A New Consolidation Theory Of Variable-load Cohesive Soils Considering The Variations Of Void Ratio And Permeability Coefficient

In the design and construction of buildings,estimating the final settlement of the foundation and understanding the time required for the foundation to reach a certain settlement or estimating the possible settlement at a certain time after the completion of buildings is of great significance to the safety and the reliability of engineering construction.Regarding the relationship between settlement and time,it is currently based on Terzaghi’s one-dimensional consolidatin theory of saturated soil.However,it is well known that the void ratio and permeability coefficient of saturated cohesive soil are constantly changing during consolidation process.In Terzaghi’s theory,these variations are ignored to obtain an easy-to-solve linear governing equation.Therefore,how to consider the various complex conditions of soil more reasonably in the consolidation process is a crucial issue.In addition,the soil layer has different consolidation states because of the natural soil layer has gone through a long process during the formation process.The foundation soil may be in a normal consolidated state of in an over-consolidated state.In order to study the influence of the actual consolidation state on the consolidation process of the soil,the consolidation history of the soil must be studied.Because of the obvious difference in compressibility and permeability between normal consolidated soil and over-consolidated soil,it is necessary to take into account the consolidation state of cohesive soil and the changes in void ratio and permeability coefficient during soil consolidation into consolidation theory.Taking Xi’an and Luochuan saturated silty cohesive loess as well as saturated artificial clay as the reseach objects,a new one-dimensional consolidation theory of saturated clay considering the changes of void ratio and permeability in the process of soil consolidation is established based on Terzaghi’s one-dimensional consolidation theory.On this basis,indoor experimental methods and numerical methods are used to verify the correctness and effectiveness of the proposed new theory.It is expected to provide a scientific basis for establishing the theory of saturated clay consolidation,revealing the law of saturated clay consolidation and permeability,and exploring the prevention and control methods of saturated clay foundation instability and settlement.The main research contents and results are as follows:(1)The influence of bound water on permeability characteristics of cohesive soil.Indoor permeability tests and thermogravimetric analysis tests were carried out on cohesive soils in three different regions.According to the results of indoor tests,the permeability coefficients of the clays in three different regions and the relevant parameters for calculating the permeability coefficients were determined.At the same time,the proportional coefficient α0 of the bound water in the cohesive soil to the liquid limit is determined.Then the effective void ratios of the cohesive soils in the three regions is calculated according to α0.Substituting the effective void ratio into the formula of permeability coefficient of coarse-grained soil,the formula of permeability coefficient suitable for cohesive soil is obtained.Finally,a comparative analysis based on the calculated permeability coefficient value and the actual measured value shows that:the permeability coefficient value calculated by the modified permeability coefficient formula suitable for cohesive soil is relatively close to the actual measured value,which verifies the correctness and applicability of the proposed formula.It also lays a theoretical foundation for the construction of considering the change of permeability coefficient during soil consolidation.(2)One-dimensional consolidation permeability joint experimental study of saturated cohesive soil.A series of one-dimensional consolidation and permeability tests were carried out on Xi’an and Luochuan saturated silty cohesive loess as well as saturated artificial clay using the improved GDS advanced consolidation device.In order to study the effect of pre-consolidation stress on the consolidation and permeability of saturated cohesive soils,multi-component-level loading consolidation tests and multiple sets of permeability tests with constant consolidation stresses were carried out on saturated cohesive soils under different pre-consolidation stresses.Based on the test results,a detailed analysis of the one-dimensional consolidation and permeability laws of Xi’an and Luochuan saturated silty cohesive loess as well as saturated artificial clay in the normal consolidation stage and over-consolidation stage.On this basis,the empirical formula of permeability coefficient for cohesive soil established in the previous chapter is used to fit and analyze the results of the permeability test,which is shows that the modified Kozeny-Carman permeability coefficient formula is suitable for predicting the permeability coefficient of saturated cohesive soil.It also lays a theoretical foundation for the construction of considering the change of void ratio and permeability coefficient in the process of soil consolidation.(3)A new consolidation theory for variable-load cohesive soil considering the consolidation state of the soil and the variation of void ratio and permeability coefficient.On the basis of modifying the assumptions of Terzaghi’s one-dimensional consolidation theory,a onedimensional consolidation theory of variable-load cohesive soils that considers the consolidation state of the soil and the changes in void ratio and permeability coefficient during the consolidation process is proposed,and the corresponding consolidation equation is established.(4)Research on the method of using the finite difference method(FDM)to solve the variable load consolidation equation considering the consolidation state of the soil and the variation of void ratio and permeability coefficient.Using the calculation steps of the finite difference method,the finite difference equations of the one-dimensional consolidation governing equations of normal consolidated soil,over-consolidated soil and under-consolidated soil are established.Then,the finite difference equation considering the changes of void ratio,permeability and external load in the consolidation process is solved by MATLAB programming,and the onedimensional consolidation finite difference solution of pore water pressure,foundation settlement and foundation consolidation degree in the one-dimensional consolidation process is obtained.Finally,the obtained finite difference results and the one-dimensional consolidation-permeability joint test results are compared and analyzed to verify the correctness of the consolidation theory established in this paper and the reliability of the finite difference method calculation program.(5)The semi-analytical method is used to solve the variable load consolidation equation considering the consolidation state of the soil and the change of void ratio and permeability coefficient.Introducing the idea of layered foundation,the variable load consolidation equation established in this paper considering the soil consolidation state,porosity ratio and permeability change is solved by semi-analytical method.The semi-analytical solutions of the pore water pressure,the foundation settlement and the degree of foundation consolidation in the overconsolidated soil and under-consolidated soil during the one-dimensional consolidation process were calculated by MATLAB programming.Then the obtained semi-analytic solutions and the calculation results of the existing analytical solutions were compared and analyzed.The analysis found that the calculation results obtained by the semi-analytic method were quite consistent with the analytical solutions,verifying that the analysis methods and calculation procedures in this paper are correct and reliable.In addition,comparing the calculation results of the finite difference method and t of the semi-analytical method,it can be seen that the results obtained by either of these two methods are reliable.(6)Analysis and study of one-dimensional consolidation behavior of variable-load saturated cohesive soils considering soil consolidation state,changes in void ratio and permeability coefficient.After analyzing the reliability of the two solution methods,the one-dimensional consolidation behavior of saturated clay is analyzed in detail by using the calculation program of finite difference method.On this basis,a large number of consolidation curves were drawn,and the effects of void ratio,final load,stress history,pre-consolidation pressure,external load loading rate,and the ratio of expansion index to compression index Ce/Cc on one-dimensional consolidation of normal and over-consolidated soils were discussed.The research shows that:the void ratio,the final load and the external load loading rate have a great influence on the pore water pressure dissipation rate,the foundation settlement and the degree of consolidation of the normal consolidated soil during the consolidation process.In addition,the pre-consolidation pressure,external load loading rate and the ratio of expansion index to compression index Ce/Cc both have a great influence on the consolidation properties of over-consolidated soils.If the influence of these factors on the consolidation properties is not considered,the analysis of the consolidation process(settlement,consolidation degree,and consolidation rate,etc.)of the cohesive soil will be biased.(7)Engineering case analysis.In order to verify the applicability of the proposed theory in engineering practice,a comparative analysis was carried out combining the measured value of the subgrade settlement of the Changzhou section of the Changning Expressway and the theoretical calculation of the subgrade settlement.In addition,the roadbed settlement calculated by Terzaghi theory is compared with the actual measured value and the theoretical calculation result of this paper.Compared with the theoretical calculation result of Terzaghi,the calculated roadbed settlement calculated by this paper is more consistent with the actual measured value.