Study On Microstructure Characteristics And Mechanical Properties Of Remolded Clay With Long-term High-pressure K₀ Consolidation

The biggest difficulty of exploitation of mineral resources and underground space under deep topsoil in Mid-Eastern China is the construction and long-term safety maintenance of deep shaft.The safety and stability of shaft in deep topsoil fundamentally depends on the interaction between shaft lining and surrounding soil,which is directly determined by the in-situ mechanical properties of deep soil.Deep soil was deposited in high-pressure K₀ consolidation for a long process,and its mechanical properties are significantly different from shallow soil reported by scientific research and engineering practice.Therefore,the mechanical parameters of remolded soil obtained by conventional geotechnical test methods are impossible to accurately reflect the mechanical properties of deep soil,and are disqualified in design and safety analysis of deep shaft lining.Considering above,this thesis taken remolded deep clay as the research object,and aimed at the improvement of the test method as well as the evolution of micro-structure and macro-mechanical properties with consolidation stress and time.The research based on the theory and test method of the accelerated consolidation by"mechanical loading-electroosmosis",carried out the experiments of remolded clay by the long-term high-pressure K₀ consolidation,and studied micro-structure and macro-mechanical characteristics of the consolidated clay.The main research contents and conclusions of this thesis are as follows:Firstly,based on the existing one-dimensional linear electroosmosis consolidation theory and equation,considering the nonlinearity of hydraulic permeability coefficient and electroosmosis coefficient,the one-dimensional nonlinear electroosmosis consolidation control equation considering the nonlinear variation of two permeability coefficients was established by introducing compression index and two permeability indexes.Then,the theoretical study on the law of clay electroosmotic consolidation was carried out by numerical method.It is found that increasing the electroosmotic voltage can effectively accelerate the dissipation of pore water pressure,but it is easy to induce the asymmetric distribution of pore water in soil samples and reduce the homogeneity of soil samples.When the sample is in stable consolidation,exchanging the electrode at rational time can symmetrize the distribution of the pore water pressure and efficiently improve the homogeneity of soil samples.The feasibility of accelerating the consolidation of clay samples by electroosmosis was confirmed theoretically.Secondly,a"mechanical loading-electroosmosis"accelerated consolidation test device suitable for high-pressure K₀ consolidation of remolded clay was developed.A series of high-pressure K₀ consolidation tests of remolded clay were carried out by using the device,and the electroosmotic accelerated consolidation test parameters and methods for efficiently preparing remolded clay samples with stable consolidation and high homogeneity at 2,4 and 6 Mpa were obtained.Compared with the moisture content of different parts of clay samples under single mechanical loading consolidation test,the effectiveness and rationality of electroosmosis accelerated consolidation test method and parameters were verified.Then,the high pressure K₀ consolidation test of remolded deep clay was carried out by using the"mechanical loading-electroosmosis"consolidation test device.For the high-pressure K₀ consolidated remolded clay samples with different consolidation stress and consolidation time,the bound water and its microstructure in the soil samples were tested by means of NMR,nitrogen adsorption and SEM scanning.The variation of microstructure parameters such as bound water content,bound water film thickness,moisture content variation coefficient C.V_w,specific surface area of the soil samples,pore orientation probability entropy with consolidation time and stress were obtained.The comprehensive analysis shows that the variation coefficient of water content C.V_wcan comprehensively characterize the spatial distr_<sub>ibution uniformity of bound water and pores,the average bound water film thickness hcan comprehensively characterize the content of bound water and the complexity of pore structure,and they can comprehensively reflect the microstructure characteristics of long-term high-pressure K₀ consolidated clay samples.Finally,triaxial shear tests for long-term high-pressure K₀ consolidated remolded clay samples were carried out under two stress paths of"constant axial pressure unloading confining pressure"and"constant confining pressure increasing axial pressure".The variation laws of macro-mechanical parameters such as shearing strength,secant modulus,cohesion and internal friction angle with consolidation stress and time were obtained.Combined with the micro-structure test results,a comprehensive analysis was carried out.It shows_<sub>that the above mechanical parameters of clay are negatively correl_<sub>ated with C.V_w and h,while the failure strain is positively correlated with C.V_w and_<sub>h.The shearing strength,failure strain and cohesion are significantly affected by h,while the internal friction angle is more significantly affected by C.V_w.The analysis of the variation mechanism of mechanical parameters demonstrates that under the same consolidation pressure,with the extension of consolidation time,the thickness of bound water film decreases and the distribution uniformity increases,resulting in the increase of shearing strength,secant modulus,cohesion and internal friction angle,and the decrease of failure strain.In the same consolidation time,with the increase of consolidation stress,the degree of consolidation increases,but the degree of uniformity and the combined water film thickness decreases.The shearing strength,failure strain and cohesion are more significantly affected by the combined water film thickness,the internal friction angle is more significantly affected by the uniformity of the sample,resulting in the shearing strength and cohesion increase as well as the internal friction angle and failure strain decrease with the increase of consolidation stress.The"mechanical loading-electroosmosis"high-pressure K₀ consolidation test method and parameters of deep remolded clay proposed in this thesis as well as the research results of micro-structure and mechanical properties of high-pressure K₀consolidated clay samples provide an important reference for further improving the mechanical test method of deep soil and revealing the differences between deep and shallow soil mechanical properties.In this thesis,there are 114 figures,32 tables and 144 references.