Research On Silt Subgradw Improvement Performance Based On Recycled Polyester Fiber

In the construction of traffic engineering in the southeast coastal area of China,there are usually technical difficulties in the treatment of silt subgrade.At present,one of the most important problems to be solved is about the exploration of efficient,economic and environmental protection silt improvement technology in China’s geotechnical engineering field.Recycled polyester fiber mainly comes from waste polyester bottles,which has not been effectively used in the improvement and reinforcement of silt subgrade.The new technology of subgrade silt improvement based on recycled polyester fiber described in this paper is to further explore the engineering characteristics and mechanism of recycled polyester fiber related to improving silt.The purpose is to provide a certain scientific basis for the promotion and application of the new technology.Based on the National Natural Science Foundation of China,the scientific research and innovation program of Jiangsu Province,and the excellent doctoral dissertation fund of Southeast University,the research object of this paper is the improved subgrade silt with recycled polyester fiber.The purpose of this paper is to show the engineering characteristics and mechanism of its improvement,the main research contents and achievements are as follows:（1）This paper systematically explores the properties of recycled polyester fiber and the status quo of its application and resource utilization,and expounds the application prospect of recycled polyester fiber in soil improvement;The application of fiber in soil improvement,micro properties,physicochemical properties,mechanical properties,constitutive model and improvement mechanism of improved soil are explored,and some existing problems and problems to be solved are pointed out.（2）This paper systematically explores the physical properties of recycled polyester fiber improved silt,and explores the influence of recycled polyester fiber dosage and curing age on the mechanical properties and physical properties of fiber improved silt.The results show that compared with fiber incorporation,the influence of curing age on the liquid limit and plastic limit of improved silt is weak,And the plasticity index decreases with age;The linear shrinkage ratio of plain soil is about 1.04～1.45 times of that of fiber soil,which indicates that fiber has a good effect on inhibiting soil shrinkage,but the linear shrinkage ratio of fiber soil is about 1.04～1.45 times of that of fiber soilρThe difference ofρ_dmax was not significant,and the OMC increased slightly with the increase of fiber content;The recycled polyester fiber has a positive effect on the road function of subgrade silt.After 28 days of curing,its resilient modulus and CBR value meet the design requirements of highway subgrade;With the increase of recycled polyester fiber age and content,the unconfined compressive strength of the corresponding recycled polyester modified silt also increases.When the content is higher than 0.2%,the strength decreases,and the maximum value can reach about 900 k Pa;For Jiangsu silt,the optimal content of fiber under renewable polyester is about 0.2%.Under the action of dynamic load,the transverse cracks of plain improved soil expand obviously,accompanied by inclined cracks,and the brittle failure is obvious.The increase of fiber makes the inclined cracks of improved soil decrease,and gradually develop to ductile failure.（3）The dry wet cycle effect of recycled polyester fiber on the strength and microstructure of the improved silt was studied,and the micro macro mechanism of the improved silt was discussed.The results showed that the macro mechanical properties and shear strength of the improved silt were deteriorated to varying degrees under the dry wet environment,the shear strength lost sharply,and the cohesion and internal friction angle decreased in a parabolic trend;With the development of wetting drying cycle,the local contact form of improved soil particles presents the form of edge edge or point surface,which is the development of surface surface form and edge surface form;The repeated action of drying and wetting cycles leads to the cyclic expansion and contraction of clay minerals（causing expansion potential）and the reciprocating dissolution of cementitious substances（accompanied by dissolution potential）by micro pore water,which leads to the fatigue damage of soil microstructure,that is,the hydrolysis of cementitious substances and pore coarsening,which leads to the deterioration of macro strength of improved soil;With the increase of dry density,the final degradation degree of the corresponding cohesion will decrease,which indicates that in order to reduce the cyclic degradation of dry and wet,we can achieve it by enhancing the compaction degree of the improved soil;The model of dry wet cycle strength degradation of improved silt was established considering three factors of dry wet cycle amplitude,dry density and lower limit moisture content of dry wet cycle.（4）Based on the unsteady thermal probe technology,the thermal conductivity of bentonite based mixture,sand mixture,silt and recycled polyester fiber modified silt was tested.The effects of water content,particle size,dry density,saturation and fine content on the thermal conductivity were studied.A generalized calculation model for predicting the thermal conductivity of soil was proposed,the results show that the thermal conductivity of soil increases with the increase of water content,dry density,particle size,saturation and fine particle content;The thermal conductivity of modified soil with recycled polyester fiber decreases with the increase of fiber content and curing age.After 60days of oxidation age,the thermal conductivity of modified soil with different fiber content is the same;Recycled polyester fiber can significantly improve the mechanical properties of soil,and the generated cementitious materials not only inhibit the thermal conductivity of soil,but also include particles and filling pores;There is a positive correlation between the thermal conductivity of recycled polyester fiber modified soil and its gravity,but when the modulus of resilience and compressive strength increase,the thermal conductivity will decrease,showing a negative correlation.（5）By means of mercury intrusion analysis（MIP）,X-ray diffraction（XRD）,energy dispersive spectroscopy（EDS）,apparent resistivity,scanning electron microscopy（SEM）and infrared spectroscopy（FTIR）,the mechanism of recycled polyester fiber and inorganic curing agent improving subgrade silt is revealed,and the reinforcement mechanism of fiber in improved soil is analyzed,The results show that the fibers are distributed in the soil alternately,forming a three-dimensional network structure,and then produce"interweaving"effect.The fibers distributed in the soil can make the fiber soil present a state of"cracking but not breaking";The improved silt mainly depends on the friction between fiber and interface and the generation of cementation material,which connects,Twines and encapsulates silt particles,and fills the pores of soil skeleton to form a more stable and dense structural system,without producing some mineral components or new functional groups.（6）Based on the fiber random distribution function and the strength yield criterion,the two-phase constitutive model of the reinforced silt is modified.The modified model truly reflects the stress-strain development characteristics of the basic phase and fiber phase of the improved silt.The research results show that:in the initial stage of strain change,the fiber dispersed in the soil is in the state of non-extension,it’s not in a state of complete tension;Or there is a relatively low level of stress,there is a relative slip between the fiber phase and the basic phase,which makes it difficult to play the role of fiber phase reinforcement.The results show that the fiber phase extends with the increase of strain,and the relative slip between the fiber phase and the basic phase decreases with the increase of stress level,and begins to play a role of reinforcement and gradually tends to be stable;Compared with the diambra model,the two-phase constitutive model considering fiber random distribution and basic phase nonlinearity proposed in this paper can simulate the non-linearity of stress-strain curve of fiber reinforced subgrade silt,and the strength criterion can analyze the yield of fiber reinforced soil under large strain Some necessary analysis models are provided for the instability of slope and foundation.