Short Note on Blanketing Material

Blanketing Materia :- A layer of specified coarse, granular material of designed thickness provided over full width of formation between sub-grade and ballast is called BLANKET.

The main functions of the Blanket layer are as under:-

i. It reduces the traffic induced stresses at top of sub-grade to a tolerable limit to avoid track foundation failure.

ii. It prevents penetration of ballast into the sub-grade and also prevents the upward migration of fine particles from sub-grade into the ballast. iii. It intercepts downward percolation of water, facilitates run off of water from top surface of blanket layer and provide drainage path for upward movement of water from sub- grade ensuring dissipation of excess pore water pressure.

SPECIFICATIONS OF BLANKET MATERIAL:-

The blanket material should generally conform to the specifications prescribed as under:-

i. It should be coarse, granular and well graded. ii. The co-efficient of uniformity Cu should be > 7 and Cc between 1and 3.

iii. Fines (passing 75 microns) should be limited 3% to 10%.

iv. Minimum CBR value 25 of the blanket material compacted at 100% of MDD.

v. The size gradation should be within the specified range or enveloping curves.

vi. Los Angeles Abrasion value < 35-40%

vii. Filter criteria should be satisfied with sub-grade layer, as given below: (OPTIONAL)

CRITERIA-1 : D15 (Blanket) < 5xD85 (Sub-grade)

CRITERIA-2 : D15 (Blanket) > 4 to 5 D15 (Sub-grade)

CRITERIA-1 : D50 (Blanket)< 25xD50 (Sub-grade)