Behavior evaluation of geogrid-reinforced ballast-subballast interface under shear condition

The effective functioning of a railway track under operating conditions depends largely on the performance of various rail track interfaces (e.g. ballast-subballast interface, subballast-subgrade interface). In this context, a series of large-scale direct shear tests were conducted to investigate the shear behavior of unreinforced and geogrid-reinforced ballast-subballast interfaces at different normal stresses (σn) and rates of shearing (Sr). Fresh granite ballast and subballast having average particle size (D50) of 42 mm and 3.5 mm respectively, and five geogrids with different aperture shapes and sizes were used in this study. Tests were performed at different normal stresses (σn) ranging from 20 to 100 kPa and shearing rates (Sr) ranging from 2.5 to 10.0 mm/min. The laboratory test results confirmed that the shear strength of ballast-subballast interface was highly influenced by the applied normal stress (σn) and rate of shearing (Sr). The friction angle (φ) of unreinforced ballast-subballast interface was found to decrease from 63.24° to 47.82° and dilation angle (ψ) from 14.56° to 5.23° as the values of σn and Sr increased from 20 to 100 kPa and 2.5-10.0 mm/min, respectively. Further, the breakage of ballast (Bg) was found to increase from 2.84 to 6.69%. However, geogrid inclusions enhanced the shear strength of the ballast-subballast interface and also reduced the extent of Bg. The results indicate that it is possible to establish a relationship between the friction angle (φ) and breakage of ballast (Bg), wherein the friction angle (φ) of both unreinforced and geogrid-reinforced interfaces reduces with the increase in breakage (Bg). The interface efficiency factor, defined as the ratio of the shear strength of the geogrid-reinforced ballast-subballast interface to the original shear strength of ballast-subballast interface varies from 1.04 to 1.22. Moreover, the current study revealed that the shear behavior of ballast-subballast interface was influenced by geogrid aperture size (A).