FORMATION Master Copy for IRTMTCALD by IRICEN DEFINITIONS

FORMATION Master Copy for IRTMTC/ALD by IRICEN

DEFINITIONS CESS TRACK STR. B A L L A S T BLANKET FORMATION SUB - GRADE SUB - SOIL 2 TRACKFOUNDATION

FORMATION

FORMATION • The prepared flat surface or platform, which is ready to receive the ballast, sleepers, and rails, is called the formation. • It is important constituent of the track as it supports the entire track structure. • Formation consists of embankment fill. the layers comprising blanket, sub-grade , and

GENERAL DESCRIPTION OF FORMATION • Prepared either by doing additional earth work over the existing ground or by excavating existing surface and making cutting. • Can be either in the shape of embankment or a cutting. • Height of formation depends upon the ground contours and the gradient adopted. • The side slopes depends upon the shearing strength of soil. • The width of the formation depends upon the number of tracks to be laid & gauge of track.

TYPES OF FORMATION • Embankment • Cutting

EMBANKMENT • That part of the Railway land where originally ground is low, subsequently raised by earth work to give the adjacent level which was decided during the survey. • Side slope of the embankment is 2: 1 (Horizontal : Vertical)

CUTTING • That part of the Railway land where originally ground is high, subsequently lowered by earth cutting to give the adjacent level which was decided during the survey. • Side slope of the cutting is 2: 1 or even steeper in case of rock. • Catch water drain to be provided in the top cutting wall and Side drain to be provided to the side of the track to ensure smooth flow of water.

FUNCTIONS OF FORMATION • To provide a smooth and uniform bed for laying the track. • To bear the load transmitted to it from the moving load through the ballast. • To facilitate drainage. • To provide stability to the track.

DISTRIBUTION OF STRESSES • From Rail wheel contact stresses of the order of 10, 000 Kg/cm 2 to Ballast blanket contact stresses of the order of 0. 5 Kg/cm 2 i. e. at the top of the formation • AIM – To have a formation which is able to take repetitive load without distress

STRESS DISTRIBUTION

TERMS ASSOCIATED WITH FORMATION • Track Foundation- Constitutes ballast, sub-ballast, blanket and sub-grade, which is placed / exist below track structure to transmit load to subsoil. • Formation Top- Boundary ( interface) between ballast and top of blanket or sub-grade (where blanket layer is not provided). • Formation-Width- It is the distance between the edges of the prepared surface. • Cess -Portion at top of formation level, extends from toe of ballast to edge of formation. • Ballast-Crushed stones with desired specifications placed directly below the sleepers.

TERMS ASSOCIATED WITH FORMATION • Sub-Soil- It is the soil immediately under the natural ground level. • Sub-Grade- It is the upper part of embankment/cutting provided above subsoil by borrowed soil of suitable quality up to bottom of blanket/ballast. For embankment, sub-grade may be of imported soil whereas in cuttings it is the naturally occurring soil of sufficient strength. • Prepared Sub-grade-The upper part of the sub-grade is formed into a prepared Sub-grade layer, which normally has a cross-fall. • Side-Slope-It is the inclined surface of an embankment on cutting.

TERMS ASSOCIATED WITH FORMATION • Sub-ballast is a layer of coarse-grained material provided between blanket/sub-grade and ballast and confined to width of ballast section only. • To reduce the effect of loads transmitted to formation through ballast and • To guard against the possibility of ballast penetration into the formation. • It is desirable to provide a sub-ballast of 15 cm depth below the ballast layer. • The need to provide this sub-ballast may be decided by Chief Engineer (Const. ) in-charge of the project”.

TERMS ASSOCIATED WITH FORMATION • Blanket is a layer of specified coarse, granular material of designed thickness provided over full width of formation between sub-grade and ballast. Blanket may be required over the formation where • The soil is of poor quality, • Rainfall is heavy and • Traffic density is high. • The absence of blanket in such cases can lead to problems in service, such as swelling or heaving of formation.

TERMS ASSOCIATED WITH FORMATION • Blanket Whether the blanket should be provided in a particular length and if so, its thickness (which should not be less than 30 cm) should be decided by the Chief Engineer (Const. ), in-charge of the project, duly taking into account the type of soil, rainfall and density of traffic and other factors relevant to the site conditions.

TERMS ASSOCIATED WITH FORMATION Functions of Blanket A blanket/sub-ballast layer fulfil following important functions : • Primary Function : Stress Reduction Function - It reduces the traffic induced stresses on top of sub-grade to a tolerable limit. This function must be fulfilled to avoid track foundation failures. • Secondary Function : Separation Function : It prevents penetration of ballast into sub-grade and also prevents upward migration of fine particles from sub-grade into ballast.

TERMS ASSOCIATED WITH FORMATION Function of Blanket Contd. . • Drainage Function : It should intercept water coming from ballast away from sub-grade and at the same time, prevent drainage of water that is flowing upward from the sub-grade. • Prevention of Mud Pumping : It prevents mud pumping by checking attrition of sub-grade particles by ballast. The quality of blanket material should be such that it is able to carry out the above functions satisfactorily

As per SOD Formation width for single line straight track for existing works ( ACS 26 ) Straight Track Minimum width In Embankment 6850 mm In Cutting excluding side drains 6250 mm

As per SOD Formation width for Double line straight track ( ACS 26 ) Straight Track Minimum width In Embankment 12150 mm In Cutting excluding side drains 11550 mm

As per SOD Formation width for single line straight track for new works / alterations to existing works (ACS 26) Straight Track Minimum width In Embankment. In Cutting excluding side drains 7850 mm

As per SOD Formation width for double line straight track for new works / alterations to existing works (ACS 26) Straight Track Minimum width In Embankment 13160 mm In. Cutting excluding side drains 13160 mm

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