Experimental study on the direct shear characteristics of crushed stones subjected to vibrational normal stress

Crushed stones are widely used in infrastructure construction, yet their frictional response under time-varying normal stress remains insufficiently understood because of limitations in dynamic testing technology. In this study, a series of large-scale direct shear tests was conducted under constant normal load (CNL) and dynamic normal load (DNL) conditions to examine the effects of normal stress vibration frequency, vibration amplitude, and shear velocity. The results show that shear stress, friction coefficient, and normal displacement fluctuate periodically under vibrational normal stress. The shear stress evolution can be divided into three stages, and there is a phase lag between the peak normal stress and peak shear stress that tends to be constant as shear displacement increases. The shear resistance is controlled jointly by vf ratios and amplitude of normal stress vibration. Shear responses under three seismic normal stress further indicate that seismic-wave-induced normal stress generally weakens the shear strength of crushed stones. These findings provide experimental evidence for understanding the stability of crushed stone foundations subjected to dynamic disturbances.