Experimental study on safety failure of clay subgrade filler under vibration-shear coupling subjected to traffic loads

Under long-term traffic loading, the clay subgrade of mountainous highways is prone to form shear bands due to the expansion of internal initial damage, which can subsequently trigger sudden instability disasters. To reveal the dynamic shear failure characteristics of damaged subgrades, this study employs a multifunctional soil-structure interface cyclic shear tester. By simulating the original defects of the subgrade with prefabricated shear surfaces, the study focuses on investigating the influence of traffic loading frequency, subgrade soil moisture content, and normal stress amplitude on shear safety performance. The results indicate that an increase in loading frequency slightly enhances the peak shear stress but significantly increases its corresponding shear displacement. The effect of moisture content on the peak shear stress exhibits a non-monotonic trend, decreasing first and then increasing. A notable stress weakening phenomenon is observed at low moisture content. An increase in normal stress amplitude significantly enhances the peak shear stress. The difference between the upper and lower peak shear stresses (Δτ) increases in an “upward convex” manner with shear displacement, and decreases with increasing frequency during the large displacement stage. Additionally, Δτ exhibits a significant exponential growth characteristic with the increase of water content and normal stress. The equivalent internal friction angles corresponding to the upper and lower peak states range from 23° to 45° and 15° to 40°, respectively, and both increase with the increase of normal stress amplitude. The research findings can provide data support for early warning of subgrade dynamic instability and safety enhancement.