| Net-like red soil of red matrix and white vein is widely distributed in Southern China.With the development of China's economy and modernization,to ensure the long-term stability and safety of important projects such as slopes and dams,higher requirements are put forward for the study of rheological properties of net-like red soil,in which the study on stress relaxation characteristics and responding constitutive model of soil is a prominent and urgent problem.In this paper,on the basis of summarizing researches on the stress relaxation characteristics of soil at home and abroad,the composition and percentage content of clay minerals of red matrix and white vein in the net-like red soil in Xuancheng area were analyzed and compared by X-ray diffraction test,which obtained the material basis of mechanical properties of net-like red soil.The triaxial strength tests and stress relaxation tests of net-like red soil were conducted by the GDS unsaturated soil triaxial apparatus to investigate the effects of strain level,matrix suction and net confining pressure on initial stress,stress relaxation,residual stress and residual stress ratio.Moreover,SEM experiments for quantitative and qualitative analysis on the microstructure of samples before and after relaxation process were carried out to analyze the relaxation mechanism of soil from the micro scale.Based on the fractional calculus theory and damage theory,constitutive models cosndiering the nonlinear relaxation characteristics were further established.Comparisons of the calculated results obtained by the proposed models with other models were also conducted to verify the reliability of the models proposed here.The main conclusions were as follows:(1)Results of XRD and EDS tests indicate that illite,kaolinite,chlorite and montmorillonite are the major clay minerals in the red matrix and white vein,but the percentage content is slightly different.Main chemical elements are oxygen,silicon,aluminum,iron and a small amount of potassium.And net-like red soil has a high structure due to the cementation in the soil particles.(2)Triaxial strength test results of unsaturated soil indicated that the shear strength of the specimen under suctions of 50 k Pa and 100 k Pa,increased with the increasing net confining pressure,and the curve of deviator stress axial strain behaved strain hardening feature;the specimen under the suction of 150 k Pa showed strain softening characteristics with the net confining pressures of 55 k Pa and 105 k Pa.The triaxial shear strength of specimens subjected to the same net confining pressure was increased with the incresement of matrix suction.(3)Measured relaxation curves obtained from the unsaturated triaxial stress relaxation tests under stage loading and step loading were of a basically similar pattern belonged to incomplete relaxation.And the volume strain was no longer a constant value under different strain levels,and the changing trend was similar to that stress relaxation under the same strain level,which could reflect the process of internal stress relaxation and weakening with time.Using the empirical mode decomposition theory to analyze the fluctuation of the stress relaxation curve,it was concluded that the change of temperature difference between day and night was the main reason for the fluctuation of the relaxation curve.(4)Changes in the microstructure before and after stress relaxation tests were analyzed qualitatively and quantitatively.The results indicated that the main structure of the net-like red soil was flocculent structure,honeycomb structure and sponge structure.After stress relaxation tests,the contact form between particles transferred from edge and edge,edge and face to point and face,face and face;the change of pores followed the principle of pore homogenization,which the decrease of large pores was more,the decrease of small pores was less,and the distribution of pores was more and more uniform.And the particle abundance from about 0.50 of the initial sample gradually decreased,while the pore abundance gradually increased to 0.50,which also showed that the particles pattern after relaxation tests changed from "equiaxed" to "long strip",and the pattern of pores changed from "long strip" to "equiaxed".Compared with that of the initial sample,the complexities of particles at the strain level of 4.0% increased by about 50%,and the variation tendency of the complexities of pores was opposite;the orientation probability entropy and orientation fractal dimension of particles and pores both decreased,and the variation range was within-0.5%?0.And the orientation probability radar map of the particles and the pores both showed obvious advantage interval,which indicated that the orientation of the net-like red soil was enhanced by stress relaxation tests.The three-dimensional model of bulk porosity was improved to analyze the change tendency of volume porosity after stress relaxation tests,and the evolution equation of damage under various strain levels was established by combining the expression of relaxation damage variable and measured data.(5)Based on the fractional calculus theory and soil rheology theory,FVMS model and FVMP model that can reflect the nonlinear relaxation characteristics of net-like red soil under different strain levels were established,and the sensitivity of fractional order was also discussed;the generalized fractional Maxwell model under different net confining pressure relaxation test conditions was built in the paper,and the applicability and accuracy of generalized fractional order Maxwell relaxation model of two,four and six units were analyzed;the variable order fractional order damage relaxation model under different suction conditions was put forward.Combined with the measured data,the results that compared with other models indicated that the proposed models in the paper can better describe the non-linear characteristics of stress relaxation with higher accuracy,which provided an indispensable theoretical basis for the long-term stability analysis of the important engineering of net-like red soil. |
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