Soil-water Coupled Elasto-plastic Unified Numerical Analysis On Bearing Capacity And Settlement Of Soft Ground

The bearing capacity and settlement of soft soil foundation have always been traditional and still important research topics in geotechnical engineering.The development of numerical methods that can uniformly evaluate the bearing capacity and settlement characteristics of natural soft soils and soft soil composite foundations under different loading and drainage conditions is of great significance for improving the prediction level of soft soil foundation failure and deformation.Although there are many research results on these issues,there is still a lack of methods that can uniformly evaluate these two issues:bearing capacity and settlement.The numerical research on the bearing capacity of structured soft soil foundation,the settlement of sand compaction piles composite soft soil layer and the long-term settlement of soft soil layer after earthquake are conducted based on the unified elastoplastic constitutive model(Unified model)coupled with soil-water finite element algorithm.The advantage of this constitutive model is that the overconsolidation,structure,and anisotropy of the soil can be considered,and the single set of material parameters can be used to describe the mechanical properties of the soil under different loading and drainage conditions.In the course of the research,the results of laboratory triaxial tests and consolidation tests are used to determine the material parameters and initial state parameters of the materials.Then the reliability and accuracy of the numerical methods are verified by comparing field model tests,centrifugal model tests,and field measured data.When analyzing the calculation results,the effective stress path,stress-strain relationship and structural evolution of soft soil are analyzed in order to reveal the influence of different loading and drainage conditions on the strength and deformation behaviour of the soft soil.The main research contents and results of this research are as follows:(1)Analyze the influence of consolidation conditions on the bearing capacity of shallow foundations on natural soft soil,and reveal the mechanical mechanism that the bearing capacity increases with consolidation.The triaxial test data of high-quality undisturbed soil samples were used to determine the material parameters and initial state parameters for the unified constitutive model.Then,a field model test is used as the research object to establish a finite element model to carry out soil–water coupled numerical analysis under unconsolidated and undrained(UU)and consolidated undrained(CU)conditions.The initial conditions as well as material parameters for the two conditions of UU and CU are the same.In the UU condition continuous load is applied to instability,while for the CU condition the foundation is left for 540 days after the first stage of loading and then loaded to instability.The calculated bearing capacity and settlement of the foundation are in good agreement with the field measurements,which verifies the correctness of the numerical method.The soil-water coupled finite element algorithm based on the unified model reveals the increase in the strength of the foundation caused by consolidation,which in turn leads to an increase in bearing capacity.By analyzing the stress-strain relationship,stress path,and soil structure degradation of the soft soil at different depths,the mechanical responses of the soft soil under UU and CU conditions are obtained.The propagation of structural decay zones also reveals the possible instability mechanism of soft soil foundation bearing capacity.Then the influence of the initial structure of soft soil on the bearing capacity of the foundation is further discussed.Adopting a unified model based soil-water coupling numerical approach can promote in understanding the bearing capacity changes of shallow foundations on structured soft clay strata under short-term / long-term loads,and provide a useful reference for design and construction.(2)Analysis of settlement differences between floating and fixed sand compaction piles(SCPs)in deep soft soil layers is performed with additional concept of floating and fixed sand piles combination.Based on the results of numerical analysis,a simple prediction method for the corresponding settlement is proposed.Initially,based on the triaxial test results of clay and sand,the unified constitutive model is employed to simulate the mechanical properties of these two different soils,and the relevant material parameters are determined.Then a finite element model of SCPs composite ground is established to carry out numerical analysis for the centrifugal test of floating and fixed SCPs composite ground.Numerical simulation results show that the unified model based soil-water coupling numerical method can well reproduce the variation of the settlement and excess pore water pressure of the both composite ground under different stages of loadings and consolidations.In addition alternate floating combinations,and floating and fixed combination SCPs are also studied at varying displacement area ratios and lengths of floating SCPs.For an alternate fixed and floating SCPs combination analyzed results in lower settlement compared to an alternate floating SCPs combination under embankment loading and long-term consolidation at the area ratio of 31%.Settlements are evaluated at different area ratios and different combinations.Furthermore,the settlement reduction factors proposed are evaluated for ease in computing the settlement for different cases.This research results help to understand the SCPs behaviour so that it can be used for ground improvement in soft clay,contributing to the economical design of SCPs improved soft clay ground under various loading and consolidation conditions in a pragmatic engineering approach.(3)Based on the unified elastoplastic constitutive model,combined with the soil-water coupled dynamic finite element algorithm,the 2007 Niigata earthquake in Japan causing ground subsidence even after the earthquake is used as the background to analyze the longterm settlement of soft soil foundations after earthquake loading.The structured soft soil parameters and initial state parameters used in the calculation are obtained by simulating and tracing the laboratory test results.The dynamic calculation uses the Newmark-? implicit algorithm where the seismic wave is incident from the bottom of the model.The ground acceleration response obtained from the numerical calculation is consistent with the K-Net record.The calculated value of the ground settlement is the same as the measured value,which shows that the settlement caused by the earthquake lasts about 3 years.The mechanical characteristics of the soft soil under seismic loading,including the variation in the excess pore water pressure and the changes in soil shear stiffness along the depth are well reproduced.The above research results show that the soil-water coupled finite element algorithm based on unified elastoplastic constitutive model can better evaluate the bearing capacity and settlement of soft soil layers,including different load conditions(static and dynamic loads),different stratum(structured clay and sandy soil)and different drainage conditions.The accuracy and applicability of the proposed method have been verified by the results of laboratory tests,field model tests,centrifugal model tests and field measured values.The research results will promote the continuous improvement and development of numerical calculations in the field of geotechnical engineering.