Study On The Mechanical Properties Of Fiber And Cement Reinforced Soil Under Freeze-thaw Cycles

In seasonal freezing soil regions,subgrade fillings exposed to natural environment experienced freeze-thaw cycle at least once a year.Under a large number of freeze-thaw cycles,the mechanical properties of subgrade fillings will change,and the subgrade will suffer from uneven settlement,mud boiling and strength reduction,which seriously affects the running safety of railways and highways.Therefore,it is of great significance to find a kind of improved material with good mechanical properties under freeze-thaw cycles.With the aim of selecting a single subgrade filling material with most significant freeze-thaw durability on the basis of traditional cement improvement methods,a composite improvement method of adding basalt fiber to cemented soil is proposed.In order to obtain the best ratio of basalt fiber content,the stress-strain relationship,failure strength,shear strength properties,dynamic shear modulus,damping ratio and other physical properties of cemented soil samples reinforced with different basalt fiber contents were compared by a series of freeze-thaw cycle tests,unconsolidated undrained(UU)triaxial compression tests and Scanning Electron Microscope tests.The research shows as follows:(1)In the static triaxial test,the results showed that both the shear strength properties and failure strength of cemented soil reinforced with basalt fiber increased comparing with the samples without fiber.Under the freeze-thaw cycles,the reduction rate of mechanical properties of fiber-cemented soil was less than that of cemented soil.The cohesion decreased gradually with an increase in the number of freeze-thaw cycles,while the friction angle decreased initially and then increased.(2)In the Scanning Electron Microscope(SEM)test,it could be seen from SEM pictures of the air-dried samples that the cemented soil particles and fibers showed a close bonding state.The mutual encapsulation of cemented soil particles and fibers reduced the freeze-thaw cyclic deterioration effect,and greatly improved its cohesion and shear strength.(3)In the dynamic triaxial test,the dynamic strength of cement soil could be significantly improved by adding basalt fiber.Under different test conditions,the damping ratio of the reinforced soil increased with the increase of dynamic shear strain,while the dynamic elastic modulus decreased with the increase of dynamic shear strain.The damping ratio and dynamic elastic modulus of cemented soil would increase due to the addition of fiber;with the increase of confining pressure,the dynamic elastic modulus increased gradually,but the effect of confining pressure on the damping ratio was not obvious;with the increase of freeze-thaw cycles,the damping ratio and dynamic elastic modulus of samples decreased significantly.(4)Based on the dynamic triaxial test results of reinforced soil,the dynamic stress-strain relationship and dynamic shear modulus of dynamic triaxial test were fitted by Davidenkov dynamic constitutive model.In summary,the basalt fiber cemented soil has higher road potential.Compared with the reinforcement effects of basalt fiber at 0.25%and 0.75%,fiber-reinforced cemented soil with fiber content of 0.5%demonstrated the highest strength and performed well in minimizing the effect of freeze-thaw cycles.It is therefore recommended that ratio of 6%cement and 0.5%basalt fiber should be used to enhance the integrity of subgrade filling materials on silty clay in seasonal freezing soil regions.