Triaxial Test Of Unsaturated Soil Mixed With Polypropylene Fiber And Meso-Characteristi

With the rapid improvement of China’s economic level,the high-speed railway network has developed rapidly.By 2030,it is planned to build a "eight vertical and eight horizontal" high-speed railway network in China in the new era.There are a lot of soft clay in the eastern coastal cities of China.When building high-speed railway subgrade in soft soil area,soft clay needs to be improved before it can be used as filler.Coarse-grained soil filler can give more powerful support to the foundation,while fiber material can improve the shear strength and tensile strength of soil.It is a common soil improvement material.Therefore,the mechanical properties of soft clay are enhanced by adding coarse particles such as sand and gravel and using fiber as reinforcement material.In this paper,the variation law of mechanical strength of unsaturated soil is studied by adding polypropylene fiber with different content.It provides reference for practical engineering design and stability analysis.The main research contents of this paper are as follows:(1)Through compaction test and screening test,some parameters of coarse-grained soil filler are measured,such as optimal water content,maximum dry density and so on.According to the standard of highway geotechnical test,the scale gradation is determined by equivalent substitution method.(2)Indoor consolidation and drainage triaxial tests are used to study the effects of different polypropylene fiber content,different net confining pressure and different matrix suction on the mechanical properties of coarse-grained soil.The main research indexes are stress-strain curve,internal friction angle and cohesion of shear strength index.The test results show that the net confining pressure is constant,the fiber content is constant,and the greater the matrix suction,the greater the growth rate of eccentric stress.When the confining pressure is constant and the matrix suction is constant,the eccentric stress axial strain curve of soil particles is obviously higher than that without fiber.The more fiber is added,the higher the eccentric stress curve and the greater the shear strength.The results of shear strength index show that with the increase of matrix suction,the internal friction angle and cohesion increase,especially the increase of cohesion is obvious.(3)Combining triaxial test with CT test,industrial CT equipment was used to scan polypropylene fiber soil with different content before and after shearing,and the microscopic characteristic parameters of polypropylene fiber coarse-grained soil were studied by CT scanning technology and image processing technology,and the effect of polypropylene fiber on strengthening the deformation resistance of coarse-grained soil was discussed from the microscopic point of view.The three-dimensional reconstruction model of coarse-grained soil is built by Avizo software,and the three-dimensional void structure is constructed.Through the calculation of porosity,the strength changes of coarse-grained soil before and after shearing under the action of different content of polypropylene fiber are analyzed.The experimental results show that the porosity of polypropylene fiber with different content decreases with the increase of fiber material before and after shearing,which shows that the addition of fiber material improves the deformation resistance and strength of soil.Comparing the polypropylene fiber with the same content before and after shearing,it is found that the porosity is increasing.This result shows that cracks may occur in the sample after shearing compression,and these cracks and holes are connected together to cause the porosity to increase,and then the porosity is increasing.With the increase of fiber materials before and after shearing,the void volume decreases,which shows that the internal skeleton of soil particles is more dense with the addition of fiber materials than without any materials,and the shear strength of soil samples is improved.