Blanket and Blanketing Material
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
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.
quarry grit, gravel, and other non-cohesive materials
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%.
material should be properly graded and its particle size distribution curve
should lie within the standard enveloping curves.
liquid limit should not exceed 35 and the plasticity index should be below 10.
uniformity coefficient should be above 4, preferably above 7. The coefficient
of curvature, which is D230/D60
× D10, should be between 1 and 3.
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.