Blanket Functions Specifications and RDSO Guidelines 1 Commonly

Blanket : Functions, Specifications and RDSO Guidelines

1. Commonly used Terms Track Structure: Rail, Sleeper & Fastenings Track Foundation: Ballast, Blanket & Fill Material and Sub-soil

2. Pressure on Formation and Sub-soil Pressure on Formation at bottom of ballast, as good design practice, should not exceed 0. 3 MN/m 2 or 3 kg/cm 2 and Pressure on Sub-soil should not generally exceed 0. 1 MN/m 2 or 1 kg/cm 2.

2. 1 Blanket Layer Ø A layer of Coarse Granular Material laid over entire top width of formation between Sub-grade & Ballast. Ø Functions of Blanket Layer: Brings induced stress (imposed stress) within permissible stress (threshold stress) on top of sub -grade soil (Stress Reduction / Reinforcement Function). Prevents penetration of Ballast into sub-grade and also prevents upward migration of fine particles from sub-grade into ballast (Separation Function). Intercepts water coming to sub-grade and also permits drainage of water flowing upward from sub-grade due to consolidation (Drainage Function).

2. 2 Blanket Thickness as per GE: G-1(July’ 2003) Ø Depends on Axle Load (up to 22. 5 t) & Type of Soil: Minimum 100 cm for ML, ML-CL, MI & CI. Minimum 60 cm for GC, SM, SC & SM-SC. Minimum 45 cm for GP & SP (with Cu <2), GM and GM- GC. Not required for Rocky bed, SW, GW and soils conforming specification of Blanket Material. Ø Additional blanket thickness: Axle load above 22. 5 t & upto 25 t = 30 cm Axle load above 25 t & upto 30 t = 45 cm

2. 3 Guidelines and Specifications for Design of Formation for Heavy Axle Load (GE: G-14) were framed in November’ 2009 for adoption on New Projects For Axle Load of 25 t, 30 t and 32. 5 t All new formations to be constructed for 25 t Axle Load.

Rationalisation of Formation Layer Thickness on Indian Railway Track RDSO/2018/GE: IRS 0004 (D) Part-IV - July’ 2019

Suggested /Further Reference RDSO Guidelines for Earthwork in Railway Projects (GE: G-1, July 2003). RDSO Guidelines and Specifications for Design of Formation for heavy Axle Load (No. GE: G-14, November 2009). RDSO Specifications : RDSO/2018/GE: IRS-0004 (D) Part-IV, July’ 2019 “Rationalisation of Formation Layer Thickness on Indian Railway Track”.

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