Experimental investigation on direct shear mechanical characteristics of ballast under dynamic normal loading

The shear resistance of ballast under dynamic loading directly affects the stability and bearing capacity of the ballast bed; however, the influence of load characteristics on direct shear parameters remains insufficiently understood. A direct shear testing scheme for premium-grade ballast was presented in this study, utilising a dynamic direct shear apparatus configured to accommodate varying characteristics of dynamic loading. Twelve loading conditions were tested, including constant normal load shear and dynamic normal load shear at different frequencies and amplitudes. The effects of these parameters on shear strength, vertical deformation, particle breakage, vertical dynamic stiffness, and damping were analyzed. Results indicate that, under identical static loads, dynamic normal loading exerts little influence on the initial shear force but either strengthens or weakens shear strength. High-frequency loading weakens shear strength, whereas high-amplitude loading strengthens it. Dynamic normal loading suppresses the shear dilation effect, leading the ballast towards a stable compressive state during shearing and causing greater particle breakage. Both compressive deformation and particle breakage increase nonlinearly with the frequency and amplitude of dynamic loading. In addition, ballast dissipates energy through intrinsic damping, achieving a relatively stable dynamic stiffness in the later stages of shearing. These findings provide theoretical support for the scientific maintenance and operation of railway ballast beds.