Effect of sub-freezing temperatures on ballast strength: A laboratory study

Ice formation within the ballast layer of railroad track is common in regions that experience consistent temperatures below the freezing point of water. Freely draining ballast cannot retain as much moisture as saturated fouled ballast, which may develop ice-bonded particles depending on the nature of fouling material and moisture present. This paper quantifies and compares the effect of sub-freezing [-17 ± 5°C (0 ± 10°F)] temperatures on ballast strength to non-frozen [21 ± 5°C (70 ± 10°F)] conditions. Ballast specimens were tested in a large-scale direct shear apparatus at gravimetric moisture contents, ranging from 0 % to 12 % of the dry weight of fine material [smaller than 9.5 mm (3/8-in.)], and fouling index (FI) levels ranging from 0 to 40. In non-frozen conditions, addition of moisture and fouling typically reduces the shear strength of ballast, whereas the presence of fouling and absence of moisture typically increases the strength. In a frozen condition, however, the presence of moisture and fouling increased the strength of the ballast due to ice-bonding within the ballast matrix. An increased moisture content yielded higher strengths of moderately and heavily fouled specimens in a nonlinear fashion. Non-fouled samples reduced strength due to less ice-bonding. Interestingly, higher fouling levels nonuniformly changed the strength of the ballast depending upon whether mechanical friction and aggregate interlock or ice-bonding of fine material generated higher strength. Ballast resistance is a key parameter for quantifying the stress state present within the rail, thus requiring accurate assessment of ballast strength in a multitude of environments.