Determination of Ballast Layers Shear Interlocking Stiffness by Laboratory Method

Ballast shear interlocking is phenomenon that occurs between adjacent ballast masses under sleepers due to axial loading. This phenomenon is caused when three factors: 1) large thickness of the ballast layer, 2) large stress distribution angle at ballast mass of under sleeper, 3) small sleeper spacing are available at ballasted track. Thus, despite these three factors together and based on the assumption of transfer load from sleeper into ballast mass in accordance with a stress distribution pyramid, caused an overlap between two adjacent pyramids that this intersection would mobilize shear stiffness and shear damping in addition to stiffness and damping ballast layer in the vertical direction. This stiffness and damping is called the shear coupling. Considering the fact that so far, few studies have investigated about these parameters, but the method for calculating them is not provided in the technical literature. Therefore, in this research by developing a laboratory method in order to determine the ballast shear stiffness parameter and shear strength, shear tests of ballast in the vertical direction in the locking zone under static overheads 200, 300 and 400 kg in the relative density is 70.95% done and the effect of relative densities of 40, 70 and 94.44 percent on the shearing of the ballast is investigated. For this purpose, a device for shear of ballast in Railway Superstructure Laboratory of the railway engineering school was designed and built. Finally, based on the results and the integrate them, in order to use the results a mathematical relationship for ballast shear stiffness parameter has been determined which includes the axial load and vehicle speed and high density relative of the ballast.