CE-363

Lecture 11: Ballast

Dr. Ankit Gupta, Assistant Professor

Department of Civil Engineering

National Institute of Technology Hamirpur

Lecture Outline

Ballast

Functions and Requirements

Types of ballast

Design of section

Ballast

Definition

It is a layer of broken stone, gravel,

moorum or any other gritty material placed

and packed below and around sleepers for

distributing the load from the sleepers to

the formation and for providing drainage

as well as giving longitudinal and lateral

stability to the track.

Ballast - Functions

Provide level and hard bed for sleepers

Hold sleepers in position

Transfer and distribute load to wide area

Provide elasticity and resilience to track

Provide longitudinal and lateral stability

Provide effective drainage

Maintain level and alignment of track

Ballast - Requirements

Tough and resist wear

Hard enough

Cubical, with sharp edges

Non-porous, non water absorbent

Resist attrition

Durable

Good drainage

Cheap and economical

Ballast – Types

Broken stone

Mostly used on Indian Railways

Procured from hard stones like granite,

quartzite, hard trap etc.

Economical in long run

Ballast – Types

Sand

Mostly used for CI Pot sleepers

In low density areas with wooden and ST

sleepers

Coarse sand is preferred

Causes excessive wear on rail top and

moving parts of the rolling stock

Ballast – Types

Blast Furnace Slag, Cinders

Used in yards, sidings etc.

Used as initial ballast in new construction

Cheap and easily available

Corrosive, harmful for steel sleepers and

fittings

Ballast – Types

Moorum

Used as initial ballast in new constructions

or as sub-ballast.

Also used as blanketing material on black

cotton soil.

Others

Gravel, River Pebbles, Kankar, Brick

Ballast, etc.

Ballast – Types: Comparison

Drainage property

Cost

Packing material

Wear

Stability / resilience property of track

Maintenance of track

Ballast – Types: Comparison

Suitability / unsuitability

Quality of track

Availability / procurement

Harmful effects, if any

Ballast – Section

Ballast section

Ballast – Cushion

Minimum Depth of Ballast section

= 1/2[Sleeper Spacing – width of sleeper]

Sleepers

c/c Sleepers

Spacing

Depth 45o 45o

Width of

Sleeper

Ballast – Cushion

Minimum Depth of Ballast section

Ballast - Cushion

Based on stresses in ballast section

Depth of

ballast

(z)

Spacing of

sleeper

Bottom of

Tie Pressure at the bottom of tie = Pa

Pressure at depth ‘z’ = Pz

Pressure at distance ‘x’ for depth ‘z’

= Px

Pz

Px

Pa

P

Width of sleeper (w)

Length of

sleeper (L)

Ballast - Cushion

Based on stresses in ballast section

Pressure at the bottom of the tie (sleeper)

is given by:

Pa = P/ (w . L/2) in kg/sq.m

Where, P = isolated wheel load, in kg

w = width of the tie or sleeper, in m

L = length of the sleeper, in m

Ballast - Cushion

Based on stresses in ballast section

Pressure at depth ‘z’ (in cm) below centre

of tie width is computed as:

Pz = 5.24 Pa / z1.25 (in kg/sq.m)

Pressure at distance ‘x’ (in cm) at depth ‘z’

is given by:

Px = 0.48 (Pa / z) . 10-2.06 (x / z) (x / z)

(in kg/sq.m)

Ballast – Specifications

Quality

Hard and durable

Free from inorganic and organic residues,

inferior and harmful substances

Top width 3350 mm to 1850mm

Ballast – Specifications

Depth of Cushion

BG / MG

300mm (A, Q)

250mm (B, C, R1)

200 mm (D, R2)

150mm (E, S)

NG: 150mm

Ballast – Specifications

Physical Properties Abrasion test value Max 30%,

relaxable to 35%

Impact value Max 20%, relaxable to 25%

Elongation and Max 50% each Flakiness index

Specific gravity Min. 2.65

Water absorption Max. 1%

Ballast – Specifications

Size and Gradation

Size designated – 50 mm

Retained on 65mm – nil, 5% max

Retained on 40mm – 40% to 60%

Retained on 20mm – not less than 98%

for machine

crushed and not

less than 95% for

hand broken

Ballast – Specifications

Size and Gradation - Oversized

Rejected if more than 10% of ballast

remains on 65mm sieve

In case the retention on 40mm sieve

exceeds 70%, it is rejected

2% in machine crushed and 5% in manually

crushed permitted for passing on 20mm

sieve

Ballast – Specifications

Size and Gradation - Undersized

Rejected if retention on a 40mm sieve is

less than 40%

Rejected if retention on a 20mm sieve is

less than 98% for machine crushed ballast

or 95% for hand broken ballast

Ballast - Compaction

Packing

Compacted ballast cushion laid below

sleepers as per the gradation

Boxing

Relatively Loose ballast Placed on the side

of the sleepers to provide them lateral

stability

Ballast – Screening

Screening of Ballast – Procedure of

renewing the ballast section got degraded

due to:

Crushed material clogs the voids causing

drainage problem

Penetration of ballast in the formation

Blowing away of the ballast

Ballast – Screening

Screening of Ballast