Research On Dynamic Stability Of Cement-Improved Loess Subgrade Of High-Speed Railway

The requirement on the deformation of subgrade caused by high-speed railway is strict.As the basis of ballastless track structure, subgrade is the prerequisite for the smoothness and stability of track. With the advancement of western development, more and more problems were encountered in the construction of high-speed railway in loess area, especially lack of optimum filling materials A and B. In order to meet the requirements of high-speed railway on subgrade filling, the engineering property of loess must be improved. And the best solution is to add hardening agents to loess, for example cement. But, there has little research on the long-term dynamic stability of cement-improved loess subgrade. Neither relevant experience data nor relative mature theory can be used. Thus, the long-term dynamic stability of cement-improved loess subgrade under cyclic loading is studied by dynamic triaxial test and numerical simulation in this paper. And the main conclusions were carried out as follows:(1) The compaction test, liquid plastic limit test, unconfined compressive strength test and standard triaxial test were carried out in order to obtain the engineering properties of remolded loess and cement-developed loess. The cement ratios are 3%, 5% and 7%,respectively. The results show that the unconfined compressive strength,cohesion and internal friction angle increase with the increase of cement content. And these three kinds of cement-improved loess can meet the strength requirement of high-speed railway on subgrade filling.(2) In order to study the deformation characteristic of remolded loess and cement-developed loess under the effect of train load which is long-term and low amplitude,the actual stress state of high-speed railway subgrade was simulated by GDS dynamic triaxial tester. The results show that elastic deformation and accumulated plastic deformation decrease with the increase of cement mixing ratio and decrease gradually along the depth direction. Then the prediction model of accumulated plastic deformation was obtained.Considering the construction quality and economy, it is recommended that the cement ratio is 5%.(3) Based on short-time and fatigue cycle triaxial tests, the short-time and fatigue dynamic shear strain threshold and critical dynamic stress of 5% content cement-improved loess were studied. The results show that short-time dynamic shear strain threshold ?tvS,fatigue dynamic shear strain threshold ?tvL and critical dynamic stress ?dcr increase with the increase of confining pressure and consolidation ratio. Then the fitting formulas of these three parameters were obtained by linear fitting mehod. Through analysis, the fitting factor of fatigue dynamic shear strain ?tvL divided by short-time dynamic shear strain ?tvS is 0.25-0.35.(4) Based on the dynamic equation of vertical vehicle-track-subgrade coupled system under secondary suspension, the varying regularity of vertical displacement, dynamic acceleration, dynamic stress and dynamic shear strain were obtained by finite element analysis software MIDAS/GTS. The results show that these four parameters decrease rapidly along depth direction, but increase with the increase of train velocity.(5) Combined with the numerical simulation results, the critical dynamic stress method and dynamic shear strain method were used to evaluate the dynamic stability of cement-improved loess subgrade. The result shows that dynamic shear strain method is more reasonable and accurate. Based on unconfined compressive strength test, the relationship between shear threshold and the unconfined compressive strength was established. Then the evaluation parameters of dynamic stability can be obtained through the simple unconfined compressive strength test, using equations ?tvSO = (0.188 ? 0.212)?cu, ?tvS = (0.73?3 +14.60Kc+ ?tvSO)×105 and ?tvL=(0.25?0.35)?tvS.