Experimental Study on Dynamic Strength of Subgrade Loess under Continuous and Intermittent Loads

The cyclic dynamic stress generated during train operation presents a significant challenge to the dynamic strength of subgrade fill materials. Existing research has mostly simulated train loads using continuous loading methods, which fails to fully reflect the intermittency of these loads. To investigate the differences in dynamic strength of loess subgrade under continuous and intermittent loading, a series of consolidated undrained tests under continuous and intermittent loading conditions were conducted using a GDS dynamic triaxial apparatus. The influences of confining pressure and dynamic stress amplitude on the dynamic strength of the soil were examined. The effects of different loading methods on the dynamic strength and strength parameters of the subgrade loess were compared. The experimental results indicate that the dynamic strength of the loess subgrade increases with higher confining pressure, but the growth rate diminishes gradually. Both dynamic cohesion ( c _d) and dynamic friction angle ($ {\varphi _{\text{d}}} $) decrease with the increase in the failure cycles (lg N _f), showing an overall linear relationship. Under intermittent loading, the soil exhibits a marked increase in c _dand$ {\varphi _{\text{d}}} $compared to continuous loading, with c _dincreasing by 2.18%–5.09% and $ {\varphi _{\text{d}}} $ by 4.03%–13.78%. By normalizing the dynamic strength using the static triaxial shear strength, an empirical formula for the dynamic strength of the loess subgrade based on static strength is proposed, which provides a critical basis for assessing the stability of the subgrade under dynamic loads.