Dynamic Properties Of Coarse-grained Fillings Used Under High-speed Railway Subjected To Freeze-thaw Cycles

High-speed railway is characterized by its high speed,low energy consumption and light pollution.It has become the trend of high-speed traffic development in the world.In recent years,the construction of high-speed railways in the cold regions of China has developed rapidly.And the total mileage of the high-speed railways that built or currently under construction in cold regions will exceed 7000 km.The dynamic characteristics of the subgrade coarse-grained fillings have evolved during the action of freeze-thaw cycles and dynamics load,and so does the performance of the subgrade.At present,many scholars at home and abroad have conducted many studies on the dynamic and static characteristics of coarse-grained soils.The study of freeze-thaw effect is mainly focused on the static characteristics of coarse-grained soils,while the study on its dynamic characteristics under freeze-thaw conditions needs to be further strengthened.Therefore,this paper relies on high-speed railways in the cold regions in China.The B-group fillings of the high-speed railway subgrade is chosen as the research object.The dynamic parameters of coarse-grained soils,such as dynamic shear modulus,damping ratio and threshold shear strain and their influencing factors are studied through the combination of indoor experiments and theoretical analysis in this article,which provides suggestions for the design and the analysis dynamic response of subgrade in seasonally frozen areas.The main contents and results of this paper are as follows:(1)The particle-size distribution curve,maximum dry density and optimal moisture content of the Group B fillings were obtained by performing indoor particle analysis,compaction tests and freeze-thaw time index tests,which providing important control indexes for dynamic triaxial tests.The results showed that the fillings were classified as fine gravel and group B fillings;the maximum dry density and the optimum moisture content were found to increase with the increase of fines content;The freezing time of 200 min and thawing time of 150 min were taken as indexes to complete a freeze-thaw cycle.(2)A series of dynamic triaxial tests were performed on the coarse-grained soil fillings using GDS dynamic triaxial apparatus.The dynamic stress-strain relationship,dynamic shear modulus and damping ratio under different confining pressure,freezethaw cycles and fines content were obtained.The dynamic shear modulus model considering the effect of confined pressure,freeze-thaw cycles and fines content and the unified formula for the damping ratio were given.The results showed that the dynamic stress-strain relationship of coarse-grained soil became steeper with the increase of confining pressure,and it became more smooth with the increase of the freeze-thaw cycles and fines content;the dynamic shear modulus increased with the increase of confining pressure,and it decreased with the increase of freeze-thaw cycles and fines content;the dynamic shear modulus developed nonlinearly with the dynamic shear strain and its linear threshold shear strain is about 10-3;different influence factors had little effects on the damping ratio.(3)The deformation characteristics of coarse-grained soil under short-time dynamic loading were studied for different influencing factors.The effects of different factors such as confining pressure,number of freeze-thaw cycles,and fines content on the deformation threshold shear strain were analyzed.The results showed that the residual strain and threshold shear strain decreased with the increase of confining pressure,and they increased with the increase of freeze-thaw cycles and fines content;the dynamic shear strain over 10-6~10-5 resulted in the residual deformation in the fillings.