A blanket can be defined as an intervening layer of superior material that is provided in the body of the bank just underneath the ballast cushion. It is different from the sub-ballast, which is provided above the formation. The functions of the blanket are twofold:
(a) to minimize the puncturing of the stone ballast into the formation soil and
(b) to reduce the ingress of rain water in to the formation soil.
The blanket should generally cover the entire width of the formation from the shoulder, except in the case of sand or similar erodable material, where it should be confined within berms of a width of 60-75 cm. The depth of the blanket should normally be about 30 cm in ordinary clayey soil. However, if the formation soil is particularly weak, a thicker layer of up to 60 cm may be necessary, depending on the shear properties of the formation soil. The blanket material should have the following properties.
For sand, quarry grit, gravel, and other non-cohesive materials
(a) The blanket material should be coarse and granular.
(b) If the material contains plastic fines, the percentage of fines (particles measuring up to 75 microns) should not exceed 5. If the fines are non-plastic, then they can be allowed up to a maximum of 12%.
(c) The material should be properly graded and its particle size distribution curve should lie within the standard enveloping curves.
(a) The liquid limit should not exceed 35 and the plasticity index should be below 10.
(b) The uniformity coefficient should be above 4, preferably above 7. The coefficient of curvature, which is D2 30/D60 x D10, should be between 1 and 3.
(c) When macadam is used as the blanketing material, it should be compacted in a suitable number of layers at or near the optimum moisture content so as to achieve not less than 90% of the maximum dry density as determined by Proctor's test using heavy compaction.
(d) If an erodable material is used as a blanket, it should be confined in a trench and sand drains should be provided across the track and the blanket. These cross sand drains with adequate slope should be 5-30 cm below the bottom of the blanket and spaced 2-4 m apart.