Ballast functions in permanent way: Which set best summarizes the primary purposes of ballast under and around sleepers?

Introduction / Context:
Ballast is a critical layer in conventional track structure. Its engineering role is multifaceted: to distribute loads, provide resilience, and ensure adequate drainage. Good ballast performance helps maintain track geometry and reduces maintenance frequency.

Given Data / Assumptions:

• Conventional ballasted track with sleepers and rails.
• Ballast graded and clean to ensure permeability and interlock.

Concept / Approach:
Proper ballast achieves three essential aims. First, it spreads wheel loads from sleepers to a broader area of formation. Second, it forms an elastic support, moderating impact and vibration. Third, its permeability drains water away from sleepers, reducing fouling and pumping.

Step-by-Step Solution:
1) Load distribution: ballast transmits and spreads concentrated sleeper loads to the subgrade.2) Elasticity: angular particles interlock yet permit slight movement, providing resilience.3) Drainage: voids in ballast facilitate water flow, protecting the formation and minimizing soft spots.

Verification / Alternative check:
Track design guides specify gradations to balance interlock and drainage; poor gradation or fouling undermines all three functions.

Why Other Options Are Wrong:

• Single-function choices ignore the multi-role nature of ballast.
• Maintenance-only help is a secondary effect; the primary functions are structural and hydraulic.

Common Pitfalls:

• Using poorly graded or fouled ballast compromises drainage and stiffness.
• Inadequate depth leads to subgrade overstress and track geometry deterioration.

Final Answer:
All of the above (load transfer, elasticity, and drainage).