Download - Lecture 11 Final
CE-363
Lecture 11: Ballast
Dr. Ankit Gupta, Assistant Professor
Department of Civil Engineering
National Institute of Technology Hamirpur
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 – 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
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