| The loess in China is widely distributed and thick,and it is mainly distributed in the central and western regions with frequent earthquakes.It has the special structural characteristics of large pores,weak cementation and rich soluble salts,and has a great risk of catastrophe under external force.Therefore,improving the engineering characteristics of loess is an important way for foundation subgrade treatment,tunnel protection and slope treatment in loess area.Based on the hollow cylinder torsional shear apparatus(compared with the dynamic triaxial apparatus,the stress state is closer to the engineering practice and has better measurement accuracy),this paper selects four different materials to compare the dynamic characteristics of the improved loess,and selects the optimal improved material and its optimal mixing ratio.The influence of mixing ratio,compaction degree,water content and consolidation confining pressure is considered from three aspects of engineering characteristics,physical characteristics and static characteristics.The backbone curve,dynamic shear modulus,damping ratio and dynamic strength index of improved loess are mainly studied.The research results can provide some theoretical reference for subgrade design,tunnel protection and slope treatment under earthquake in loess area.The main conclusions are as follows:(1)According to the comprehensive research results,the optimal dosage of each material to improve the dynamic characteristics of loess is:fly ash 25%,lignin fiber 1%,polypropylene fiber 0.75%,bentonite 8%,and the maximum dynamic shear stress of the improved soil is the largest when the lignin fiber content is 1%,and the improvement effect is the best.(2)The dynamic shear stress-strain curve of lignin fiber modified loess moves up with the increase of compaction degree and consolidation confining pressure,and decreases with the increase of water content.The dynamic shear modulus increases with the increase of compaction degree and confining pressure,and decreases with the increase of water content.The damping ratio increases with the increase of water content and confining pressure,and decreases with the increase of compaction degree.(3)The dynamic strength curve of lignin fiber modified loess under cyclic torsional shear load is obtained.It is found that the dynamic strength decreases with the increase of water content and increases with the increase of confining pressure,and shows obvious hardness after 200kPa.(4)The dynamic strength index of the modified loess with 1%lignin fiber incorporation ratio was analyzed,and the variation law with water content and vibration times was analyzed.The dynamic cohesion and dynamic internal friction angle decrease with the increase of water content,and decrease with the increase of vibration times.Different from the dynamic cohesion,the dynamic internal friction angle of the improved loess changes abruptly when the water content reaches 17%,and its decreasing rate increases abruptly.(5)Under the condition of single-stage loading,the failure appearance of lignin fiber modified loess sample is different from that of step-by-step loading.There is no or only one set of dynamic shear failure surface,and the structure outside the failure surface is complete. |
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