Study On Engineering Performance Of Lignin-Fiber Reinforced Loess Subgrade Considering Freeze-Thaw Effect

Freeze-thaw cycles caused by climate change can change the mechanical behaviors and structure of the soil.Thus,the influence of the freeze-thaw effect should be considered when using improved loess as subgrade material in seasonal frozen soil areas.Meanwhile,as the global environmental problems have become increasingly serious,exploring and finding environmental-friendly materials has become one of the hotspots in soil performance improvement research.The purpose of this thesis is to investigate the engineering performance and the strengthen mechanism of the lignin-fiber improved loess under freeze-thaw cycles.Based on the freeze-thaw cycle tests,unconfined compressive strength tests,triaxial shear tests,dynamic triaxial tests,and scanning electron microscope tests,the static strength,dynamic deformation,and microstructure characteristics of lignin-fiber reinforced loess under the condition of different cycles of freeze and thaw are studied.The influence of freeze-thaw cycles on the static characteristics and long-term cumulative deformation behaviors of the lignin-fiber reinforced loess was analyzed using the combination of macro and micro soil mechanics.The reinforcement mechanism of the lignin-fiber to reinforce the freeze-thaw resistance of the loess was discussed,and the suitable freeze-thaw resistance improvement method of the loess subgrade was proposed.The main results and innovation as shown below:(1)Under different confining pressure,the stress-strain relationship of the lignin-fiber reinforced loess shows strain-softening behaviors.When the content of the lignin-fiber is less than 1%,the compressive strength,shear strength,cohesion,and friction angle of the reinforced loess increase significantly.The lignin-fiber reinforced loess began to enter a stable stage after 15 freeze-thaw cycles.The freeze-thaw effect reduces the compressive strength,deformation modulus,cohesion,and elastic modulus,and increases the shear strength,friction angle,and deformation modulus of the lignin-fiber reinforced loess with the lignin-fiber content of 1%.(2)With the increase of vibration times,the cumulative residual strain and cumulative maximum strain of the lignin-fiber reinforced loess increase continuously,and remain stable after 4000 vibrations.After freeze-thaw cycles,the cumulative residual strain of the lignin-fiber reinforced loess with the lignin-fiber content of 1%is similar to that of the compacted loess.During the freeze-thaw cycles,the increased rate of the cumulative residual strain and the cumulative maximum strain of the lignin-fiber reinforced loess with the lignin-fiber content of 1% is minimum.The cumulative residual strain and the cumulative maximum strain of the reinforced loess are negative linear correlation with the deformation modulus.The cumulative residual strain increase rate is linear correlation with the compressive strength loss rate.(3)With the increase of lignin-fiber dosage in the reinforced loess,the apparent porosity decreases,the proportion of small and micropore area increases,and the average pore area decreases.After the freeze-thaw cycles,the apparent porosity and average pore area of the lignin-fiber reinforced loess increase.The proportion of small and micropore of the lignin-fiber reinforced loess increases with the lignin-fiber content of 0 and 1%,while the proportion of the lignin-fiber reinforced loess decreases of other lignin-fiber content.(4)The filling effect of lignin-fiber on loess pores and the bridging effect formed by the close combination between lignin-fiber and loess particles are the reasons for lignin-fiber reinforced loess strength.The adsorption of fiber to water and the redistribution of water in the loess by freeze-thaw cycles are the reasons for the changing of the strength enhancement efficiency of the lignin-fiber reinforced loess from a rapid decline to a gradual decline.(5)The freeze-thaw effect has the minimum influence on the static strength,dynamic deformation,and microstructure of the lignin-fiber reinforced loess with the lignin-fiber content of 1%.The freeze-thaw resistance of the lignin-fiber reinforced loess with the lignin-fiber content of 1% is the strongest,which is suggested as the reinforcement ratio of loess subgrade in seasonal frozen soil areas.