model for soft soil is developed by considering the geometrical nonlinearity,the non-linear compression and permeability characteristics and the load effect with time.The analytical solution of the consolidation model is given by variable substituting.Then the reliability of the analytical solution is verified by the degradation of the solution and the comparison with the numerical solution and the laboratory test results.The main research contents are as follows:(1)Based on a large number of test data of permeability and permeability of soft clay,the adaptability analysis of the non-linear relationship between effective stress versus void ratio and permeability coefficient versus void ratio of soft clay under different strain conditions is carried out,and the advantages and disadvantages of several common compression models and permeability models in describing the non-linear compression and permeability characteristics of soft soil are compared.Then,the compression and permeability curves are fitted in semi-logarithmic coordinate system and bi-logarithmic coordinate system respectively,and the results show that the linear relationships in the bilogarithmic coordinate system is more applicable than that in the semi-logarithmic coordinate system.The variation ranges of parameter Ic in the bilogarithmic compression model and parameter a in the bilogarithmic permeability model are obtained on the basis of fully summarizing the test data.Finally,the conversion relationship between Ic,α and indexes of the classical semi-logarithmic model for compressibility and permeability is established.Based on the data analysis of several groups of combined tests of consolidation and permeability,the variation ranges of Icα,Ic(α-1)and Ic(α-2)are also studied,respectively.(2)Based on the lg(1+e)-lgσ’,and lg(1+e)-lg/kv relationships,a one-dimensional large strain non-linear consolidation model of high compressible soft soil under instantaneous loading is established by combining the variation range of parameters of the lg(1+e)-lgσ’and lg(1+e)-lgkv bilogarithmic model.The analytical solutions of the consolidation model under three cases of Ic(α-2)=1,Ic(α-2)≠1 and Ic(α-1)≠1,and Ic(α-1)=1 are given by variable substituting.The corresponding calculation program is compiled according to the analytical solution,and the theoretical settlement curve is compared with that of laboratory test and finite difference solution to verify the reliability of the analytical solution.Finally,the influence of the large and small strain geometric assumptions on the consolidation rate of soils are analyzed in terms of excess pore pressure,settlement and average consolidation degree,and the conditions applying the geometric assumption of large strain are given(3)Based on the bi-logarithmic linear relationship of lg(1+e)-lgσ’ and lg(1+e)-lgkv,a one-dimensional nonlinear large strain consolidation model of high compressible soft soil under a ramp loading is established,and the analytical solutions of the consolidation model under three conditions are obtained by variable substituting.According to the analytical solution,the corresponding calculation program is compiled,and the reliability of the analytical solution is verified by comparing settlement curve between the results calculated by theory and that measured by test.Then the reliability of the analytical solution is verified again by degenerating the analytical solution of the consolidation model under a ramp loading into that under instantaneous loading.Finally,an example is used to analyze the influences of soil parameter,final external load and loading rate on consolidation behavior.