B - Ballast, Fundamental Properties

Question 1

What causes particles to re-orientate?

Answer 1

Although the crushing strength of a ballast is over 100 MPa, the stress at the contact between sleeper and particle could be over this
If an individual particle is overstressed it will fracture and the ballast particles will re-orientate under the stress at contacts reduce

Question 2

What is the approximate value of Resilient Modulus (Young’s) of a railway ballast?

Answer 2

100MPa

Steel - 210GPa
Concrete - 30GPa

Question 3

What is the key form of deformation in ballast?

Answer 3

Frictional Slip

Question 4

What are the properties of ideal ballast?

Answer 4

High stiffness (Less Important)
High strength (Very Important)
Crushing Resistance (Very Important)
Not affected by: dirt, contamination from below, rainfall, frost and chemical weathering
Absorbs noise
Allows easy maintenance

Question 5

What is the angle of friction of ballast?

Answer 5

50 degrees

Question 6

What is the result of slip in ballast? (Discuss dependance on load size)

Answer 6

If shear is not too great, almost all slips return, negligible deformation
More shear and some will never recover, some deformation every load cycle
Approach failure stress and many will not recover, rapid accumulation of deformation

Question 7

How does crushing affect slip?

Answer 7

If a rock is crushed it will be harder for slips to recover