Mechanical Properties And Damage Characteristics Of Basalt Fiber Reinforced Cemented Silty Sand In Seasonally Frozen Zone

Seasonally frozen zone was widely distributed in China,accounting for more than half of the country's territory.Subgrade would inevitably experience freeze-thaw cycles in seasonally frozen zone.Freeze-thaw cycles would result in road diseases,which seriously affected the service life of roads,increased the maintenance costs in later stages and greatly threated the driving safety of drivers.The essence of road diseases in seasonally frozen zone was that the mechanical properties of the subgrade soil was attenuated under the long-term service state and the influence of the external environment.Effective improvement of the subgrade soil to obtain the subgrade materials with stable and excellent performance was the main measure to prevent subgrade diseases.It was of great significance to the construction of road engineering and the prevention and control of subgrade diseases in seasonally frozen zones.Basalt fiber was a new type of high-performance inorganic green environmentally friendly material produced with excellent characteristics,such as high tensile strength,good chemical stability,high temperature resistance,acid and alkali corrosion resistance,etc.With the development of basalt fiber manufacturing industry in Jilin province was in full swing.The promotion and application of basalt fiber was not only beneficial to the economic development of our province,but also to obtain an environmentally friendly subgrade material.As common subgrade materials in the northwest of Jilin Province,silty sand had low strength and poor freeze-thaw resistance.Cement was the most widely used for its improvement.but the tensile strength and flexural strength of cement stabilized silty soil was low,and road diseases such as subgrade deformation and road rutting were easily generated under freeze-thaw cycles.In view of the special environment of the seasonally frozen zone,this paper used basalt fiber to reinforce the cemented silty sand for subgrade.The freeze-thaw damage properties and strength attenuation law of silty sand,fiber reinforced silty sand,cemented silty sand and fiber reinforced cemented silty soil were compared and analyzed,and the toughening mechanism of fibers was revealed through the laboratory test,acoustic emission test and scanning electron microscope test.The main work to be carried out were as follows:(1)The effects of fiber content,fiber length and curing time on the unconfined compressive strength(UCS)and splitting tensile strength(STS)of fiber reinforced cemented silty sand were studied based on the UCS test and STS test.The optimal ratio of fiber reinforced cement silty sand was determined as fiber content of 0.2%,fiber length of 12 mm,and curing time of 14 days.The research showed that with the increase of fiber length,the UCS of fiber reinforced cement silty sand decreased,while the STS increased.With the increase of curing time,the UCS and STS were both increased.There was a good linear relationship between UCS and STS.The prediction models of UCS and STS of fiber reinforced silty sand and fiber reinforced cemented silty sand soil based on the test data provided a quantitative basis for the estimation of the UCS and STS of subgrade material.(2)Based on the UCS test,STS test,the unconsolidated and undrained triaxial(UU)test and the California bearing ratio(CBR)test,the road performance and mechanical damage properties of the improved soil with the optimal mix ratio under freeze-thaw cycles were systematically studied.The results indicated that the CBR value of fiber reinforced cemented silty sand was the largest and the expansion was the smallest.The CBR value of the improved soil at 100% compaction was greater than that at 96% compaction,and the changing laws of water absorption and expansion were the opposite.The increase of mechanical properties of fiber reinforced cemented silty sand increased the most under freeze-thaw cycles compared with the improvement of single fiber or single cement.The research results of the damage process of reinforced cemented silty sand based on the Logistic model demonstrated that the freeze-thaw damage degree of cohesion was the largest,and the freeze-thaw damage rate of static elastic modulus was the fastest.(3)The effects of different factors on the cumulative ringing count and cumulative energy,energy,amplitude and RA and AF values of fiber reinforced cemented silty sand under uniaxial compression were studied by Acoustic Emission(AE)technology,revealed the reinforced mechanism of fibers.Based on the AE characteristic parameters,the damage stages of fiber reinforced cemented silty sand were divided.The results manifested that the incorporation of fibers could effectively reduce the growth rate of micro-cracks,help to alleviate the brittle failure of cemented silty sand,and enhance the ductility of cemented silty sand.With the increase of fiber content and fiber length,the damage formation time of improved soil was delayed,the critical failure load increased and both reached the maximum at the optimal mix ratio.With the increase of curing time,the AE energy signal and amplitude signal intensity weakened.The freeze-thaw cycle caused local damage to occur in fiber reinforced silty sand in advance.With the increase of freeze-thaw cycles,the AE energy signal and the AE amplitude intensity of the critical failure point increase.The amplitude of the critical failure point of 65 d B after 6 freeze-thaw cycles could be used as the damage warning value.(4)The effects of confining pressure and freeze-thaw cycles on the dynamic stress-strain relationship curve,dynamic elastic modulus and cumulative plastic deformation of the improved soil were studied by using hierarchical loading dynamic triaxial test.The dynamic stress-strain curves of the improved soil were all in line with the Hardin-Drnevich hyperbolic model.The maximum dynamic elastic modulus and the final dynamic stress amplitude were used as model parameters to verify the application of the Hardin-Drnevich hyperbolic model.Based on the nonlinear least squares method,prediction models were proposed to estimate the normalized maximum dynamic elastic modulus,final dynamic stress amplitude and cumulative plastic deformation of the improved soil effected by different factors,which provided a quantitative basis for the dynamic response analysis of the improved soil.(5)Scanning electron microscope(SEM)was used to obtain the microscopic images of the improved soil under freeze-thaw cycles.The images were denoised,binarized and morphologically processed by MATLAB.The characteristic parameters of particles and pores were extracted by Image-Pro Plus(IPP),and quantitatively analyzed the extracted microscopic data.At the same time,the microscopic images were qualitatively analyzed.The interaction mechanism of fiber and cement was clarified.Subsequently,the influence of microcharacteristic parameters on macro-mechanical properties was analyzed by grey relational theory,and revealed the microscopic mechanism of freeze-thaw loss of macroscopic mechanical properties.