Study On Liquefaction Characteristics Of Silty-Fine Sand And Liquefiable Ground Treated By Gravel Pile Composite Foundation

The newly built Kunming-Hekou railway from Kunming,Yunnan Province to Hekou,the border city of China and Vietnam,with a total length of 380 km,is the trunk line that connect China and Vietnam.The construction of Kunming-Hekou railway is of great significance in lightening the serious nervous situation for a long time of transportation of the existing narrow gauge track railway,forming Chins's medium and long-term railway network programming, and promoting China's economic and social development.There is large area saturated silty-fine sand ground along the Kun-He railway.Most of these areas are located in 8 degree earthquake region, and the ground is liquefiable with the acting of earthquake load.The liquefaction,the lateral flow of the liquefiable ground,and the embankment settlement generated by embankment damage are the main reason for the damage railway. So we must clearly know the earthquake liquefaction mechanism of saturated silty-fine sand ground,and we can quantitative assessment the influence of earthquake liquefaction for the railway subgrade.And building one or more economic and appropriate earthquake-resistance reinforcement method is very necessary and urgent.Firstly,the dynamic triaxial test were performed under different density and confining pressure.The strength resisting liquefaction,the development rule of dynamic pore water pressure during sand liquefaction process are studied based on the dynamic triaxial test.Besides,the relationship between the modulus and strain is studied.The result shows that the strength of silty-fine sand increase with the augmentation of density.The maximum excess pore water pressure approximately equivalent to 90 percent of confining pressure under undrained conditions,which is only reach about 70 percent of confining pressure under partial drained conditions.In this study,it was found that the strength of silty-fine sand under under undrained conditions was less than that under partial drained conditions,which is enhanced about 22.8%.Dynamic shear modulus decreases with the augmentation of dynamic strain,and increases with the augmentation of confining pressure. And then, to verify the appropriateness of dynamic model and the reasonableness of the parameters,undrained dynamic triaxial tests were simulated under different experiment conditions using FLAC software.The numerical predictions and laboratory measurement shows a similar tendency.Finally,a finite difference dynamic analysis was implemented on a silt-fine sandy layer with and without gravel pile treatment according to the Kun-He Railway DK23 working section.The analysis was conducted incorporated with dynamic pore-water pressure calculation and fluid flow modeling.The development rule of excess pore water pressure, the settlement and the lateral displacement of the ground were discussed during different seismic intensity.Numerical results with and without gravel pile improvement were then compared to illustrate the improvement effect of gravel pile on high liquefaction potential sandy stratum.The result indicated that the foundation without gravel pile treatment does not meet seismic-resistant requirements of Kun-He railway under the condition of 6 degree seismic fortifications,it is required reinforcement.The increase of excess pore water pressure can be restrained and dissipated effectively by the gravel pile composite foundations to improve the anti-liquefaction ability of ground.The settlement and the horizontal displacement of ground can be reduced greatly by the gravel pile composite foundation to enhance the stability of ground.Under the condition of 8 degree seismic fortifications,the design meets seismic-resistant requirements of Kun-He railway.