What sort of sub-ballast, blanket, and subgrade layers are provided in the track formation?

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What sort of sub-ballast, blanket, and subgrade layers are provided in the track formation?

July 18, 2019, 2:48 PM

IR generally does not use a separate sub-ballast layer below the ballast layer. A blanket layer of coarse, granular material is usually provided directly below the ballast layer. Blanket layers are not provided for tracks on rocky beds, or on well-graded gravelly or sandy beds.

Blankets of at least 45cm thickness are provided for tracks laid on poorly graded gravel or sand beds, or on silty gravel or silty / clayey gravel beds. Blanket layers of 60cm are required for clayey gravel, clayey sand, silty sand, or clayey / silty sand beds. A 1m-thick blanket is provided for silt, silty clay, or clay of low plasticity or in conditions where the underlying rocks are of a type known to be excessively susceptible to weathering. The blanket layer is generally composed of well-graded sandy gravel or crushed rock with specified distributions of size and curvature. Mixtures of fines (metal, plastic, etc.) from industrial applications are used in specific proportions in some cases, as are certain other waste materials that conform to specified mechanical, chemical, and geometric requirements.

The subgrade is generally built up from a mixture of soil and stone fragments, cobbles, and waste materials, crushed brick, etc. The blanket and subgrade are built up at a slope of about 2:1. The entire embankment may rise to 6m with most ordinary kinds of materials used for the blanket and subgrade. In case the subgrade is thicker than 1m or so, usually a 30cm layer of compacted soil is provided for every 1m-3m of the subgrade thickness.

Source – IFRCA.org

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