Abstract
A series of large-scale direct shear tests was conducted to capture the interface shear behaviour of ballast with and without geogrids. A fresh granite ballast with a maximum particle size (Dmax) and average particle size (D50) of 65 and 42 mm, respectively, and five geogrids having different aperture shapes and sizes were used in this study. Laboratory experiments were performed at different applied normal stresses (σn) and rates of shearing (Sr) ranging from 35 to 140 kPa and 2·5 to 10·0 mm/min, respectively. The test results confirmed that the shear strength of the ballast was highly influenced by σnand Sr. The internal friction angle of the ballast (ϕ) was found to decrease from 66·5 to 58° and the dilation angle (ψ) from 25·24 to 8·05° with an increase of σnand Sr. Moreover, sieve analysis of ballast samples after testing revealed that a significant amount of particle breakage occurred during shearing. The breakage of the ballast, evaluated in terms of Marsal's breakage index (Bg), increased from 5·12 to 13·24% with an increase in σnand Sr. However, the inclusion of geogrids was found to have a profound influence on the interface shear strength and reducing the extent of the dilation angle and particle breakage.
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