Formation

Track structure –

• Track foundation – constitutes the ballast, blanket and sub grade, which placed / exist below track structure to transmit load to subsoil.

• Sub-grade – The part of embankment or of cutting above subsoil by borrowed soil of suitable quality upto bottom of blanket/ballast

• Ballast – crushed stone with desired specifications placed directly below sleepers.

• Sub ballast – The layer of coarse-grained material provided between blanket/sub grade and ballast & confined to width of ballast section.

• Blanket – The layer of specified coarse-grained material of designed thickness provided over full width of formation between subgrade and ballast.

• Formation top – the boundary between ballast and top of blanket or sub-grade • Subsoil – the soil below natural ground level.

• Formation – the general term referring to whole of blanket, sub-grade and subsoil.

Very Bad Formation : Where either speed restriction is imposed on this account or numbers of normal attention to track in a year is more than 12 

Bad Formation : Where number of normal attention to track is between 6 to 12 in a year

Design of Railway Formation

• A stable formation should be able to sustain the track geometry under anticipated traffic densities and axle loads during service under most adverse conditions of weather & maintenance of track structure, which are likely to be encountered.

• The formation should be structurally sound and the settlements should be within limits.

Blanket

• The layer between the ballast & the sub grade is the blanket

• Functions :

1. Reduce stress to sub grade

2. Keep sub grade & ballast separate

3. Prevent upward sub grade fines migration

4. Prevent sub grade attrition by ballast

5. shed water from above

6. Drain water from below

• Ballast fulfills function (1) only

• Blanket fulfills all functions and including function (1), it reduces the otherwise required greater thickness of the ballast.

• In the absence of a blanket layer a high maintenance effort can be expected • In addition, blanket dampens vibration.

Properties of Blanket Material

• Reduce stress to sub-grade :

• To serve as a structural material, it must have a 

High enough resilient modulus

Stable plastic strain accumulation characteristic under repeated wheel load

• To achieve these properties

The material must be permeable enough to avoid significant positive pore pressure build up under repeated load

Must consist of durable particles

Must not be sensitive to changes in moisture content

• Such a material is represented by mixtures of sand & gravel particles composed of crushing and abrasion resistant materials.

• PARTICLE SEPARATION: - MUST PREVENT

Intermixing of ballast & sub-grade

Upward migration of sub-grade particles into the ballast

– These properties can be achieved by proper gradation.

SEPARATION CRITERIA

• D15(filter)<5D85 (protected soil) ……. (i)

• D50(filter)<25 D50 (protected soil) ……. (ii)

– The criterion in (i) Causes the particles at the coarsest end of the protected soil (D85)to be blocked by the particles at the finest end of the filter(D15).

– The criterion in (ii) helps to avoid gap graded filters and create a filter gradation that is somewhat parallel to that of the protected soil.

• % Fines (Passing 75μ) upto 5% plastic fines & upto12% non-plastic fines  

• No skip grading ,coarse granular & well graded & more or less within enveloping curve.

• The material –well graded with Cu & Cc as below:

- uniformity coefficient, Cu = D60 /D10 > 4(preferably >7)

- coefficient of curvature, Cc = (D30)²/D60 /D10 within 1& 3

Required Ballast/Blanket Depth

• A min. ballast layer thickness is needed to provide for maintenance tamping & for void storage space

• A min. sub-ballast layer thickness is required for performing the functions of a separation/filter layer

• In addition, the combined ballast/blanket thickness must be sufficient to prevent progressive shear sub-grade failure, and excessive rate of settlement through plastic strain accumulation in the sub-grade

• As per RDSO guide lines, thickness of blanket required is 0 to one meter as per soil used in top one meter of sub-grade & Axle load.

Depth of Blanket Layer

• For axle load upto 22.5 t for different types of sub-grade soils (in top one meter)

– No need of blanket for soils

• Rocky beds except shales & other soft rock, which are susceptible to weathering or becomes muddy on contact with water

• GW – well graded gravel SW – well graded sand

• Soil confirming to blanket material

• Soil having grain size distribution curve lying on right side of enveloping curve of blanket material in consultation with RDSO

• 45 cm thick blanket for soils

– 1.GP having Cu > 2, 2.SP having Cu > 2,

– 3.GM, 4.GM-GC

• 60 cm thick blanket for soils

– 1.GC, 2.SM, 3.SC, 4.SM-SC

– 5.Should increase to one meter if PI > 7

• 100 cm thick blanket from soils

– 1.ML, 2.ML-CL, 3.CL, 4.MI, 5.CI

– Rocks which are very susceptible to weathering

• Soils having fines between 5 to 12% having dual symbol e.g. GP-GC, SW-SM etc. provide thickness as per second symbol

• Geo – synthetics can be used in consultation with RDSO as it reduce requirement of thickness of blanket.

• Blanket should be provided in new construction on all lines (even with light passenger traffic)

– In cohesive subgrade even 100 cycles of repeated load in excess of threshold strength will cause failure of formation.

• In case more than one type of soil in top one of subgrade, soil requiring higher thickness of blanket will govern.

• For other types of soils not covered above, RDSO may be consulted for deciding thickness of blanket 

• For higher axle loads

– Above 22.5t upto 25 t

• Add 30 cm thickness over & above as given for 22.5 t

– Above 25 t upto 30t

• Add 45 cm thickness over & above the given for 22.5t