Railroad Eng., ballast and subballast

1. Chapter 21
Ballast
Subballast

2. Roadway and Track
Ballast
Sub-Ballast

3. Ballast & Sub-Ballast Cross-Section*
• Ballast and sub-ballast are the final stages in load distribution
• In addition to distributing vertical loads, ballast has a critical role maintaining
longitudinal and lateral stability of track.
• Ballast and sub-ballast must provide adequate drainage.
• Ballast is subject to pulverization from loading and unloading as trains pass over,
thereby generating fine particles that clog the ballast
3
Ballast
Sub-Ballast
Tie
Rails
* AREMA recommended practice.

4. Subballast
• Similar to highway
base material (DGA)
• Fine grained -- smaller
size than ballast and more
fine-size particles
• Compacts tight and
dense with low % voids
• Separates and
Waterproofs
Use AREMA Recommended Practices

5. Ballast
Transmits Loadings
Anchors the Track
Drains the Track
Provides Resilience
Must be Adjustable

6. Ballast – permeable, granular material placed
under and around the ties to promote track
stability -- should be hard, tough, abrasion
resistant, and angular

7. Ballast
• Use highest quality material possible
• Obtain to proper gradation
• Fill cribs to 1 in. below rail base
• Minimum shoulder width
– 9 in. jointed rail
– 12 in. CWR
• Minimum depth below bottom of tie
– Wood/Composite ties
• 8 in. if track carries < 10 MGT annually
• 12 in. otherwise
– Concrete ties --12 in.

8. Types of Ballast
– Crushed Granite, Basalt,
Traprock & Slag are common
• high tonnage and mainlines
– Dolomite, Limestone
• low tonnage lines & yard tracks
– Gravel & Sand
• yard tracks, maybe?
Use AREMA Recommended
Practices

9. Basic Purposes
– Transmits Loadings Vertically
– Anchors – Lateral & Longitudinal
– Drains
– Reduces Frost Heaving & Capillary
– Facilitates Adjusting
– Retards Vegetation
– Provides Resilience
Ballast View

10.  FacilitatesAdjusting

11. – Retards Vegetation
– Provides Resilience

12. • Thickness Design
– Talbot
•Pc = 16.8 Pa / h1.25
Subgrade Tie
– Arbitrary Standard
– Rational Calculation???

13. • If Thickness is Inadequate
Overloads Subgrade
Increases Pore Water Pressure
Weakens Soil

14. If Thickness is
Inadequate:
• Shorter Maintenance
Cycles
• More Slow Orders
• Track Wears Quicker
• Safety a Concern

15. Surface Problem
(Cross level)
Track
Settlement and
Pumping

16. Profile Trouble Spots

18. • Characteristics
– Functionally Adequate & Economical
– Provides Some Resiliency
– New Track Needs to be Seasoned (Densified)
– Provides Confinement for Track
– Provides Lateral Stability for Track

19. • Specifications
– Strength
– Toughness
– Durability
– Stability
– Drainability
– Cleanability
– Workability/Adjustability
– Availability
– Min. Purchase Price
– Resist Deformation
– Overall Economy

20. Ballast Gradations
Similar to ASTM Specifications for Aggregate

21. • Quality (Deleterious Materials)
– Soft & Friable Particles (max 5%)
– Clay Lumps (max 0.05%)
– Minus No. 200 Sieve (max 1%)

22. • Soundness (Durability)
– Sulfate (max 7% loss)
• Abrasion Resistant (Strength/Toughness)
– Los Angeles & Wet Mill (max 40% loss)

23. • Weight (Mass) per Unit Volume
– Standard Unit Weight Test (lb/ft3)
• Other Tests
– New Ones
– Experience
• Performance Specifications are Largely Lacking
(Use Qualitative & Relative)

27. • Application
– Design
• Estimate ‘h’, shoulder width, slope
• Full crib minus 1 in., top ½ h(t) high quality
• Quantities minus tie volume – new construction
don’t deduct for raise
• 15 to 20% shrinkage
– Construction
• Place Subballast, Build Track
• Unload Ballast, Raise (Pull Track), Tamp Ballast
• Regulate and Broom

28. – Maintenance – Four Reasons
• Fill Cribs
• Restore X-Section
• Replace
• Adjust Geometry

29. • Compaction (Tamping) – under bearing
areas
– Hand (spot)
– Mechanized
• Maintenance – used to adjust geometry
– Smooth
– Smooth/Surface

31. Ballast Compactor/Stabilizer

35. • Frequency of Surfacing Track
CL = 10 x [(T-A) / B]
• T = 50 MGT/yr
• CL = 2 to 5 years typical (can be more frequent)
• A and B = constants, depend on many factors

36. • Dirty Ballast – not desirable
– Sources
• Subgrade Intrusion (from Below)
• Internal (Ballast breakdown)
• External (from Above)
– Cleaning and Rehabilitation
• Shoulder
• Crib Out (Backhoe)
• Undercutting (6 to 8 in. below ties)
• Dirt Disposal – dump over side on fill

37. Track Undercutting